[RFC,1/3] kcov: remote coverage support

Currently kcov can only collect coverage for syscalls that are issued
from the current process. This patch adds support for KCOV_REMOTE_ENABLE,
that makes it possible to collect coverage for arbitrary parts of the
kernel code, provided that this part is annotated with kcov_remote_start
and kcov_remote_stop.

This allows to collect coverage from two types of kernel background
threads: the global ones, that are spawned during kernel boot and are
always running (e.g. USB hub_event); and the local ones, that are spawned
when a user interacts with some kernel interfaces (e.g. vhost).

To enable collecting coverage from a global background thread, a unique
global id must be assigned and passed to the corresponding
kcov_remote_start annotation call. Then a userspace process can pass this
id to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
kcov_remote_arg struct. This will attach kcov device to the code section,
that is referenced by this id. Multiple ids can be targeted with the same
kcov device simultaneously.

Since there might be many local background threads spawned from different
userspace processes, we can't use a single global id per annotation.
Instead, the userspace process passes an id through the common_handle
field of the kcov_remote_arg struct. This id gets saved to the kcov_handle
field in the current task_struct and needs to be passed to the newly
spawned threads via custom annotations. Those threads should be in turn
annotated with kcov_remote_start/kcov_remote_stop.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 Documentation/dev-tools/kcov.rst |  99 +++++++
 include/linux/kcov.h             |   9 +
 include/linux/sched.h            |   6 +
 include/uapi/linux/kcov.h        |  11 +
 kernel/kcov.c                    | 434 ++++++++++++++++++++++++++++---
 5 files changed, 524 insertions(+), 35 deletions(-)

On Thu, Oct 17, 2019 at 7:44 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> Currently kcov can only collect coverage for syscalls that are issued
> from the current process. This patch adds support for KCOV_REMOTE_ENABLE,
> that makes it possible to collect coverage for arbitrary parts of the
> kernel code, provided that this part is annotated with kcov_remote_start
> and kcov_remote_stop.

Nitpick: () after function names is the common C style.

> This allows to collect coverage from two types of kernel background
> threads: the global ones, that are spawned during kernel boot and are
> always running (e.g. USB hub_event); and the local ones, that are spawned
> when a user interacts with some kernel interfaces (e.g. vhost).
>
> To enable collecting coverage from a global background thread, a unique
> global id must be assigned and passed to the corresponding
> kcov_remote_start annotation call. Then a userspace process can pass this
> id to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
> kcov_remote_arg struct. This will attach kcov device to the code section,
> that is referenced by this id. Multiple ids can be targeted with the same
> kcov device simultaneously.
>
> Since there might be many local background threads spawned from different
> userspace processes, we can't use a single global id per annotation.
> Instead, the userspace process passes an id through the common_handle
> field of the kcov_remote_arg struct. This id gets saved to the kcov_handle
> field in the current task_struct and needs to be passed to the newly
> spawned threads via custom annotations. Those threads should be in turn
> annotated with kcov_remote_start/kcov_remote_stop.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kcov.rst |  99 +++++++
>  include/linux/kcov.h             |   9 +
>  include/linux/sched.h            |   6 +
>  include/uapi/linux/kcov.h        |  11 +
>  kernel/kcov.c                    | 434 ++++++++++++++++++++++++++++---
>  5 files changed, 524 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 42b612677799..1cf0af95a801 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -34,6 +34,7 @@ Profiling data will only become accessible once debugfs has been mounted::
>
>  Coverage collection
>  -------------------
> +
>  The following program demonstrates coverage collection from within a test
>  program using kcov:
>
> @@ -128,6 +129,7 @@ only need to enable coverage (disable happens automatically on thread end).
>
>  Comparison operands collection
>  ------------------------------
> +
>  Comparison operands collection is similar to coverage collection:
>
>  .. code-block:: c
> @@ -202,3 +204,100 @@ Comparison operands collection is similar to coverage collection:
>
>  Note that the kcov modes (coverage collection or comparison operands) are
>  mutually exclusive.
> +
> +Remote coverage collection
> +--------------------------
> +
> +With KCOV_ENABLE coverage is collected only for syscalls that are issued from
> +the current process. With KCOV_REMOTE_ENABLE it's possible to collect coverage
> +for arbitrary parts of the kernel code, provided that this part is annotated
> +with kcov_remote_start/kcov_remote_stop.

Same here.

> +This allows to collect coverage from two types of kernel background threads:
> +the global ones, that are spawned during kernel boot and are always running
> +(e.g. USB hub_event); and the local ones, that are spawned when a user
> +interacts with some kernel interfaces (e.g. vhost).
> +
> +To enable collecting coverage from a global background thread, a unique global
> +id must be assigned and passed to the corresponding kcov_remote_start annotation
> +call. Then a userspace process can pass this id to the KCOV_REMOTE_ENABLE ioctl
> +in the handles array field of the kcov_remote_arg struct. This will attach kcov
> +device to the code section, that is referenced by this id. Multiple ids can be
> +targeted with the same kcov device simultaneously.
> +
> +Since there might be many local background threads spawned from different
> +userspace processes, we can't use a single global id per annotation. Instead,
> +the userspace process passes an id through the common_handle field of the
> +kcov_remote_arg struct. This id gets saved to the kcov_handle field in the
> +current task_struct and needs to be passed to the newly spawned threads via
> +custom annotations. Those threads should be in turn annotated with
> +kcov_remote_start/kcov_remote_stop.
> +
> +.. code-block:: c
> +
> +    struct kcov_remote_arg {
> +       unsigned        trace_mode;
> +       unsigned        area_size;
> +       unsigned        num_handles;
> +       uint64_t        common_handle;
> +       uint64_t        handles[0];
> +    };
> +
> +    #define KCOV_REMOTE_MAX_HANDLES            0x10000

This is unused. I would remove it. I assume you just copied all of the
current declarations. But they are extended over time, so later it
will be just some random subset of used and unused declarations.

> +
> +    #define KCOV_INIT_TRACE                    _IOR('c', 1, unsigned long)
> +    #define KCOV_ENABLE                        _IO('c', 100)

Same here and for few others below.

> +    #define KCOV_DISABLE                       _IO('c', 101)
> +    #define KCOV_REMOTE_ENABLE         _IOW('c', 102, struct kcov_remote_arg)
> +
> +    #define COVER_SIZE (64 << 10)
> +
> +    #define KCOV_TRACE_PC      0
> +    #define KCOV_TRACE_CMP     1
> +
> +    #define KCOV_REMOTE_ID     0x42
> +
> +    int main(int argc, char **argv)
> +    {
> +       int fd;
> +       unsigned long *cover, n, i;
> +       uint64_t handle;
> +
> +       fd = open("/sys/kernel/debug/kcov", O_RDWR);
> +       if (fd == -1)
> +               perror("open"), exit(1);
> +       if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
> +               perror("ioctl"), exit(1);
> +       cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
> +                                    PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       if ((void*)cover == MAP_FAILED)
> +               perror("mmap"), exit(1);
> +       /* Enable coverage collection from the USB bus #1. */

This is a bit confusing because KCOV_REMOTE_ID name is not related to
USB in any way. And the real USB id is actually 1.
I think we also need to show an example for the common id functionality.


> +       arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
> +       if (!arg)
> +               perror("calloc"), exit(1);
> +       arg->trace_mode = KCOV_TRACE_PC;
> +       arg->area_size = COVER_SIZE;
> +       arg->num_handles = 1;
> +       arg->handles[0] = KCOV_REMOTE_ID;
> +       if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
> +               perror("ioctl"), free(arg), exit(1);
> +       free(arg);
> +
> +       /*
> +        * The user needs to trigger execution of kernel code section that is
> +        * annotated with KCOV_REMOTE_ID.
> +        */
> +       sleep(2);
> +
> +       n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> +       for (i = 0; i < n; i++)
> +               printf("0x%lx\n", cover[i + 1]);
> +       if (ioctl(fd, KCOV_DISABLE, 0))
> +               perror("ioctl"), exit(1);
> +       if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
> +               perror("munmap"), exit(1);
> +       if (close(fd))
> +               perror("close"), exit(1);
> +       return 0;
> +    }
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index b76a1807028d..702672d98d35 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -27,6 +27,13 @@ enum kcov_mode {
>  void kcov_task_init(struct task_struct *t);
>  void kcov_task_exit(struct task_struct *t);
>
> +/*
> + * Reserved handle ranges:
> + * 0000000000000000 - 0000ffffffffffff : common handles
> + */

It can make sense to make this scheme slightly more generic and
officially reserve high bits for "subsystem id" and low bits for "id
within the subsystem". We probably won't use different subsystems for
common handles for syzkaller, but still it may be reasonable to be
able to separate coverage from different subsystems. We could reserve
0x00 as a "generic" subsystem, but allow introductions of other
subsystems later.


> +void kcov_remote_start(u64 handle);
> +void kcov_remote_stop(void);
> +
>  #define kcov_prepare_switch(t)                 \
>  do {                                           \
>         (t)->kcov_mode |= KCOV_IN_CTXSW;        \
> @@ -41,6 +48,8 @@ do {                                          \
>
>  static inline void kcov_task_init(struct task_struct *t) {}
>  static inline void kcov_task_exit(struct task_struct *t) {}
> +static inline void kcov_remote_start(u64 handle) {}
> +static inline void kcov_remote_stop(void) {}
>  static inline void kcov_prepare_switch(struct task_struct *t) {}
>  static inline void kcov_finish_switch(struct task_struct *t) {}
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..7417e0207b48 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1211,8 +1211,14 @@ struct task_struct {
>         /* Buffer for coverage collection: */
>         void                            *kcov_area;
>
> +       /* KCOV sequence number: */

This comment does not add much. Extend it to explain why we need it
and how it is used.

> +       int                             kcov_sequence;
> +
>         /* KCOV descriptor wired with this task or NULL: */
>         struct kcov                     *kcov;
> +
> +       /* KCOV handle for remote coverage collection: */
> +       u64                             kcov_handle;
>  #endif
>
>  #ifdef CONFIG_MEMCG
> diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> index 9529867717a8..46f78f716ca9 100644
> --- a/include/uapi/linux/kcov.h
> +++ b/include/uapi/linux/kcov.h
> @@ -4,9 +4,20 @@
>
>  #include <linux/types.h>
>
> +struct kcov_remote_arg {
> +       unsigned int    trace_mode;
> +       unsigned int    area_size;
> +       unsigned int    num_handles;
> +       __u64           common_handle;

We need some docs here.
In particular we never spell out that common_handle == 0 means no
handle (can't be used for real handles).

> +       __u64           handles[0];
> +};
> +
> +#define KCOV_REMOTE_MAX_HANDLES                0x10000
> +
>  #define KCOV_INIT_TRACE                        _IOR('c', 1, unsigned long)
>  #define KCOV_ENABLE                    _IO('c', 100)
>  #define KCOV_DISABLE                   _IO('c', 101)
> +#define KCOV_REMOTE_ENABLE             _IOW('c', 102, struct kcov_remote_arg)
>
>  enum {
>         /*
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 2ee38727844a..49d1668a4570 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -9,6 +9,7 @@
>  #include <linux/types.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
> +#include <linux/hashtable.h>
>  #include <linux/init.h>
>  #include <linux/mm.h>
>  #include <linux/preempt.h>
> @@ -23,6 +24,8 @@
>  #include <linux/refcount.h>
>  #include <asm/setup.h>
>
> +#define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
> +
>  /* Number of 64-bit words written per one comparison: */
>  #define KCOV_WORDS_PER_CMP 4
>
> @@ -44,19 +47,94 @@ struct kcov {
>          * Reference counter. We keep one for:
>          *  - opened file descriptor
>          *  - task with enabled coverage (we can't unwire it from another task)
> +        *  - each code section for remote coverage collection
>          */
>         refcount_t              refcount;
>         /* The lock protects mode, size, area and t. */
>         spinlock_t              lock;
>         enum kcov_mode          mode;
> -       /* Size of arena (in long's for KCOV_MODE_TRACE). */
> -       unsigned                size;
> +       /* Size of arena (in long's). */
> +       unsigned int            size;
>         /* Coverage buffer shared with user space. */
>         void                    *area;
>         /* Task for which we collect coverage, or NULL. */
>         struct task_struct      *t;
> +       /* Collecting coverage from remote threads. */
> +       bool                    remote;
> +       /* Size of remote arena (in long's). */
> +       unsigned int            remote_size;
> +       /* Sequence is incremented each time kcov is reenabled. */
> +       int                     sequence;
> +};
> +
> +struct kcov_remote_area {
> +       struct list_head        list;
> +       unsigned int            size;
> +};
> +
> +struct kcov_remote {
> +       u64                     handle;
> +       struct kcov             *kcov;
> +       struct hlist_node       hnode;
>  };
>
> +static DEFINE_SPINLOCK(kcov_remote_lock);
> +static DEFINE_HASHTABLE(kcov_remote_map, 4);
> +static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);
> +
> +static struct kcov_remote *kcov_remote_find(u64 handle)
> +{
> +       struct kcov_remote *remote;
> +
> +       hash_for_each_possible(kcov_remote_map, remote, hnode, handle) {
> +               if (remote->handle == handle)
> +                       return remote;
> +       }
> +       return NULL;
> +}
> +
> +static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
> +{
> +       struct kcov_remote *remote;
> +
> +       if (kcov_remote_find(handle))
> +               return ERR_PTR(-EEXIST);
> +       remote = kmalloc(sizeof(*remote), GFP_ATOMIC);
> +       if (!remote)
> +               return ERR_PTR(-ENOMEM);
> +       remote->handle = handle;
> +       remote->kcov = kcov;
> +       hash_add(kcov_remote_map, &remote->hnode, handle);
> +       return remote;
> +}
> +
> +static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
> +{
> +       struct kcov_remote_area *area;
> +       struct list_head *pos;
> +
> +       kcov_debug("size = %u\n", size);
> +       list_for_each(pos, &kcov_remote_areas) {
> +               area = list_entry(pos, struct kcov_remote_area, list);
> +               if (area->size == size) {
> +                       list_del(&area->list);
> +                       kcov_debug("rv = %px\n", area);
> +                       return area;
> +               }
> +       }
> +       kcov_debug("rv = NULL\n");
> +       return NULL;
> +}
> +
> +static void kcov_remote_area_put(struct kcov_remote_area *area,
> +                                       unsigned int size)
> +{
> +       kcov_debug("area = %px, size = %u\n", area, size);
> +       INIT_LIST_HEAD(&area->list);
> +       area->size = size;
> +       list_add(&area->list, &kcov_remote_areas);
> +}
> +
>  static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
>  {
>         unsigned int mode;
> @@ -73,7 +151,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
>          * in_interrupt() returns false (e.g. preempt_schedule_irq()).
>          * READ_ONCE()/barrier() effectively provides load-acquire wrt
>          * interrupts, there are paired barrier()/WRITE_ONCE() in
> -        * kcov_ioctl_locked().
> +        * kcov_start().
>          */
>         barrier();
>         return mode == needed_mode;
> @@ -227,6 +305,69 @@ void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
>  EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
>  #endif /* ifdef CONFIG_KCOV_ENABLE_COMPARISONS */
>
> +static void kcov_start(struct task_struct *t, unsigned int size,
> +                       void *area, enum kcov_mode mode, int sequence)
> +{
> +       kcov_debug("t = %px, size = %u, area = %px\n", t, size, area);
> +       /* Cache in task struct for performance. */
> +       t->kcov_size = size;
> +       t->kcov_area = area;
> +       /* See comment in check_kcov_mode(). */
> +       barrier();
> +       WRITE_ONCE(t->kcov_mode, mode);
> +       t->kcov_sequence = sequence;
> +}
> +
> +static void kcov_stop(struct task_struct *t)
> +{
> +       WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
> +       barrier();
> +       t->kcov_size = 0;
> +       t->kcov_area = NULL;
> +}
> +
> +static void kcov_task_reset(struct task_struct *t)
> +{
> +       kcov_stop(t);
> +       t->kcov = NULL;
> +       t->kcov_sequence = 0;
> +       t->kcov_handle = 0;
> +}
> +
> +void kcov_task_init(struct task_struct *t)
> +{
> +       kcov_task_reset(t);
> +       t->kcov_handle = current->kcov_handle;
> +}
> +
> +static void kcov_reset(struct kcov *kcov)
> +{
> +       kcov->t = NULL;
> +       kcov->mode = KCOV_MODE_INIT;
> +       kcov->remote = false;
> +       kcov->remote_size = 0;
> +       kcov->sequence++;
> +}
> +
> +static void kcov_remote_reset(struct kcov *kcov)
> +{
> +       int bkt;
> +       struct kcov_remote *remote;
> +       struct hlist_node *tmp;
> +
> +       spin_lock(&kcov_remote_lock);
> +       hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) {
> +               if (remote->kcov != kcov)
> +                       continue;
> +               kcov_debug("removing handle %llx\n", remote->handle);
> +               hash_del(&remote->hnode);
> +               kfree(remote);
> +       }
> +       /* Do reset before unlock to prevent races with kcov_remote_start(). */
> +       kcov_reset(kcov);
> +       spin_unlock(&kcov_remote_lock);
> +}
> +
>  static void kcov_get(struct kcov *kcov)
>  {
>         refcount_inc(&kcov->refcount);
> @@ -235,20 +376,12 @@ static void kcov_get(struct kcov *kcov)
>  static void kcov_put(struct kcov *kcov)
>  {
>         if (refcount_dec_and_test(&kcov->refcount)) {
> +               kcov_remote_reset(kcov);
>                 vfree(kcov->area);
>                 kfree(kcov);
>         }
>  }
>
> -void kcov_task_init(strucKCOV_TRACE_PCt task_struct *t)
> -{
> -       WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
> -       barrier();
> -       t->kcov_size = 0;
> -       t->kcov_area = NULL;
> -       t->kcov = NULL;
> -}
> -
>  void kcov_task_exit(struct task_struct *t)
>  {
>         struct kcov *kcov;
> @@ -256,15 +389,27 @@ void kcov_task_exit(struct task_struct *t)
>         kcov = t->kcov;
>         if (kcov == NULL)
>                 return;
> +
>         spin_lock(&kcov->lock);
> +       kcov_debug("t = %px, kcov->t = %px\n", t, kcov->t);
> +       /*
> +        * If !kcov->remote, this checks that t->kcov->t == t.
> +        * If kcov->remote == true then the exiting task is either:
> +        * 1. a remote task between kcov_remote_start() and kcov_remote_stop(),
> +        *    in this case t != kcov->t and we'll print a warning; or
> +        * 2. the task that created kcov exiting without calling KCOV_DISABLE,
> +        *    in this case t == kcov->t and no warning is printed.
> +        */
>         if (WARN_ON(kcov->t != t)) {
>                 spin_unlock(&kcov->lock);
>                 return;
>         }
>         /* Just to not leave dangling references behind. */
> -       kcov_task_init(t);
> -       kcov->t = NULL;
> -       kcov->mode = KCOV_MODE_INIT;
> +       kcov_task_reset(t);
> +       if (kcov->remote)
> +               kcov_remote_reset(kcov);
> +       else
> +               kcov_reset(kcov);

These 5 lines are also repeated in KCOV_DISABLE. Looks like a good
candidate for a helper function.

>         spin_unlock(&kcov->lock);
>         kcov_put(kcov);
>  }
> @@ -313,6 +458,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
>         if (!kcov)
>                 return -ENOMEM;
>         kcov->mode = KCOV_MODE_DISABLED;
> +       kcov->sequence = 1;
>         refcount_set(&kcov->refcount, 1);
>         spin_lock_init(&kcov->lock);
>         filep->private_data = kcov;
> @@ -325,6 +471,20 @@ static int kcov_close(struct inode *inode, struct file *filep)
>         return 0;
>  }
>
> +static int kcov_get_mode(unsigned long arg)
> +{
> +       if (arg == KCOV_TRACE_PC)
> +               return KCOV_MODE_TRACE_PC;
> +       else if (arg == KCOV_TRACE_CMP)
> +#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> +               return KCOV_MODE_TRACE_CMP;
> +#else
> +               return -ENOTSUPP;
> +#endif
> +       else
> +               return -EINVAL;
> +}
> +
>  /*
>   * Fault in a lazily-faulted vmalloc area before it can be used by
>   * __santizer_cov_trace_pc(), to avoid recursion issues if any code on the
> @@ -345,9 +505,13 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>  {
>         struct task_struct *t;
>         unsigned long size, unused;
> +       int mode, i;
> +       struct kcov_remote_arg *remote_arg;
> +       struct kcov_remote *remote;
>
>         switch (cmd) {
>         case KCOV_INIT_TRACE:
> +               kcov_debug("KCOV_REMOTE_ENABLE\n");
>                 /*
>                  * Enable kcov in trace mode and setup buffer size.
>                  * Must happen before anything else.
> @@ -366,6 +530,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>                 kcov->mode = KCOV_MODE_INIT;
>                 return 0;
>         case KCOV_ENABLE:
> +               kcov_debug("KCOV_ENABLE\n");
>                 /*
>                  * Enable coverage for the current task.
>                  * At this point user must have been enabled trace mode,
> @@ -378,29 +543,20 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>                 t = current;
>                 if (kcov->t != NULL || t->kcov != NULL)
>                         return -EBUSY;
> -               if (arg == KCOV_TRACE_PC)
> -                       kcov->mode = KCOV_MODE_TRACE_PC;
> -               else if (arg == KCOV_TRACE_CMP)
> -#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> -                       kcov->mode = KCOV_MODE_TRACE_CMP;
> -#else
> -               return -ENOTSUPP;
> -#endif
> -               else
> -                       return -EINVAL;
> +               mode = kcov_get_mode(arg);
> +               if (mode < 0)
> +                       return mode;
>                 kcov_fault_in_area(kcov);
> -               /* Cache in task struct for performance. */
> -               t->kcov_size = kcov->size;
> -               t->kcov_area = kcov->area;
> -               /* See comment in check_kcov_mode(). */
> -               barrier();
> -               WRITE_ONCE(t->kcov_mode, kcov->mode);
> +               kcov->mode = mode;
> +               kcov_start(t, kcov->size, kcov->area, kcov->mode,
> +                               kcov->sequence);
>                 t->kcov = kcov;
>                 kcov->t = t;
> -               /* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
> +               /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
>                 kcov_get(kcov);
>                 return 0;
>         case KCOV_DISABLE:
> +               kcov_debug("KCOV_DISABLE\n");
>                 /* Disable coverage for the current task. */
>                 unused = arg;
>                 if (unused != 0 || current->kcov != kcov)
> @@ -408,11 +564,52 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>                 t = current;
>                 if (WARN_ON(kcov->t != t))
>                         return -EINVAL;
> -               kcov_task_init(t);
> -               kcov->t = NULL;
> -               kcov->mode = KCOV_MODE_INIT;
> +               kcov_task_reset(t);
> +               if (kcov->remote)
> +                       kcov_remote_reset(kcov);
> +               else
> +                       kcov_reset(kcov);
>                 kcov_put(kcov);
>                 return 0;
> +       case KCOV_REMOTE_ENABLE:
> +               kcov_debug("KCOV_REMOTE_ENABLE\n");
> +               if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
> +                       return -EINVAL;
> +               t = current;
> +               if (kcov->t != NULL || t->kcov != NULL)
> +                       return -EBUSY;
> +               remote_arg = (struct kcov_remote_arg *)arg;
> +               mode = kcov_get_mode(remote_arg->trace_mode);
> +               if (mode < 0)
> +                       return mode;
> +               kcov->mode = mode;
> +               t->kcov = kcov;
> +               kcov->t = t;
> +               kcov->remote = true;
> +               kcov->remote_size = remote_arg->area_size;

I think we need some sanity check on area_size, or it can lead to
overflows later.

> +               spin_lock(&kcov_remote_lock);
> +               for (i = 0; i < remote_arg->num_handles; i++) {
> +                       remote = kcov_remote_add(kcov, remote_arg->handles[i]);
> +                       if (IS_ERR(remote)) {
> +                               spin_unlock(&kcov_remote_lock);
> +                               kcov_remote_reset(kcov);
> +                               return PTR_ERR(remote);
> +                       }
> +               }
> +               if (remote_arg->common_handle) {
> +                       remote = kcov_remote_add(kcov,
> +                                       remote_arg->common_handle);
> +                       if (IS_ERR(remote)) {
> +                               spin_unlock(&kcov_remote_lock);
> +                               kcov_remote_reset(kcov);
> +                               return PTR_ERR(remote);
> +                       }
> +                       t->kcov_handle = remote_arg->common_handle;
> +               }
> +               spin_unlock(&kcov_remote_lock);
> +               /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> +               kcov_get(kcov);
> +               return 0;
>         default:
>                 return -ENOTTY;
>         }
> @@ -422,11 +619,35 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>  {
>         struct kcov *kcov;
>         int res;
> +       struct kcov_remote_arg *remote_arg = NULL;
> +       unsigned int remote_num_handles;
> +       unsigned long remote_arg_size;
> +
> +       if (cmd == KCOV_REMOTE_ENABLE) {
> +               if (get_user(remote_num_handles, (unsigned __user *)(arg +
> +                               offsetof(struct kcov_remote_arg, num_handles))))
> +                       return -EFAULT;
> +               if (remote_num_handles > KCOV_REMOTE_MAX_HANDLES)
> +                       return -EINVAL;
> +               remote_arg_size = sizeof(*remote_arg) +
> +                       remote_num_handles * sizeof(u64);

Looks like a good candidate for struct_size().
This can't overflow due to KCOV_REMOTE_MAX_HANDLES, but still more
readable and bulletproof for future.

> +               remote_arg = memdup_user((void __user *)arg, remote_arg_size);
> +               if (IS_ERR(remote_arg))
> +                       return PTR_ERR(remote_arg);
> +               if (remote_arg->num_handles != remote_num_handles) {
> +                       kfree(remote_arg);
> +                       return -EINVAL;
> +               }
> +               arg = (unsigned long)remote_arg;
> +       }
>
>         kcov = filep->private_data;
>         spin_lock(&kcov->lock);
>         res = kcov_ioctl_locked(kcov, cmd, arg);
>         spin_unlock(&kcov->lock);
> +
> +       kfree(remote_arg);
> +
>         return res;
>  }
>
> @@ -438,6 +659,149 @@ static const struct file_operations kcov_fops = {
>         .release        = kcov_close,
>  };
>
> +void kcov_remote_start(u64 handle)
> +{
> +       struct kcov_remote *remote;
> +       void *area;
> +       struct task_struct *t;
> +       unsigned int size;
> +       enum kcov_mode mode;
> +       int sequence;
> +
> +       if (WARN_ON(!in_task()))
> +               return;
> +       t = current;
> +       /*
> +        * Check that kcov_remote_start is not called twice
> +        * nor called by user tasks (with enabled kcov).
> +        */
> +       if (WARN_ON(t->kcov))
> +               return;
> +
> +       kcov_debug("handle = %llx\n", handle);
> +
> +       spin_lock(&kcov_remote_lock);
> +       remote = kcov_remote_find(handle);
> +       if (!remote) {
> +               kcov_debug("no remote found");
> +               spin_unlock(&kcov_remote_lock);
> +               return;
> +       }
> +       /* Put in kcov_remote_stop(). */
> +       kcov_get(remote->kcov);
> +       t->kcov = remote->kcov;
> +       /*
> +        * Read kcov fields before unlock to prevent races with
> +        * KCOV_DISABLE / kcov_remote_reset().
> +        */
> +       size = remote->kcov->remote_size;
> +       mode = remote->kcov->mode;
> +       sequence = remote->kcov->sequence;
> +       area = kcov_remote_area_get(size);
> +       spin_unlock(&kcov_remote_lock);
> +
> +       if (!area) {
> +               area = vmalloc(size * sizeof(unsigned long));
> +               if (!area) {

Reset t->kcov?

> +                       kcov_put(remote->kcov);
> +                       return;
> +               }
> +       }
> +       /* Reset coverage size. */
> +       *(u64 *)area = 0;
> +
> +       kcov_debug("area = %px, size = %u", area, size);
> +
> +       kcov_start(t, size, area, mode, sequence);
> +
> +}
> +
> +static void kcov_move_area(enum kcov_mode mode, void *dst_area,
> +                               unsigned int dst_area_size, void *src_area)
> +{
> +       u64 word_size = sizeof(unsigned long);
> +       u64 count_size, entry_size;
> +       u64 dst_len, src_len;
> +       void *dst_entries, *src_entries;
> +       u64 dst_occupied, dst_free, bytes_to_move, entries_moved;
> +
> +       kcov_debug("%px %u <= %px %lu\n",
> +               dst_area, dst_area_size, src_area, *(unsigned long *)src_area);
> +
> +       switch (mode) {
> +       case KCOV_MODE_TRACE_PC:
> +               dst_len = READ_ONCE(*(unsigned long *)dst_area);
> +               src_len = *(unsigned long *)src_area;
> +               count_size = sizeof(unsigned long);
> +               entry_size = sizeof(unsigned long);
> +               break;
> +       case KCOV_MODE_TRACE_CMP:
> +               dst_len = READ_ONCE(*(u64 *)dst_area);
> +               src_len = *(u64 *)src_area;
> +               count_size = sizeof(u64);
> +               entry_size = sizeof(u64) * KCOV_WORDS_PER_CMP;
> +               break;
> +       default:
> +               WARN_ON(1);
> +               return;
> +       }
> +
> +       if (dst_len > (dst_area_size * word_size - count_size) / entry_size)
> +               return;
> +       dst_occupied = count_size + dst_len * entry_size;
> +       dst_free = dst_area_size * word_size - dst_occupied;
> +       bytes_to_move = min(dst_free, src_len * entry_size);
> +       dst_entries = dst_area + dst_occupied;
> +       src_entries = src_area + count_size;
> +       memcpy(dst_entries, src_entries, bytes_to_move);
> +       entries_moved = bytes_to_move / entry_size;
> +
> +       switch (mode) {
> +       case KCOV_MODE_TRACE_PC:
> +               WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved);
> +               break;
> +       case KCOV_MODE_TRACE_CMP:
> +               WRITE_ONCE(*(u64 *)dst_area, dst_len + entries_moved);
> +               break;
> +       default:
> +               break;
> +       }
> +}
> +
> +void kcov_remote_stop(void)
> +{
> +       struct task_struct *t = current;
> +       struct kcov *kcov = t->kcov;
> +       void *area = t->kcov_area;
> +       unsigned int size = t->kcov_size;
> +       int sequence = t->kcov_sequence;
> +
> +       if (!kcov) {
> +               kcov_debug("no kcov found\n");
> +               return;
> +       }
> +
> +       kcov_stop(t);
> +       t->kcov = NULL;
> +
> +       spin_lock(&kcov->lock);
> +       /*
> +        * KCOV_DISABLE could have been called between kcov_remote_start()
> +        * and kcov_remote_stop(), hence the check.
> +        */
> +       kcov_debug("move if: %d == %d && %d\n",
> +               sequence, kcov->sequence, (int)kcov->remote);
> +       if (sequence == kcov->sequence && kcov->remote)
> +               kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
> +       spin_unlock(&kcov->lock);
> +
> +       spin_lock(&kcov_remote_lock);
> +       kcov_remote_area_put(area, size);
> +       spin_unlock(&kcov_remote_lock);
> +
> +       kcov_put(kcov);
> +}
> +
>  static int __init kcov_init(void)
>  {
>         /*
> --
> 2.23.0.866.gb869b98d4c-goog
>

On Mon, Oct 21, 2019 at 3:53 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, Oct 17, 2019 at 7:44 PM Andrey Konovalov <andreyknvl@google.com> wrote:
> >
> > Currently kcov can only collect coverage for syscalls that are issued
> > from the current process. This patch adds support for KCOV_REMOTE_ENABLE,
> > that makes it possible to collect coverage for arbitrary parts of the
> > kernel code, provided that this part is annotated with kcov_remote_start
> > and kcov_remote_stop.
>
> Nitpick: () after function names is the common C style.
>
> > This allows to collect coverage from two types of kernel background
> > threads: the global ones, that are spawned during kernel boot and are
> > always running (e.g. USB hub_event); and the local ones, that are spawned
> > when a user interacts with some kernel interfaces (e.g. vhost).
> >
> > To enable collecting coverage from a global background thread, a unique
> > global id must be assigned and passed to the corresponding
> > kcov_remote_start annotation call. Then a userspace process can pass this
> > id to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
> > kcov_remote_arg struct. This will attach kcov device to the code section,
> > that is referenced by this id. Multiple ids can be targeted with the same
> > kcov device simultaneously.
> >
> > Since there might be many local background threads spawned from different
> > userspace processes, we can't use a single global id per annotation.
> > Instead, the userspace process passes an id through the common_handle
> > field of the kcov_remote_arg struct. This id gets saved to the kcov_handle
> > field in the current task_struct and needs to be passed to the newly
> > spawned threads via custom annotations. Those threads should be in turn
> > annotated with kcov_remote_start/kcov_remote_stop.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > ---
> >  Documentation/dev-tools/kcov.rst |  99 +++++++
> >  include/linux/kcov.h             |   9 +
> >  include/linux/sched.h            |   6 +
> >  include/uapi/linux/kcov.h        |  11 +
> >  kernel/kcov.c                    | 434 ++++++++++++++++++++++++++++---
> >  5 files changed, 524 insertions(+), 35 deletions(-)
> >
> > diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> > index 42b612677799..1cf0af95a801 100644
> > --- a/Documentation/dev-tools/kcov.rst
> > +++ b/Documentation/dev-tools/kcov.rst
> > @@ -34,6 +34,7 @@ Profiling data will only become accessible once debugfs has been mounted::
> >
> >  Coverage collection
> >  -------------------
> > +
> >  The following program demonstrates coverage collection from within a test
> >  program using kcov:
> >
> > @@ -128,6 +129,7 @@ only need to enable coverage (disable happens automatically on thread end).
> >
> >  Comparison operands collection
> >  ------------------------------
> > +
> >  Comparison operands collection is similar to coverage collection:
> >
> >  .. code-block:: c
> > @@ -202,3 +204,100 @@ Comparison operands collection is similar to coverage collection:
> >
> >  Note that the kcov modes (coverage collection or comparison operands) are
> >  mutually exclusive.
> > +
> > +Remote coverage collection
> > +--------------------------
> > +
> > +With KCOV_ENABLE coverage is collected only for syscalls that are issued from
> > +the current process. With KCOV_REMOTE_ENABLE it's possible to collect coverage
> > +for arbitrary parts of the kernel code, provided that this part is annotated
> > +with kcov_remote_start/kcov_remote_stop.
>
> Same here.
>
> > +This allows to collect coverage from two types of kernel background threads:
> > +the global ones, that are spawned during kernel boot and are always running
> > +(e.g. USB hub_event); and the local ones, that are spawned when a user
> > +interacts with some kernel interfaces (e.g. vhost).
> > +
> > +To enable collecting coverage from a global background thread, a unique global
> > +id must be assigned and passed to the corresponding kcov_remote_start annotation
> > +call. Then a userspace process can pass this id to the KCOV_REMOTE_ENABLE ioctl
> > +in the handles array field of the kcov_remote_arg struct. This will attach kcov
> > +device to the code section, that is referenced by this id. Multiple ids can be
> > +targeted with the same kcov device simultaneously.
> > +
> > +Since there might be many local background threads spawned from different
> > +userspace processes, we can't use a single global id per annotation. Instead,
> > +the userspace process passes an id through the common_handle field of the
> > +kcov_remote_arg struct. This id gets saved to the kcov_handle field in the
> > +current task_struct and needs to be passed to the newly spawned threads via
> > +custom annotations. Those threads should be in turn annotated with
> > +kcov_remote_start/kcov_remote_stop.
> > +
> > +.. code-block:: c
> > +
> > +    struct kcov_remote_arg {
> > +       unsigned        trace_mode;
> > +       unsigned        area_size;
> > +       unsigned        num_handles;
> > +       uint64_t        common_handle;
> > +       uint64_t        handles[0];
> > +    };
> > +
> > +    #define KCOV_REMOTE_MAX_HANDLES            0x10000
>
> This is unused. I would remove it. I assume you just copied all of the
> current declarations. But they are extended over time, so later it
> will be just some random subset of used and unused declarations.
>
> > +
> > +    #define KCOV_INIT_TRACE                    _IOR('c', 1, unsigned long)
> > +    #define KCOV_ENABLE                        _IO('c', 100)
>
> Same here and for few others below.
>
> > +    #define KCOV_DISABLE                       _IO('c', 101)
> > +    #define KCOV_REMOTE_ENABLE         _IOW('c', 102, struct kcov_remote_arg)
> > +
> > +    #define COVER_SIZE (64 << 10)
> > +
> > +    #define KCOV_TRACE_PC      0
> > +    #define KCOV_TRACE_CMP     1
> > +
> > +    #define KCOV_REMOTE_ID     0x42
> > +
> > +    int main(int argc, char **argv)
> > +    {
> > +       int fd;
> > +       unsigned long *cover, n, i;
> > +       uint64_t handle;
> > +
> > +       fd = open("/sys/kernel/debug/kcov", O_RDWR);
> > +       if (fd == -1)
> > +               perror("open"), exit(1);
> > +       if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
> > +               perror("ioctl"), exit(1);
> > +       cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
> > +                                    PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> > +       if ((void*)cover == MAP_FAILED)
> > +               perror("mmap"), exit(1);
> > +       /* Enable coverage collection from the USB bus #1. */
>
> This is a bit confusing because KCOV_REMOTE_ID name is not related to
> USB in any way. And the real USB id is actually 1.
> I think we also need to show an example for the common id functionality.
>
>
> > +       arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
> > +       if (!arg)
> > +               perror("calloc"), exit(1);
> > +       arg->trace_mode = KCOV_TRACE_PC;
> > +       arg->area_size = COVER_SIZE;
> > +       arg->num_handles = 1;
> > +       arg->handles[0] = KCOV_REMOTE_ID;
> > +       if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
> > +               perror("ioctl"), free(arg), exit(1);
> > +       free(arg);
> > +
> > +       /*
> > +        * The user needs to trigger execution of kernel code section that is
> > +        * annotated with KCOV_REMOTE_ID.
> > +        */
> > +       sleep(2);
> > +
> > +       n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> > +       for (i = 0; i < n; i++)
> > +               printf("0x%lx\n", cover[i + 1]);
> > +       if (ioctl(fd, KCOV_DISABLE, 0))
> > +               perror("ioctl"), exit(1);
> > +       if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
> > +               perror("munmap"), exit(1);
> > +       if (close(fd))
> > +               perror("close"), exit(1);
> > +       return 0;
> > +    }
> > diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> > index b76a1807028d..702672d98d35 100644
> > --- a/include/linux/kcov.h
> > +++ b/include/linux/kcov.h
> > @@ -27,6 +27,13 @@ enum kcov_mode {
> >  void kcov_task_init(struct task_struct *t);
> >  void kcov_task_exit(struct task_struct *t);
> >
> > +/*
> > + * Reserved handle ranges:
> > + * 0000000000000000 - 0000ffffffffffff : common handles
> > + */
>
> It can make sense to make this scheme slightly more generic and
> officially reserve high bits for "subsystem id" and low bits for "id
> within the subsystem". We probably won't use different subsystems for
> common handles for syzkaller, but still it may be reasonable to be
> able to separate coverage from different subsystems. We could reserve
> 0x00 as a "generic" subsystem, but allow introductions of other
> subsystems later.
>
>
> > +void kcov_remote_start(u64 handle);
> > +void kcov_remote_stop(void);
> > +
> >  #define kcov_prepare_switch(t)                 \
> >  do {                                           \
> >         (t)->kcov_mode |= KCOV_IN_CTXSW;        \
> > @@ -41,6 +48,8 @@ do {                                          \
> >
> >  static inline void kcov_task_init(struct task_struct *t) {}
> >  static inline void kcov_task_exit(struct task_struct *t) {}
> > +static inline void kcov_remote_start(u64 handle) {}
> > +static inline void kcov_remote_stop(void) {}
> >  static inline void kcov_prepare_switch(struct task_struct *t) {}
> >  static inline void kcov_finish_switch(struct task_struct *t) {}
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 2c2e56bd8913..7417e0207b48 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1211,8 +1211,14 @@ struct task_struct {
> >         /* Buffer for coverage collection: */
> >         void                            *kcov_area;
> >
> > +       /* KCOV sequence number: */
>
> This comment does not add much. Extend it to explain why we need it
> and how it is used.
>
> > +       int                             kcov_sequence;
> > +
> >         /* KCOV descriptor wired with this task or NULL: */
> >         struct kcov                     *kcov;
> > +
> > +       /* KCOV handle for remote coverage collection: */
> > +       u64                             kcov_handle;
> >  #endif
> >
> >  #ifdef CONFIG_MEMCG
> > diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> > index 9529867717a8..46f78f716ca9 100644
> > --- a/include/uapi/linux/kcov.h
> > +++ b/include/uapi/linux/kcov.h
> > @@ -4,9 +4,20 @@
> >
> >  #include <linux/types.h>
> >
> > +struct kcov_remote_arg {
> > +       unsigned int    trace_mode;
> > +       unsigned int    area_size;
> > +       unsigned int    num_handles;
> > +       __u64           common_handle;
>
> We need some docs here.
> In particular we never spell out that common_handle == 0 means no
> handle (can't be used for real handles).
>
> > +       __u64           handles[0];
> > +};
> > +
> > +#define KCOV_REMOTE_MAX_HANDLES                0x10000
> > +
> >  #define KCOV_INIT_TRACE                        _IOR('c', 1, unsigned long)
> >  #define KCOV_ENABLE                    _IO('c', 100)
> >  #define KCOV_DISABLE                   _IO('c', 101)
> > +#define KCOV_REMOTE_ENABLE             _IOW('c', 102, struct kcov_remote_arg)
> >
> >  enum {
> >         /*
> > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > index 2ee38727844a..49d1668a4570 100644
> > --- a/kernel/kcov.c
> > +++ b/kernel/kcov.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/types.h>
> >  #include <linux/file.h>
> >  #include <linux/fs.h>
> > +#include <linux/hashtable.h>
> >  #include <linux/init.h>
> >  #include <linux/mm.h>
> >  #include <linux/preempt.h>
> > @@ -23,6 +24,8 @@
> >  #include <linux/refcount.h>
> >  #include <asm/setup.h>
> >
> > +#define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
> > +
> >  /* Number of 64-bit words written per one comparison: */
> >  #define KCOV_WORDS_PER_CMP 4
> >
> > @@ -44,19 +47,94 @@ struct kcov {
> >          * Reference counter. We keep one for:
> >          *  - opened file descriptor
> >          *  - task with enabled coverage (we can't unwire it from another task)
> > +        *  - each code section for remote coverage collection
> >          */
> >         refcount_t              refcount;
> >         /* The lock protects mode, size, area and t. */
> >         spinlock_t              lock;
> >         enum kcov_mode          mode;
> > -       /* Size of arena (in long's for KCOV_MODE_TRACE). */
> > -       unsigned                size;
> > +       /* Size of arena (in long's). */
> > +       unsigned int            size;
> >         /* Coverage buffer shared with user space. */
> >         void                    *area;
> >         /* Task for which we collect coverage, or NULL. */
> >         struct task_struct      *t;
> > +       /* Collecting coverage from remote threads. */
> > +       bool                    remote;
> > +       /* Size of remote arena (in long's). */
> > +       unsigned int            remote_size;
> > +       /* Sequence is incremented each time kcov is reenabled. */
> > +       int                     sequence;
> > +};
> > +
> > +struct kcov_remote_area {
> > +       struct list_head        list;
> > +       unsigned int            size;
> > +};
> > +
> > +struct kcov_remote {
> > +       u64                     handle;
> > +       struct kcov             *kcov;
> > +       struct hlist_node       hnode;
> >  };
> >
> > +static DEFINE_SPINLOCK(kcov_remote_lock);
> > +static DEFINE_HASHTABLE(kcov_remote_map, 4);
> > +static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);
> > +
> > +static struct kcov_remote *kcov_remote_find(u64 handle)
> > +{
> > +       struct kcov_remote *remote;
> > +
> > +       hash_for_each_possible(kcov_remote_map, remote, hnode, handle) {
> > +               if (remote->handle == handle)
> > +                       return remote;
> > +       }
> > +       return NULL;
> > +}
> > +
> > +static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
> > +{
> > +       struct kcov_remote *remote;
> > +
> > +       if (kcov_remote_find(handle))
> > +               return ERR_PTR(-EEXIST);
> > +       remote = kmalloc(sizeof(*remote), GFP_ATOMIC);
> > +       if (!remote)
> > +               return ERR_PTR(-ENOMEM);
> > +       remote->handle = handle;
> > +       remote->kcov = kcov;
> > +       hash_add(kcov_remote_map, &remote->hnode, handle);
> > +       return remote;
> > +}
> > +
> > +static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
> > +{
> > +       struct kcov_remote_area *area;
> > +       struct list_head *pos;
> > +
> > +       kcov_debug("size = %u\n", size);
> > +       list_for_each(pos, &kcov_remote_areas) {
> > +               area = list_entry(pos, struct kcov_remote_area, list);
> > +               if (area->size == size) {
> > +                       list_del(&area->list);
> > +                       kcov_debug("rv = %px\n", area);
> > +                       return area;
> > +               }
> > +       }
> > +       kcov_debug("rv = NULL\n");
> > +       return NULL;
> > +}
> > +
> > +static void kcov_remote_area_put(struct kcov_remote_area *area,
> > +                                       unsigned int size)
> > +{
> > +       kcov_debug("area = %px, size = %u\n", area, size);
> > +       INIT_LIST_HEAD(&area->list);
> > +       area->size = size;
> > +       list_add(&area->list, &kcov_remote_areas);
> > +}
> > +
> >  static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
> >  {
> >         unsigned int mode;
> > @@ -73,7 +151,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
> >          * in_interrupt() returns false (e.g. preempt_schedule_irq()).
> >          * READ_ONCE()/barrier() effectively provides load-acquire wrt
> >          * interrupts, there are paired barrier()/WRITE_ONCE() in
> > -        * kcov_ioctl_locked().
> > +        * kcov_start().
> >          */
> >         barrier();
> >         return mode == needed_mode;
> > @@ -227,6 +305,69 @@ void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
> >  EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
> >  #endif /* ifdef CONFIG_KCOV_ENABLE_COMPARISONS */
> >
> > +static void kcov_start(struct task_struct *t, unsigned int size,
> > +                       void *area, enum kcov_mode mode, int sequence)
> > +{
> > +       kcov_debug("t = %px, size = %u, area = %px\n", t, size, area);
> > +       /* Cache in task struct for performance. */
> > +       t->kcov_size = size;
> > +       t->kcov_area = area;
> > +       /* See comment in check_kcov_mode(). */
> > +       barrier();
> > +       WRITE_ONCE(t->kcov_mode, mode);
> > +       t->kcov_sequence = sequence;
> > +}
> > +
> > +static void kcov_stop(struct task_struct *t)
> > +{
> > +       WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
> > +       barrier();
> > +       t->kcov_size = 0;
> > +       t->kcov_area = NULL;
> > +}
> > +
> > +static void kcov_task_reset(struct task_struct *t)
> > +{
> > +       kcov_stop(t);
> > +       t->kcov = NULL;
> > +       t->kcov_sequence = 0;
> > +       t->kcov_handle = 0;
> > +}
> > +
> > +void kcov_task_init(struct task_struct *t)
> > +{
> > +       kcov_task_reset(t);
> > +       t->kcov_handle = current->kcov_handle;
> > +}
> > +
> > +static void kcov_reset(struct kcov *kcov)
> > +{
> > +       kcov->t = NULL;
> > +       kcov->mode = KCOV_MODE_INIT;
> > +       kcov->remote = false;
> > +       kcov->remote_size = 0;
> > +       kcov->sequence++;
> > +}
> > +
> > +static void kcov_remote_reset(struct kcov *kcov)
> > +{
> > +       int bkt;
> > +       struct kcov_remote *remote;
> > +       struct hlist_node *tmp;
> > +
> > +       spin_lock(&kcov_remote_lock);
> > +       hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) {
> > +               if (remote->kcov != kcov)
> > +                       continue;
> > +               kcov_debug("removing handle %llx\n", remote->handle);
> > +               hash_del(&remote->hnode);
> > +               kfree(remote);
> > +       }
> > +       /* Do reset before unlock to prevent races with kcov_remote_start(). */
> > +       kcov_reset(kcov);
> > +       spin_unlock(&kcov_remote_lock);
> > +}
> > +
> >  static void kcov_get(struct kcov *kcov)
> >  {
> >         refcount_inc(&kcov->refcount);
> > @@ -235,20 +376,12 @@ static void kcov_get(struct kcov *kcov)
> >  static void kcov_put(struct kcov *kcov)
> >  {
> >         if (refcount_dec_and_test(&kcov->refcount)) {
> > +               kcov_remote_reset(kcov);
> >                 vfree(kcov->area);
> >                 kfree(kcov);
> >         }
> >  }
> >
> > -void kcov_task_init(strucKCOV_TRACE_PCt task_struct *t)
> > -{
> > -       WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
> > -       barrier();
> > -       t->kcov_size = 0;
> > -       t->kcov_area = NULL;
> > -       t->kcov = NULL;
> > -}
> > -
> >  void kcov_task_exit(struct task_struct *t)
> >  {
> >         struct kcov *kcov;
> > @@ -256,15 +389,27 @@ void kcov_task_exit(struct task_struct *t)
> >         kcov = t->kcov;
> >         if (kcov == NULL)
> >                 return;
> > +
> >         spin_lock(&kcov->lock);
> > +       kcov_debug("t = %px, kcov->t = %px\n", t, kcov->t);
> > +       /*
> > +        * If !kcov->remote, this checks that t->kcov->t == t.
> > +        * If kcov->remote == true then the exiting task is either:
> > +        * 1. a remote task between kcov_remote_start() and kcov_remote_stop(),
> > +        *    in this case t != kcov->t and we'll print a warning; or
> > +        * 2. the task that created kcov exiting without calling KCOV_DISABLE,
> > +        *    in this case t == kcov->t and no warning is printed.
> > +        */
> >         if (WARN_ON(kcov->t != t)) {
> >                 spin_unlock(&kcov->lock);
> >                 return;
> >         }
> >         /* Just to not leave dangling references behind. */
> > -       kcov_task_init(t);
> > -       kcov->t = NULL;
> > -       kcov->mode = KCOV_MODE_INIT;
> > +       kcov_task_reset(t);
> > +       if (kcov->remote)
> > +               kcov_remote_reset(kcov);
> > +       else
> > +               kcov_reset(kcov);
>
> These 5 lines are also repeated in KCOV_DISABLE. Looks like a good
> candidate for a helper function.
>
> >         spin_unlock(&kcov->lock);
> >         kcov_put(kcov);
> >  }
> > @@ -313,6 +458,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
> >         if (!kcov)
> >                 return -ENOMEM;
> >         kcov->mode = KCOV_MODE_DISABLED;
> > +       kcov->sequence = 1;
> >         refcount_set(&kcov->refcount, 1);
> >         spin_lock_init(&kcov->lock);
> >         filep->private_data = kcov;
> > @@ -325,6 +471,20 @@ static int kcov_close(struct inode *inode, struct file *filep)
> >         return 0;
> >  }
> >
> > +static int kcov_get_mode(unsigned long arg)
> > +{
> > +       if (arg == KCOV_TRACE_PC)
> > +               return KCOV_MODE_TRACE_PC;
> > +       else if (arg == KCOV_TRACE_CMP)
> > +#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> > +               return KCOV_MODE_TRACE_CMP;
> > +#else
> > +               return -ENOTSUPP;
> > +#endif
> > +       else
> > +               return -EINVAL;
> > +}
> > +
> >  /*
> >   * Fault in a lazily-faulted vmalloc area before it can be used by
> >   * __santizer_cov_trace_pc(), to avoid recursion issues if any code on the
> > @@ -345,9 +505,13 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> >  {
> >         struct task_struct *t;
> >         unsigned long size, unused;
> > +       int mode, i;
> > +       struct kcov_remote_arg *remote_arg;
> > +       struct kcov_remote *remote;
> >
> >         switch (cmd) {
> >         case KCOV_INIT_TRACE:
> > +               kcov_debug("KCOV_REMOTE_ENABLE\n");
> >                 /*
> >                  * Enable kcov in trace mode and setup buffer size.
> >                  * Must happen before anything else.
> > @@ -366,6 +530,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> >                 kcov->mode = KCOV_MODE_INIT;
> >                 return 0;
> >         case KCOV_ENABLE:
> > +               kcov_debug("KCOV_ENABLE\n");
> >                 /*
> >                  * Enable coverage for the current task.
> >                  * At this point user must have been enabled trace mode,
> > @@ -378,29 +543,20 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> >                 t = current;
> >                 if (kcov->t != NULL || t->kcov != NULL)
> >                         return -EBUSY;
> > -               if (arg == KCOV_TRACE_PC)
> > -                       kcov->mode = KCOV_MODE_TRACE_PC;
> > -               else if (arg == KCOV_TRACE_CMP)
> > -#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> > -                       kcov->mode = KCOV_MODE_TRACE_CMP;
> > -#else
> > -               return -ENOTSUPP;
> > -#endif
> > -               else
> > -                       return -EINVAL;
> > +               mode = kcov_get_mode(arg);
> > +               if (mode < 0)
> > +                       return mode;
> >                 kcov_fault_in_area(kcov);
> > -               /* Cache in task struct for performance. */
> > -               t->kcov_size = kcov->size;
> > -               t->kcov_area = kcov->area;
> > -               /* See comment in check_kcov_mode(). */
> > -               barrier();
> > -               WRITE_ONCE(t->kcov_mode, kcov->mode);
> > +               kcov->mode = mode;
> > +               kcov_start(t, kcov->size, kcov->area, kcov->mode,
> > +                               kcov->sequence);
> >                 t->kcov = kcov;
> >                 kcov->t = t;
> > -               /* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
> > +               /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> >                 kcov_get(kcov);
> >                 return 0;
> >         case KCOV_DISABLE:
> > +               kcov_debug("KCOV_DISABLE\n");
> >                 /* Disable coverage for the current task. */
> >                 unused = arg;
> >                 if (unused != 0 || current->kcov != kcov)
> > @@ -408,11 +564,52 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> >                 t = current;
> >                 if (WARN_ON(kcov->t != t))
> >                         return -EINVAL;
> > -               kcov_task_init(t);
> > -               kcov->t = NULL;
> > -               kcov->mode = KCOV_MODE_INIT;
> > +               kcov_task_reset(t);
> > +               if (kcov->remote)
> > +                       kcov_remote_reset(kcov);
> > +               else
> > +                       kcov_reset(kcov);
> >                 kcov_put(kcov);
> >                 return 0;
> > +       case KCOV_REMOTE_ENABLE:
> > +               kcov_debug("KCOV_REMOTE_ENABLE\n");
> > +               if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
> > +                       return -EINVAL;
> > +               t = current;
> > +               if (kcov->t != NULL || t->kcov != NULL)
> > +                       return -EBUSY;
> > +               remote_arg = (struct kcov_remote_arg *)arg;
> > +               mode = kcov_get_mode(remote_arg->trace_mode);
> > +               if (mode < 0)
> > +                       return mode;
> > +               kcov->mode = mode;
> > +               t->kcov = kcov;
> > +               kcov->t = t;
> > +               kcov->remote = true;
> > +               kcov->remote_size = remote_arg->area_size;
>
> I think we need some sanity check on area_size, or it can lead to
> overflows later.
>
> > +               spin_lock(&kcov_remote_lock);
> > +               for (i = 0; i < remote_arg->num_handles; i++) {
> > +                       remote = kcov_remote_add(kcov, remote_arg->handles[i]);
> > +                       if (IS_ERR(remote)) {
> > +                               spin_unlock(&kcov_remote_lock);
> > +                               kcov_remote_reset(kcov);
> > +                               return PTR_ERR(remote);
> > +                       }
> > +               }
> > +               if (remote_arg->common_handle) {
> > +                       remote = kcov_remote_add(kcov,
> > +                                       remote_arg->common_handle);
> > +                       if (IS_ERR(remote)) {
> > +                               spin_unlock(&kcov_remote_lock);
> > +                               kcov_remote_reset(kcov);
> > +                               return PTR_ERR(remote);
> > +                       }
> > +                       t->kcov_handle = remote_arg->common_handle;
> > +               }
> > +               spin_unlock(&kcov_remote_lock);
> > +               /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> > +               kcov_get(kcov);
> > +               return 0;
> >         default:
> >                 return -ENOTTY;
> >         }
> > @@ -422,11 +619,35 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> >  {
> >         struct kcov *kcov;
> >         int res;
> > +       struct kcov_remote_arg *remote_arg = NULL;
> > +       unsigned int remote_num_handles;
> > +       unsigned long remote_arg_size;
> > +
> > +       if (cmd == KCOV_REMOTE_ENABLE) {
> > +               if (get_user(remote_num_handles, (unsigned __user *)(arg +
> > +                               offsetof(struct kcov_remote_arg, num_handles))))
> > +                       return -EFAULT;
> > +               if (remote_num_handles > KCOV_REMOTE_MAX_HANDLES)
> > +                       return -EINVAL;
> > +               remote_arg_size = sizeof(*remote_arg) +
> > +                       remote_num_handles * sizeof(u64);
>
> Looks like a good candidate for struct_size().
> This can't overflow due to KCOV_REMOTE_MAX_HANDLES, but still more
> readable and bulletproof for future.
>
> > +               remote_arg = memdup_user((void __user *)arg, remote_arg_size);
> > +               if (IS_ERR(remote_arg))
> > +                       return PTR_ERR(remote_arg);
> > +               if (remote_arg->num_handles != remote_num_handles) {
> > +                       kfree(remote_arg);
> > +                       return -EINVAL;
> > +               }
> > +               arg = (unsigned long)remote_arg;
> > +       }
> >
> >         kcov = filep->private_data;
> >         spin_lock(&kcov->lock);
> >         res = kcov_ioctl_locked(kcov, cmd, arg);
> >         spin_unlock(&kcov->lock);
> > +
> > +       kfree(remote_arg);
> > +
> >         return res;
> >  }
> >
> > @@ -438,6 +659,149 @@ static const struct file_operations kcov_fops = {
> >         .release        = kcov_close,
> >  };
> >
> > +void kcov_remote_start(u64 handle)
> > +{
> > +       struct kcov_remote *remote;
> > +       void *area;
> > +       struct task_struct *t;
> > +       unsigned int size;
> > +       enum kcov_mode mode;
> > +       int sequence;
> > +
> > +       if (WARN_ON(!in_task()))
> > +               return;
> > +       t = current;
> > +       /*
> > +        * Check that kcov_remote_start is not called twice
> > +        * nor called by user tasks (with enabled kcov).
> > +        */
> > +       if (WARN_ON(t->kcov))
> > +               return;
> > +
> > +       kcov_debug("handle = %llx\n", handle);
> > +
> > +       spin_lock(&kcov_remote_lock);
> > +       remote = kcov_remote_find(handle);
> > +       if (!remote) {
> > +               kcov_debug("no remote found");
> > +               spin_unlock(&kcov_remote_lock);
> > +               return;
> > +       }
> > +       /* Put in kcov_remote_stop(). */
> > +       kcov_get(remote->kcov);
> > +       t->kcov = remote->kcov;
> > +       /*
> > +        * Read kcov fields before unlock to prevent races with
> > +        * KCOV_DISABLE / kcov_remote_reset().
> > +        */
> > +       size = remote->kcov->remote_size;
> > +       mode = remote->kcov->mode;
> > +       sequence = remote->kcov->sequence;
> > +       area = kcov_remote_area_get(size);
> > +       spin_unlock(&kcov_remote_lock);
> > +
> > +       if (!area) {
> > +               area = vmalloc(size * sizeof(unsigned long));
> > +               if (!area) {
>
> Reset t->kcov?
>
> > +                       kcov_put(remote->kcov);
> > +                       return;
> > +               }
> > +       }
> > +       /* Reset coverage size. */
> > +       *(u64 *)area = 0;
> > +
> > +       kcov_debug("area = %px, size = %u", area, size);
> > +
> > +       kcov_start(t, size, area, mode, sequence);
> > +
> > +}
> > +
> > +static void kcov_move_area(enum kcov_mode mode, void *dst_area,
> > +                               unsigned int dst_area_size, void *src_area)
> > +{
> > +       u64 word_size = sizeof(unsigned long);
> > +       u64 count_size, entry_size;
> > +       u64 dst_len, src_len;
> > +       void *dst_entries, *src_entries;
> > +       u64 dst_occupied, dst_free, bytes_to_move, entries_moved;
> > +
> > +       kcov_debug("%px %u <= %px %lu\n",
> > +               dst_area, dst_area_size, src_area, *(unsigned long *)src_area);
> > +
> > +       switch (mode) {
> > +       case KCOV_MODE_TRACE_PC:
> > +               dst_len = READ_ONCE(*(unsigned long *)dst_area);
> > +               src_len = *(unsigned long *)src_area;
> > +               count_size = sizeof(unsigned long);
> > +               entry_size = sizeof(unsigned long);
> > +               break;
> > +       case KCOV_MODE_TRACE_CMP:
> > +               dst_len = READ_ONCE(*(u64 *)dst_area);
> > +               src_len = *(u64 *)src_area;
> > +               count_size = sizeof(u64);
> > +               entry_size = sizeof(u64) * KCOV_WORDS_PER_CMP;
> > +               break;
> > +       default:
> > +               WARN_ON(1);
> > +               return;
> > +       }
> > +
> > +       if (dst_len > (dst_area_size * word_size - count_size) / entry_size)
> > +               return;
> > +       dst_occupied = count_size + dst_len * entry_size;
> > +       dst_free = dst_area_size * word_size - dst_occupied;
> > +       bytes_to_move = min(dst_free, src_len * entry_size);
> > +       dst_entries = dst_area + dst_occupied;
> > +       src_entries = src_area + count_size;
> > +       memcpy(dst_entries, src_entries, bytes_to_move);
> > +       entries_moved = bytes_to_move / entry_size;
> > +
> > +       switch (mode) {
> > +       case KCOV_MODE_TRACE_PC:
> > +               WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved);
> > +               break;
> > +       case KCOV_MODE_TRACE_CMP:
> > +               WRITE_ONCE(*(u64 *)dst_area, dst_len + entries_moved);
> > +               break;
> > +       default:
> > +               break;
> > +       }
> > +}
> > +
> > +void kcov_remote_stop(void)
> > +{
> > +       struct task_struct *t = current;
> > +       struct kcov *kcov = t->kcov;
> > +       void *area = t->kcov_area;
> > +       unsigned int size = t->kcov_size;
> > +       int sequence = t->kcov_sequence;
> > +
> > +       if (!kcov) {
> > +               kcov_debug("no kcov found\n");
> > +               return;
> > +       }
> > +
> > +       kcov_stop(t);
> > +       t->kcov = NULL;
> > +
> > +       spin_lock(&kcov->lock);
> > +       /*
> > +        * KCOV_DISABLE could have been called between kcov_remote_start()
> > +        * and kcov_remote_stop(), hence the check.
> > +        */
> > +       kcov_debug("move if: %d == %d && %d\n",
> > +               sequence, kcov->sequence, (int)kcov->remote);
> > +       if (sequence == kcov->sequence && kcov->remote)
> > +               kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
> > +       spin_unlock(&kcov->lock);
> > +
> > +       spin_lock(&kcov_remote_lock);
> > +       kcov_remote_area_put(area, size);
> > +       spin_unlock(&kcov_remote_lock);
> > +
> > +       kcov_put(kcov);
> > +}
> > +
> >  static int __init kcov_init(void)
> >  {
> >         /*
> > --
> > 2.23.0.866.gb869b98d4c-goog
> >

Will fix all in the next version, thanks!