[RFC,bpf-next,v4,3/7] bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall

The following patch implements a bpf queue/stack maps that
provides the peek/pop/push functions.  There is not a direct
relationship between those functions and the current maps
syscalls, hence a new MAP_LOOKUP_AND_DELETE_ELEM syscall is added,
this is mapped to the pop operation in the queue/stack maps
and it is still to implement in other kind of maps.

Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>
---
 include/linux/bpf.h      |    1 +
 include/uapi/linux/bpf.h |    1 +
 kernel/bpf/syscall.c     |   82 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+)

On Thu, Oct 04, 2018 at 07:12:39PM +0200, Mauricio Vasquez B wrote:
> The following patch implements a bpf queue/stack maps that
> provides the peek/pop/push functions.  There is not a direct
> relationship between those functions and the current maps
> syscalls, hence a new MAP_LOOKUP_AND_DELETE_ELEM syscall is added,
> this is mapped to the pop operation in the queue/stack maps
> and it is still to implement in other kind of maps.
> 
> Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>
> ---
>  include/linux/bpf.h      |    1 +
>  include/uapi/linux/bpf.h |    1 +
>  kernel/bpf/syscall.c     |   82 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 84 insertions(+)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 027697b6a22f..98c7eeb6d138 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -39,6 +39,7 @@ struct bpf_map_ops {
>  	void *(*map_lookup_elem)(struct bpf_map *map, void *key);
>  	int (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
>  	int (*map_delete_elem)(struct bpf_map *map, void *key);
> +	void *(*map_lookup_and_delete_elem)(struct bpf_map *map, void *key);
>  
>  	/* funcs called by prog_array and perf_event_array map */
>  	void *(*map_fd_get_ptr)(struct bpf_map *map, struct file *map_file,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f9187b41dff6..3bb94aa2d408 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -103,6 +103,7 @@ enum bpf_cmd {
>  	BPF_BTF_LOAD,
>  	BPF_BTF_GET_FD_BY_ID,
>  	BPF_TASK_FD_QUERY,
> +	BPF_MAP_LOOKUP_AND_DELETE_ELEM,
>  };
>  
>  enum bpf_map_type {
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index eb75e8af73ff..50957e243bfb 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -975,6 +975,85 @@ static int map_get_next_key(union bpf_attr *attr)
>  	return err;
>  }
>  
> +#define BPF_MAP_LOOKUP_AND_DELETE_ELEM_LAST_FIELD value
> +
> +static int map_lookup_and_delete_elem(union bpf_attr *attr)
> +{
> +	void __user *ukey = u64_to_user_ptr(attr->key);
> +	void __user *uvalue = u64_to_user_ptr(attr->value);
> +	int ufd = attr->map_fd;
> +	struct bpf_map *map;
> +	void *key, *value, *ptr;
> +	u32 value_size;
> +	struct fd f;
> +	int err;
> +
> +	if (CHECK_ATTR(BPF_MAP_LOOKUP_ELEM))

copy-paste error

> +		return -EINVAL;
> +
> +	f = fdget(ufd);
> +	map = __bpf_map_get(f);
> +	if (IS_ERR(map))
> +		return PTR_ERR(map);
> +
> +	if (!(f.file->f_mode & FMODE_CAN_WRITE)) {
> +		err = -EPERM;
> +		goto err_put;
> +	}
> +
> +	if (!map->ops->map_lookup_and_delete_elem) {
> +		err = -ENOTSUPP;
> +		goto err_put;
> +	}
> +
> +	key = __bpf_copy_key(ukey, map->key_size);
> +	if (IS_ERR(key)) {
> +		err = PTR_ERR(key);
> +		goto err_put;
> +	}
> +
> +	value_size = map->value_size;
> +
> +	err = -ENOMEM;
> +	value = kmalloc(value_size, GFP_USER | __GFP_NOWARN);
> +	if (!value)
> +		goto free_key;
> +
> +	err = -EFAULT;
> +	if (copy_from_user(value, uvalue, value_size) != 0)
> +		goto free_value;
> +
> +	/* must increment bpf_prog_active to avoid kprobe+bpf triggering from
> +	 * inside bpf map update or delete otherwise deadlocks are possible
> +	 */
> +	preempt_disable();
> +	__this_cpu_inc(bpf_prog_active);
> +	rcu_read_lock();
> +	ptr = map->ops->map_lookup_and_delete_elem(map, key);
> +	if (ptr)
> +		memcpy(value, ptr, value_size);
> +	rcu_read_unlock();
> +		err = ptr ? 0 : -ENOENT;
> +	__this_cpu_dec(bpf_prog_active);
> +	preempt_enable();

messed up formatting

> +
> +	if (err)
> +		goto free_value;
> +
> +	if (copy_to_user(uvalue, value, value_size) != 0)
> +		goto free_value;
> +
> +	err = 0;
> +
> +free_value:
> +	kfree(value);
> +free_key:
> +	kfree(key);
> +err_put:
> +	fdput(f);
> +	return err;
> +}
> +
>  static const struct bpf_prog_ops * const bpf_prog_types[] = {
>  #define BPF_PROG_TYPE(_id, _name) \
>  	[_id] = & _name ## _prog_ops,
> @@ -2448,6 +2527,9 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>  	case BPF_TASK_FD_QUERY:
>  		err = bpf_task_fd_query(&attr, uattr);
>  		break;
> +	case BPF_MAP_LOOKUP_AND_DELETE_ELEM:
> +		err = map_lookup_and_delete_elem(&attr);
> +		break;
>  	default:
>  		err = -EINVAL;
>  		break;
>