diff mbox series

[bpf] bpf: fix a rcu warning for bpffs map pretty-print

Message ID 20200916004401.146277-1-yhs@fb.com
State Accepted
Delegated to: BPF Maintainers
Headers show
Series [bpf] bpf: fix a rcu warning for bpffs map pretty-print | expand

Commit Message

Yonghong Song Sept. 16, 2020, 12:44 a.m. UTC 
Running selftest
  ./btf_btf -p
the kernel had the following warning:
  [   51.528185] WARNING: CPU: 3 PID: 1756 at kernel/bpf/hashtab.c:717 htab_map_get_next_key+0x2eb/0x300
  [   51.529217] Modules linked in:
  [   51.529583] CPU: 3 PID: 1756 Comm: test_btf Not tainted 5.9.0-rc1+ #878
  [   51.530346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.el7.centos 04/01/2014
  [   51.531410] RIP: 0010:htab_map_get_next_key+0x2eb/0x300
  ...
  [   51.542826] Call Trace:
  [   51.543119]  map_seq_next+0x53/0x80
  [   51.543528]  seq_read+0x263/0x400
  [   51.543932]  vfs_read+0xad/0x1c0
  [   51.544311]  ksys_read+0x5f/0xe0
  [   51.544689]  do_syscall_64+0x33/0x40
  [   51.545116]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The related source code in kernel/bpf/hashtab.c:
  709 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
  710 {
  711         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
  712         struct hlist_nulls_head *head;
  713         struct htab_elem *l, *next_l;
  714         u32 hash, key_size;
  715         int i = 0;
  716
  717         WARN_ON_ONCE(!rcu_read_lock_held());

In kernel/bpf/inode.c, bpffs map pretty print calls map->ops->map_get_next_key()
without holding a rcu_read_lock(), hence causing the above warning.
To fix the issue, just surrounding map->ops->map_get_next_key() with rcu read lock.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/inode.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Comments

Andrii Nakryiko Sept. 16, 2020, 12:57 a.m. UTC | #1 
On Tue, Sep 15, 2020 at 5:44 PM Yonghong Song <yhs@fb.com> wrote:
>
> Running selftest
>   ./btf_btf -p
> the kernel had the following warning:
>   [   51.528185] WARNING: CPU: 3 PID: 1756 at kernel/bpf/hashtab.c:717 htab_map_get_next_key+0x2eb/0x300
>   [   51.529217] Modules linked in:
>   [   51.529583] CPU: 3 PID: 1756 Comm: test_btf Not tainted 5.9.0-rc1+ #878
>   [   51.530346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.el7.centos 04/01/2014
>   [   51.531410] RIP: 0010:htab_map_get_next_key+0x2eb/0x300
>   ...
>   [   51.542826] Call Trace:
>   [   51.543119]  map_seq_next+0x53/0x80
>   [   51.543528]  seq_read+0x263/0x400
>   [   51.543932]  vfs_read+0xad/0x1c0
>   [   51.544311]  ksys_read+0x5f/0xe0
>   [   51.544689]  do_syscall_64+0x33/0x40
>   [   51.545116]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The related source code in kernel/bpf/hashtab.c:
>   709 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
>   710 {
>   711         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
>   712         struct hlist_nulls_head *head;
>   713         struct htab_elem *l, *next_l;
>   714         u32 hash, key_size;
>   715         int i = 0;
>   716
>   717         WARN_ON_ONCE(!rcu_read_lock_held());
>
> In kernel/bpf/inode.c, bpffs map pretty print calls map->ops->map_get_next_key()
> without holding a rcu_read_lock(), hence causing the above warning.
> To fix the issue, just surrounding map->ops->map_get_next_key() with rcu read lock.
>
> Reported-by: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrii Nakryiko <andriin@fb.com>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---

LGTM.

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  kernel/bpf/inode.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index fb878ba3f22f..18f4969552ac 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -226,10 +226,12 @@ static void *map_seq_next(struct seq_file *m, void *v, loff_t *pos)
>         else
>                 prev_key = key;
>
> +       rcu_read_lock();
>         if (map->ops->map_get_next_key(map, prev_key, key)) {
>                 map_iter(m)->done = true;
> -               return NULL;
> +               key = NULL;
>         }
> +       rcu_read_unlock();
>         return key;
>  }
>
> --
> 2.24.1
>
Alexei Starovoitov Sept. 16, 2020, 1:19 a.m. UTC | #2 
On Tue, Sep 15, 2020 at 5:58 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Sep 15, 2020 at 5:44 PM Yonghong Song <yhs@fb.com> wrote:
> >
> > Running selftest
> >   ./btf_btf -p
> > the kernel had the following warning:
> >   [   51.528185] WARNING: CPU: 3 PID: 1756 at kernel/bpf/hashtab.c:717 htab_map_get_next_key+0x2eb/0x300
> >   [   51.529217] Modules linked in:
> >   [   51.529583] CPU: 3 PID: 1756 Comm: test_btf Not tainted 5.9.0-rc1+ #878
> >   [   51.530346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.el7.centos 04/01/2014
> >   [   51.531410] RIP: 0010:htab_map_get_next_key+0x2eb/0x300
> >   ...
> >   [   51.542826] Call Trace:
> >   [   51.543119]  map_seq_next+0x53/0x80
> >   [   51.543528]  seq_read+0x263/0x400
> >   [   51.543932]  vfs_read+0xad/0x1c0
> >   [   51.544311]  ksys_read+0x5f/0xe0
> >   [   51.544689]  do_syscall_64+0x33/0x40
> >   [   51.545116]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > The related source code in kernel/bpf/hashtab.c:
> >   709 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> >   710 {
> >   711         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> >   712         struct hlist_nulls_head *head;
> >   713         struct htab_elem *l, *next_l;
> >   714         u32 hash, key_size;
> >   715         int i = 0;
> >   716
> >   717         WARN_ON_ONCE(!rcu_read_lock_held());
> >
> > In kernel/bpf/inode.c, bpffs map pretty print calls map->ops->map_get_next_key()
> > without holding a rcu_read_lock(), hence causing the above warning.
> > To fix the issue, just surrounding map->ops->map_get_next_key() with rcu read lock.
> >
> > Reported-by: Alexei Starovoitov <ast@kernel.org>
> > Cc: Andrii Nakryiko <andriin@fb.com>
> > Cc: Martin KaFai Lau <kafai@fb.com>
> > Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
> > Signed-off-by: Yonghong Song <yhs@fb.com>
> > ---
>
> LGTM.
>
> Acked-by: Andrii Nakryiko <andriin@fb.com>

Applied. Thanks!
diff mbox series

Patch

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index fb878ba3f22f..18f4969552ac 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -226,10 +226,12 @@  static void *map_seq_next(struct seq_file *m, void *v, loff_t *pos)
 	else
 		prev_key = key;
 
+	rcu_read_lock();
 	if (map->ops->map_get_next_key(map, prev_key, key)) {
 		map_iter(m)->done = true;
-		return NULL;
+		key = NULL;
 	}
+	rcu_read_unlock();
 	return key;
 }