[PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

[PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

[Jiachen Zhang]

Some code paths cannot guarantee the inode have any dentry alias. So
WARN_ON() all !dentry may flood the kernel logs.

For example, when an overlayfs inode is watched by inotifywait (1), and
someone is trying to read the /proc/$(pidof inotifywait)/fdinfo/INOTIFY_FD,
at that time if the dentry has been reclaimed by kernel (such as
echo 2 > /proc/sys/vm/drop_caches), there will be a WARN_ON(). The
printed call stack would be like:

    ? show_mark_fhandle+0xf0/0xf0
    show_mark_fhandle+0x4a/0xf0
    ? show_mark_fhandle+0xf0/0xf0
    ? seq_vprintf+0x30/0x50
    ? seq_printf+0x53/0x70
    ? show_mark_fhandle+0xf0/0xf0
    inotify_fdinfo+0x70/0x90
    show_fdinfo.isra.4+0x53/0x70
    seq_show+0x130/0x170
    seq_read+0x153/0x440
    vfs_read+0x94/0x150
    ksys_read+0x5f/0xe0
    do_syscall_64+0x59/0x1e0
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

So let's replace WARN_ON() with WARN_ON_ONCE() to avoid kernel log
flooding.

Reported-by: Hongbo Yin <yinhongbo@bytedance.com>
Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Signed-off-by: Tianci Zhang <zhangtianci.1997@bytedance.com>
---
 fs/overlayfs/export.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index 2eada97bbd23..3a8650442aec 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -259,7 +259,7 @@ static int ovl_encode_fh(struct inode *inode, u32 *fid, int *max_len,
 		return FILEID_INVALID;
 
 	dentry = d_find_any_alias(inode);
-	if (WARN_ON(!dentry))
+	if (WARN_ON_ONCE(!dentry))
 		return FILEID_INVALID;
 
 	bytes = ovl_dentry_to_fid(ofs, dentry, fid, buflen);
-- 
2.20.1

Re: [PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

[Miklos Szeredi]

On Thu, 28 Jul 2022 at 08:39, Jiachen Zhang
<zhangjiachen.jaycee@bytedance.com> wrote:
>
> Some code paths cannot guarantee the inode have any dentry alias. So
> WARN_ON() all !dentry may flood the kernel logs.
>
> For example, when an overlayfs inode is watched by inotifywait (1), and
> someone is trying to read the /proc/$(pidof inotifywait)/fdinfo/INOTIFY_FD,
> at that time if the dentry has been reclaimed by kernel (such as
> echo 2 > /proc/sys/vm/drop_caches), there will be a WARN_ON(). The
> printed call stack would be like:
>
>     ? show_mark_fhandle+0xf0/0xf0
>     show_mark_fhandle+0x4a/0xf0
>     ? show_mark_fhandle+0xf0/0xf0
>     ? seq_vprintf+0x30/0x50
>     ? seq_printf+0x53/0x70
>     ? show_mark_fhandle+0xf0/0xf0
>     inotify_fdinfo+0x70/0x90
>     show_fdinfo.isra.4+0x53/0x70
>     seq_show+0x130/0x170
>     seq_read+0x153/0x440
>     vfs_read+0x94/0x150
>     ksys_read+0x5f/0xe0
>     do_syscall_64+0x59/0x1e0
>     entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> So let's replace WARN_ON() with WARN_ON_ONCE() to avoid kernel log
> flooding.

Better just drop the WARN_ON() completely in that case, since it's a
normally occurring condition.

Thanks,
Miklos

[PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

[Jiachen Zhang]

On Thu, Jul 28, 2022 at 4:57 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Thu, 28 Jul 2022 at 08:39, Jiachen Zhang
> <zhangjiachen.jaycee@bytedance.com> wrote:
> >
> > Some code paths cannot guarantee the inode have any dentry alias. So
> > WARN_ON() all !dentry may flood the kernel logs.
> >
> > For example, when an overlayfs inode is watched by inotifywait (1), and
> > someone is trying to read the /proc/$(pidof inotifywait)/fdinfo/INOTIFY_FD,
> > at that time if the dentry has been reclaimed by kernel (such as
> > echo 2 > /proc/sys/vm/drop_caches), there will be a WARN_ON(). The
> > printed call stack would be like:
> >
> >     ? show_mark_fhandle+0xf0/0xf0
> >     show_mark_fhandle+0x4a/0xf0
> >     ? show_mark_fhandle+0xf0/0xf0
> >     ? seq_vprintf+0x30/0x50
> >     ? seq_printf+0x53/0x70
> >     ? show_mark_fhandle+0xf0/0xf0
> >     inotify_fdinfo+0x70/0x90
> >     show_fdinfo.isra.4+0x53/0x70
> >     seq_show+0x130/0x170
> >     seq_read+0x153/0x440
> >     vfs_read+0x94/0x150
> >     ksys_read+0x5f/0xe0
> >     do_syscall_64+0x59/0x1e0
> >     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > So let's replace WARN_ON() with WARN_ON_ONCE() to avoid kernel log
> > flooding.
>
> Better just drop the WARN_ON() completely in that case, since it's a
> normally occurring condition.
>
> Thanks,
> Miklos

OK. I will send the v2 patch later.

Thanks,
Jiachen