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    Subject[PATCH 5.10 038/545] ovl: drop WARN_ON() dentry is NULL in ovl_encode_fh()
    Date
    From: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>

    commit dd524b7f317de8d31d638cbfdc7be4cf9b770e42 upstream.

    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 drop WARN_ON() 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>
    Fixes: 8ed5eec9d6c4 ("ovl: encode pure upper file handles")
    Cc: <stable@vger.kernel.org> # v4.16
    Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/overlayfs/export.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/overlayfs/export.c
    +++ b/fs/overlayfs/export.c
    @@ -257,7 +257,7 @@ static int ovl_encode_fh(struct inode *i
    return FILEID_INVALID;

    dentry = d_find_any_alias(inode);
    - if (WARN_ON(!dentry))
    + if (!dentry)
    return FILEID_INVALID;

    bytes = ovl_dentry_to_fid(dentry, fid, buflen);

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