Re: [PATCH] 9p: don't decrement a server-owned zero nlink

[Dominique Martinet <asmadeus@codewreck.org>]

Yuanfu Xie wrote on Mon, Sep 28, 2026 at 03:25:48PM +0800:
> A 9P2000.L server can report nlink=0 for a regular file that still
> exists.  On a cache=loose or cache=fscache mount, lookup stores that
> value with set_nlink().  A later successful unlink calls
> v9fs_dec_count(), which still calls drop_nlink() for a non-directory.
> drop_nlink() warns, and the unsigned link count wraps to 4294967295.
> fstat() on a descriptor opened before the unlink shows 0 before and
> 4294967295 after.
> 
> The warning on the unpatched kernel, trimmed:
> 
> WARNING: fs/inode.c:408 at drop_nlink+0xac/0xd0, CPU#1: trigger/188
> CPU: 1 UID: 0 PID: 188 Comm: trigger Tainted: G                 N  7.3.0-rc4-00537-ga3ff15db6820 #1 PREEMPT(lazy)
> RIP: 0010:drop_nlink+0xac/0xd0
> Call Trace:
>  <TASK>
>  v9fs_remove+0x529/0x750
>  vfs_unlink+0x2f0/0xbd0
>  filename_unlinkat+0x340/0x6c0
>  __x64_sys_unlink+0x46/0x70
>  do_syscall_64+0xe8/0x4f0
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>  </TASK>
> ---[ end trace 0000000000000000 ]---
> 
> Directories whose cached nlink is at most 2 already skip this
> decrement, because the exported filesystem may not maintain nlink
> (commit ac89b2ef9b559).  cache=none skips it as well (commit
> 574aa0b479947).  cache=mmap does not take this path: it sets neither
> CACHE_META nor CACHE_LOOSE.  A regular file on cache=loose or
> cache=fscache still does.  Its count came from GETATTR and may
> already be zero, so skip the decrement when i_nlink is already 0.
> 
> Verified on 7.3-rc4-00537-ga3ff15db6820 with only this change.  A
> server that advertises nlink=0 and accepts the unlink no longer
> warns, and the open descriptor stays at nlink 0.  An honest server
> on the same mount still goes from 1 to 0.  A rejected unlink leaves
> the count unchanged.  cache=none is unchanged.  With panic_on_warn
> the unpatched kernel panics on the warning; the patched kernel does
> not.
> 
> Fixes: ac89b2ef9b559 ("9p: don't maintain dir i_nlink if the exported fs doesn't either")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yuanfu Xie <yuanfuxie@stu.pku.edu.cn>


This is the same as part of what was sent in [1], but they never
bothered resending split commits, so picking this up instead

[1] https://lore.kernel.org/r/20260919221734.3708000-1-benquike@gmail.com

> ---
>  fs/9p/vfs_inode.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
> index d7e678de0a036..347739b81513f 100644
> --- a/fs/9p/vfs_inode.c
> +++ b/fs/9p/vfs_inode.c
> @@ -495,6 +495,12 @@ static int v9fs_at_to_dotl_flags(int flags)
>   * nothing and races with concurrent metadata fetches that may already
>   * have observed the post-unlink value (nlink == 0).
>   *
> + * The same applies to a regular file whose cached nlink has already been
> + * driven to zero by the server: in the cached modes the attributes come
> + * from GETATTR and are applied verbatim, so decrementing a link count the
> + * server does not maintain only trips the drop_nlink() WARN_ON and
> + * underflows i_nlink.
> + *
>   * @inode: inode whose nlink is being dropped
>   */
>  static void v9fs_dec_count(struct inode *inode)
> @@ -503,6 +509,8 @@ static void v9fs_dec_count(struct inode *inode)
>  
>  	if (!(v9ses->cache & (CACHE_META | CACHE_LOOSE)))
>  		return;
> +	if (inode->i_nlink == 0)
> +		return;
>  	if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2)
>  		drop_nlink(inode);
>  }
> 
> 

-- 
Dominique Martinet | Asmadeus