Re: [PATCH v3 3/3] 9p: skip over-long directory entry names for legacy 9p2000 too

[Christian Schoenebeck]

On Friday, 18 September 2026 04:48:51 CEST Haobin Wu wrote:
> The legacy 9p2000 and 9p2000.u readdir path, v9fs_dir_readdir(), parses
> each entry with p9stat_read() and, unlike the 9p2000.L path, never had
> a 256-byte name limit: a name of any length up to the wire maximum was
> handed to dir_emit(). getdents64() therefore returned names longer than
> NAME_MAX that no later syscall can use, and failed with -EIO for names
> of PATH_MAX bytes or more.
> 
> Apply the same NAME_MAX check as the previous patch does for
> v9fs_dir_readdir_dotl(), so both protocol variants behave the same.
> 
> Note that this is a behaviour change for 9p2000 and 9p2000.u: entries
> with names longer than NAME_MAX used to be listed and are now skipped.

That sentence is a bit misleading, isn't it?

Name length 256..4095:
  - before: listed, but unusable (ENAMETOOLONG)
  - now: entry skipped, fetch continues

Name length >=4096:
  - before: aborts the entire fetch (similar to 9p2000.L side)
  - now: entry skipped, fetch continues

But yes, it would be a user-space change nevertheless, so ...
 
> Suggested-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/3910569.MHq7AAxBmi@weasel/
> Signed-off-by: Haobin Wu <853555@xxxxxxxxx>
> ---
>  fs/9p/vfs_dir.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/vfs_dir.c b/fs/9p/vfs_dir.c
> index 08d1a3429654..31de9ac913f0 100644
> --- a/fs/9p/vfs_dir.c
> +++ b/fs/9p/vfs_dir.c
> @@ -119,6 +119,8 @@ static int v9fs_dir_readdir(struct file *file, struct
> dir_context *ctx) rdir->tail = n;
>  		}
>  		while (rdir->head < rdir->tail) {
> +			size_t namelen;
> +
>  			err = p9stat_read(fid->clnt, rdir->buf + rdir->head,
>  					  rdir->tail - rdir->head, &st);
>  			if (err <= 0) {
> @@ -126,8 +128,16 @@ static int v9fs_dir_readdir(struct file *file, struct
> dir_context *ctx) return -EIO;
>  			}
> 
> -			over = !dir_emit(ctx, st.name, strlen(st.name),
> -					QID2INO(&st.qid), dt_type(&st));
> +			namelen = strlen(st.name);
> +			if (namelen > NAME_MAX) {

... maybe better if (namelen >= PATH_MAX) here for legacy 9p2000(.u)?

> +				p9_debug(P9_DEBUG_ERROR,
> +					 "skip dentry: name length %zu > NAME_MAX\n",
> +					 namelen);
> +				over = false;

Nit: alternative would be init over = false and dropping explicit assignment 
inside loop branch here. But that's already personal taste level.

/Christian

> +			} else {
> +				over = !dir_emit(ctx, st.name, namelen,
> +						 QID2INO(&st.qid), dt_type(&st));
> +			}
>  			p9stat_free(&st);
>  			if (over)
>  				return 0;