[PATCH v3 2/3] 9p: skip directory entries with names longer than NAME_MAX

The 9p wire format carries directory entry names of up to 65535 bytes
and nothing on the client checks them against NAME_MAX. Since the
previous patch, v9fs_dir_readdir_dotl() passes such names straight to
dir_emit(), and the VFS only rejects names of PATH_MAX bytes or more in
verify_dirent_name(), as -EIO, which again fails the whole getdents64()
call.

A name between NAME_MAX and PATH_MAX is therefore returned to userspace
even though every later operation on it fails with -ENAMETOOLONG, and
POSIX requires readdir() to only return components of at most NAME_MAX
bytes. Nothing can use such an entry, so skip it instead of returning it
or failing the listing, reusing the strlen() result that was already
computed for dir_emit(). Log the skipped entry at P9_DEBUG_ERROR, the
same level as the strscpy() failure message this replaces.

Suggested-by: Dominique Martinet <asmadeus@codewreck.org>
Link: https://lore.kernel.org/all/vz5bum547fqyxf5z4m3x7tuqkuq52jlopm65t7hvynqeulh7i3@2t4wnhfxs7qv/ (local)
Signed-off-by: Haobin Wu <redacted>
---
 fs/9p/vfs_dir.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/9p/vfs_dir.c b/fs/9p/vfs_dir.c
index af00b79d801e..08d1a3429654 100644
--- a/fs/9p/vfs_dir.c
+++ b/fs/9p/vfs_dir.c

@@ -173,6 +173,7 @@ static int v9fs_dir_readdir_dotl(struct file *file, struct dir_context *ctx)
 		}
 
 		while (rdir->head < rdir->tail) {
+			size_t namelen;
 
 			err = p9dirent_read(fid->clnt, rdir->buf + rdir->head,
 					    rdir->tail - rdir->head,

@@ -182,10 +183,14 @@ static int v9fs_dir_readdir_dotl(struct file *file, struct dir_context *ctx)
 				return -EIO;
 			}
 
-			if (!dir_emit(ctx, curdirent.d_name,
-				      strlen(curdirent.d_name),
-				      QID2INO(&curdirent.qid),
-				      curdirent.d_type)) {
+			namelen = strlen(curdirent.d_name);
+			if (namelen > NAME_MAX) {
+				p9_debug(P9_DEBUG_ERROR,
+					 "skip dentry: name length %zu > NAME_MAX\n",
+					 namelen);
+			} else if (!dir_emit(ctx, curdirent.d_name, namelen,
+					     QID2INO(&curdirent.qid),
+					     curdirent.d_type)) {
 				kfree(curdirent.d_name);
 				return 0;
 			}

-- 
2.54.0 (Apple Git-157)