[PATCH v2 1/2] 9p: skip intermediate directory entry name copy in p9dirent_read()

[Haobin Wu <853555@gmail.com>]

v9fs_dir_readdir_dotl() decodes each entry of a Rreaddir reply with
p9dirent_read(), which parses it through p9pdu_readf("Qqbs"). The 's'
conversion in p9pdu_vreadf() already allocates a NUL-terminated copy
of the name from the wire buffer; p9dirent_read() then strscpy()s that
copy into the fixed 256-byte p9_dirent::d_name and frees the original.

The wire format carries the name length in 16 bits, so a name longer
than 255 bytes is valid on the wire and may well be valid on the
server's filesystem, but it makes strscpy() return -E2BIG.
v9fs_dir_readdir_dotl() turns that into -EIO and aborts getdents64(),
so every entry after the long one disappears from the listing.

Drop the second copy: keep the string allocated by p9pdu_vreadf() in
p9_dirent and let v9fs_dir_readdir_dotl(), its only user, free it once
dir_emit() has consumed it. p9_dirent is a short-lived stack object, so
the string's lifetime does not change.

Note that the VFS only rejects names of PATH_MAX bytes or more in
verify_dirent_name(), so after this change a name between NAME_MAX and
PATH_MAX is returned by getdents64() even though any later lookup on it
fails with -ENAMETOOLONG. The next patch skips such entries.

Fixes: 7751bdb3a095 ("9p: readdir implementation for 9p2000.L")
Closes: https://github.com/microsoft/WSL/issues/41192
Assisted-by: Codex:gpt-5
Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Haobin Wu <853555@gmail.com>
---
 fs/9p/vfs_dir.c         |  6 +++++-
 include/net/9p/client.h |  2 +-
 net/9p/protocol.c       | 12 ++----------
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/9p/vfs_dir.c b/fs/9p/vfs_dir.c
index e0d34e4e9076..af00b79d801e 100644
--- a/fs/9p/vfs_dir.c
+++ b/fs/9p/vfs_dir.c
@@ -185,8 +185,12 @@ static int v9fs_dir_readdir_dotl(struct file *file, struct dir_context *ctx)
 			if (!dir_emit(ctx, curdirent.d_name,
 				      strlen(curdirent.d_name),
 				      QID2INO(&curdirent.qid),
-				      curdirent.d_type))
+				      curdirent.d_type)) {
+				kfree(curdirent.d_name);
 				return 0;
+			}
+
+			kfree(curdirent.d_name);
 
 			ctx->pos = curdirent.d_off;
 			rdir->head += err;
diff --git a/include/net/9p/client.h b/include/net/9p/client.h
index 838a94218b59..9f3079c6f386 100644
--- a/include/net/9p/client.h
+++ b/include/net/9p/client.h
@@ -268,7 +268,7 @@ struct p9_dirent {
 	struct p9_qid qid;
 	u64 d_off;
 	unsigned char d_type;
-	char d_name[256];
+	char *d_name;
 };
 
 struct iov_iter;
diff --git a/net/9p/protocol.c b/net/9p/protocol.c
index 67b0586d807f..d4335884e0c0 100644
--- a/net/9p/protocol.c
+++ b/net/9p/protocol.c
@@ -770,7 +770,7 @@ int p9dirent_read(struct p9_client *clnt, char *buf, int len,
 {
 	struct p9_fcall fake_pdu;
 	int ret;
-	char *nameptr;
+	char *nameptr = NULL;
 
 	fake_pdu.size = len;
 	fake_pdu.capacity = len;
@@ -785,15 +785,7 @@ int p9dirent_read(struct p9_client *clnt, char *buf, int len,
 		return ret;
 	}
 
-	ret = strscpy(dirent->d_name, nameptr, sizeof(dirent->d_name));
-	if (ret < 0) {
-		p9_debug(P9_DEBUG_ERROR,
-			 "On the wire dirent name too long: %s\n",
-			 nameptr);
-		kfree(nameptr);
-		return ret;
-	}
-	kfree(nameptr);
+	dirent->d_name = nameptr;
 
 	return fake_pdu.offset;
 }
-- 
2.54.0 (Apple Git-157)