[PATCH 2/3] 9p: v9fs_listxattr: fix %s null argument warning

[Dominique Martinet <asmadeus@codewreck.org>]

W=1 warns about null argument to kprintf:
In file included from fs/9p/xattr.c:12:
In function ‘v9fs_xattr_get’,
    inlined from ‘v9fs_listxattr’ at fs/9p/xattr.c:142:9:
include/net/9p/9p.h:55:2: error: ‘%s’ directive argument is null
[-Werror=format-overflow=]
   55 |  _p9_debug(level, __func__, fmt, ##__VA_ARGS__)
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use an empty string instead of :
 - this is ok 9p-wise because p9pdu_vwritef serializes a null string
and an empty string the same way (one '0' word for length)
 - since this degrades the print statements, add new single quotes for
xattr's name delimter (Old: "file = (null)", new: "file = ''")

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Link: https://lore.kernel.org/r/20231008060138.517057-1-suhui@nfschina.com
Suggested-by: Su Hui <suhui@nfschina.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
---
I've checked this works as expected (getfattr listing all user.* xattrs
after setting some), so let's fix this warning.

As pointed out by Dan this makes the message les clear, so I added
single quotes to make it clear we're dealing with an empty string; I
think it's good enough.
Su, I made you only Suggested-by because of the extra legwork and
format changes, but happy to give you authorship if it's something you
care about; I'd just like to get it out during the next merge window
in a couple of weeks so please say the word.

This makes fs/9p build warning-free with W=1 on gcc 12

 fs/9p/xattr.c   | 4 ++--
 net/9p/client.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/9p/xattr.c b/fs/9p/xattr.c
index e00cf8109b3f..d29907c378fd 100644
--- a/fs/9p/xattr.c
+++ b/fs/9p/xattr.c
@@ -68,7 +68,7 @@ ssize_t v9fs_xattr_get(struct dentry *dentry, const char *name,
 	struct p9_fid *fid;
 	int ret;
 
-	p9_debug(P9_DEBUG_VFS, "name = %s value_len = %zu\n",
+	p9_debug(P9_DEBUG_VFS, "name = '%s' value_len = %zu\n",
 		 name, buffer_size);
 	fid = v9fs_fid_lookup(dentry);
 	if (IS_ERR(fid))
@@ -139,7 +139,7 @@ int v9fs_fid_xattr_set(struct p9_fid *fid, const char *name,
 
 ssize_t v9fs_listxattr(struct dentry *dentry, char *buffer, size_t buffer_size)
 {
-	return v9fs_xattr_get(dentry, NULL, buffer, buffer_size);
+	return v9fs_xattr_get(dentry, "", buffer, buffer_size);
 }
 
 static int v9fs_xattr_handler_get(const struct xattr_handler *handler,
diff --git a/net/9p/client.c b/net/9p/client.c
index 86bbc7147fc1..9c2bc15e3cfa 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1979,7 +1979,7 @@ struct p9_fid *p9_client_xattrwalk(struct p9_fid *file_fid,
 		goto error;
 	}
 	p9_debug(P9_DEBUG_9P,
-		 ">>> TXATTRWALK file_fid %d, attr_fid %d name %s\n",
+		 ">>> TXATTRWALK file_fid %d, attr_fid %d name '%s'\n",
 		 file_fid->fid, attr_fid->fid, attr_name);
 
 	req = p9_client_rpc(clnt, P9_TXATTRWALK, "dds",
-- 
2.41.0