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

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Date:   Tue, 24 Oct 2023 14:29:54 +0200
From:   Christian Schoenebeck <linux_oss@...debyte.com>
To:     v9fs@...ts.linux.dev, ericvh@...nel.org,
        Dominique Martinet <asmadeus@...ewreck.org>
Cc:     lucho@...kov.net, linux-kernel@...r.kernel.org,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Su Hui <suhui@...china.com>,
        Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: [PATCH 2/3] 9p: v9fs_listxattr: fix %s null argument warning

On Tuesday, October 24, 2023 1:37:03 AM CEST Dominique Martinet wrote:
> 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@...ewreck.org>
> Link: https://lore.kernel.org/r/20231008060138.517057-1-suhui@nfschina.com
> Suggested-by: Su Hui <suhui@...china.com>
> Cc: Dan Carpenter <dan.carpenter@...aro.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);
>  }

As from the previous discussions on this, it might be worth to add a short
comment here that the magical empty string in the subsequent 'Txattrwalk' 9p
request causes 9p server to return a list of xattrs instead of one specific
xattr.

Up to you though:

Acked-by: Christian Schoenebeck <linux_oss@...debyte.com>

>  
>  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",
>