[PATCH] block/rnbd-clt: fix overlapping snprintf arguments

[PATCH] block/rnbd-clt: fix overlapping snprintf arguments

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

The -Wrestrict warning (disabled by default) points out undefined
behavior calling snprintf():

drivers/block/rnbd/rnbd-clt-sysfs.c: In function 'rnbd_clt_get_path_name':
drivers/block/rnbd/rnbd-clt-sysfs.c:486:8: error: 'snprintf' argument 4 overlaps destination object 'buf' [-Werror=restrict]
  486 |  ret = snprintf(buf, len, "%s@%s", buf, dev->sess->sessname);
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/block/rnbd/rnbd-clt-sysfs.c:472:67: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
  472 | static int rnbd_clt_get_path_name(struct rnbd_clt_dev *dev, char *buf,
      |                                                             ~~~~~~^~~

This can be simplified by using a single snprintf() to print the
whole buffer, avoiding the undefined behavior.

Fixes: 91f4acb2801c ("block/rnbd-clt: support mapping two devices with the same name from different servers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/block/rnbd/rnbd-clt-sysfs.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/block/rnbd/rnbd-clt-sysfs.c b/drivers/block/rnbd/rnbd-clt-sysfs.c
index d4aa6bfc9555..38251b749664 100644
--- a/drivers/block/rnbd/rnbd-clt-sysfs.c
+++ b/drivers/block/rnbd/rnbd-clt-sysfs.c
@@ -479,11 +479,7 @@ static int rnbd_clt_get_path_name(struct rnbd_clt_dev *dev, char *buf,
 	while ((s = strchr(pathname, '/')))
 		s[0] = '!';
 
-	ret = snprintf(buf, len, "%s", pathname);
-	if (ret >= len)
-		return -ENAMETOOLONG;
-
-	ret = snprintf(buf, len, "%s@%s", buf, dev->sess->sessname);
+	ret = snprintf(buf, len, "%s@%s", pathname, dev->sess->sessname);
 	if (ret >= len)
 		return -ENAMETOOLONG;
 
-- 
2.29.2

Re: [PATCH] block/rnbd-clt: fix overlapping snprintf arguments

[Jinpu Wang]

On Tue, Mar 23, 2021 at 1:55 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> The -Wrestrict warning (disabled by default) points out undefined
> behavior calling snprintf():
>
> drivers/block/rnbd/rnbd-clt-sysfs.c: In function 'rnbd_clt_get_path_name':
> drivers/block/rnbd/rnbd-clt-sysfs.c:486:8: error: 'snprintf' argument 4 overlaps destination object 'buf' [-Werror=restrict]
>   486 |  ret = snprintf(buf, len, "%s@%s", buf, dev->sess->sessname);
>       |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/block/rnbd/rnbd-clt-sysfs.c:472:67: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
>   472 | static int rnbd_clt_get_path_name(struct rnbd_clt_dev *dev, char *buf,
>       |                                                             ~~~~~~^~~
>
> This can be simplified by using a single snprintf() to print the
> whole buffer, avoiding the undefined behavior.
>
> Fixes: 91f4acb2801c ("block/rnbd-clt: support mapping two devices with the same name from different servers")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/block/rnbd/rnbd-clt-sysfs.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/drivers/block/rnbd/rnbd-clt-sysfs.c b/drivers/block/rnbd/rnbd-clt-sysfs.c
> index d4aa6bfc9555..38251b749664 100644
> --- a/drivers/block/rnbd/rnbd-clt-sysfs.c
> +++ b/drivers/block/rnbd/rnbd-clt-sysfs.c
> @@ -479,11 +479,7 @@ static int rnbd_clt_get_path_name(struct rnbd_clt_dev *dev, char *buf,
>         while ((s = strchr(pathname, '/')))
>                 s[0] = '!';
>
> -       ret = snprintf(buf, len, "%s", pathname);
> -       if (ret >= len)
> -               return -ENAMETOOLONG;
> -
> -       ret = snprintf(buf, len, "%s@%s", buf, dev->sess->sessname);
> +       ret = snprintf(buf, len, "%s@%s", pathname, dev->sess->sessname);
>         if (ret >= len)
>                 return -ENAMETOOLONG;
>
> --
> 2.29.2
>
Thanks Arnd, We have a same patch will send out soon as part of a
bigger patchset.