Re: [Qemu-devel] [PATCH] sheepdog: Replace strncpy() by g_strlcpy()

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/08/2018 04:56, Philippe Mathieu-Daudé wrote:
> Fedora 29 comes with GCC 8.1 which added the 'stringop-truncation' checks.
> 
> Replace the strncpy() calls by g_strlcpy() to avoid the following warning:
> 
>   block/sheepdog.c: In function 'find_vdi_name':
>   block/sheepdog.c:1239:5: error: 'strncpy' specified bound 256 equals
>   destination size [-Werror=stringop-truncation]
>        strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Reported-by: Howard Spoelstra <hsp.cat7@gmail.com>
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
> See http://lists.nongnu.org/archive/html/qemu-devel/2018-07/msg03723.html
> 
>  block/sheepdog.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/block/sheepdog.c b/block/sheepdog.c
> index b229a664d9..5dc3d0c39e 100644
> --- a/block/sheepdog.c
> +++ b/block/sheepdog.c
> @@ -1224,19 +1224,19 @@ static int find_vdi_name(BDRVSheepdogState *s, const char *filename,
>      SheepdogVdiReq hdr;
>      SheepdogVdiRsp *rsp = (SheepdogVdiRsp *)&hdr;
>      unsigned int wlen, rlen = 0;
> -    char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN];
> +    /* Ensures that the buffer is zero-filled,
> +     * which is desirable since we'll soon be sending those bytes, and
> +     * don't want the send_req to read uninitialized data.
> +     */
> +    char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN] = { };
>  
>      fd = connect_to_sdog(s, errp);
>      if (fd < 0) {
>          return fd;
>      }
>  
> -    /* This pair of strncpy calls ensures that the buffer is zero-filled,
> -     * which is desirable since we'll soon be sending those bytes, and
> -     * don't want the send_req to read uninitialized data.
> -     */
> -    strncpy(buf, filename, SD_MAX_VDI_LEN);
> -    strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
> +    g_strlcpy(buf, filename, SD_MAX_VDI_LEN);
> +    g_strlcpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
>  
>      memset(&hdr, 0, sizeof(hdr));
>      if (lock) {
> 

The protocol doesn't require (as far as I can see) the strings to be
NULL-terminated, therefore strncpy is the right function to use here.

However, we should have a check on the length of filename and tag, so
that no truncation is done.  This applies to both strncpy and g_strlcpy.

Indeed I find g_strlcpy to be harmful because it encourages a style
where truncations happen silently.  There are very few cases where
silent truncation is the right thing to do, and in several cases where
you _have to have_ fixed-size buffers, those buffers are sent on the
wire---and then g_strlcpy is wrong, while strncpy is just as good as
memset+strlen+memcpy (and shorter).

Paolo