Re: [Qemu-devel] [PATCH v3 2/5] block/sheepdog: Use QEMU_NONSTRING for non NUL-terminated arrays

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 18, 2018 at 06:51:19PM +0100, Philippe Mathieu-Daudé wrote:
> GCC 8 added a -Wstringop-truncation warning:
> 
>   The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
>   bug 81117 is specifically intended to highlight likely unintended
>   uses of the strncpy function that truncate the terminating NUL
>   character from the source string.
> 
> This new warning leads to compilation failures:
> 
>     CC      block/sheepdog.o
>   qemu/block/sheepdog.c: In function 'find_vdi_name':
>   qemu/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);
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   make: *** [qemu/rules.mak:69: block/sheepdog.o] Error 1
> 
> As described previous to the strncpy() calls, the use of strncpy() is
> correct here:
> 
>     /* 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);
> 
> Use the QEMU_NONSTRING attribute, since this array is intended to store
> character arrays that do not necessarily contain a terminating NUL.
> 
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  block/sheepdog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/sheepdog.c b/block/sheepdog.c
> index 0125df9d49..d4ad6b119d 100644
> --- a/block/sheepdog.c
> +++ b/block/sheepdog.c
> @@ -1224,7 +1224,7 @@ 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];
> +    QEMU_NONSTRING char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN];

In case you decide to respin anyway - this would be
a bit nicer as:
	char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN] QEMU_NONSTRING




>      fd = connect_to_sdog(s, errp);
>      if (fd < 0) {
> -- 
> 2.17.2