Comment 17 for bug 1803872

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 <email address hidden>
> Signed-off-by: Philippe Mathieu-Daudé <email address hidden>
> ---
> 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