Re: [PATCH] net/dump.c: Suppress spurious compiler warning

[Philipp Tomsich]

Note that I don't expect this to get applied. I just put it onto the list for visibility and to make the workaround available for downstream users that might run into the issue while Ubuntu 22.04 is maturing.  I won't have any time to dig into this on the GCC side until GCC12 is out.

Philipp.

On Mon, 10 Jan 2022 at 10:39, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:

Cc'ing Richard & Eric for dubious compiler warning.

On 1/9/22 21:57, Philipp Tomsich wrote:
> Compiling with gcc version 11.2.0 (Ubuntu 11.2.0-13ubuntu1) results in
> a (spurious) warning:
>
>   In function ‘dump_receive_iov’,
>       inlined from ‘filter_dump_receive_iov’ at ../net/dump.c:157:5:
>   ../net/dump.c:89:9: error: ‘writev’ specified size 18446744073709551600 exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]
>      89 |     if (writev(s->fd, dumpiov, cnt + 1) != sizeof(hdr) + caplen) {
>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   In file included from /home/ptomsich/qemu/include/qemu/osdep.h:108,
>                    from ../net/dump.c:25:
>   ../net/dump.c: In function ‘filter_dump_receive_iov’:
>   /usr/include/x86_64-linux-gnu/sys/uio.h:52:16: note: in a call to function ‘writev’ declared with attribute ‘read_only (2, 3)’
>      52 | extern ssize_t writev (int __fd, const struct iovec *__iovec, int __count)
>         |                ^~~~~~
>   cc1: all warnings being treated as errors
>
> This change helps that version of GCC to understand what is going on
> and suppresses this warning.
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
>
> ---
>
>  net/dump.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/dump.c b/net/dump.c
> index a07ba62401..c32d3bf4e6 100644
> --- a/net/dump.c
> +++ b/net/dump.c
> @@ -86,7 +86,7 @@ static ssize_t dump_receive_iov(DumpState *s, const struct iovec *iov, int cnt)
>      dumpiov[0].iov_len = sizeof(hdr);
>      cnt = iov_copy(&dumpiov[1], cnt, iov, cnt, 0, caplen);
> 
> -    if (writev(s->fd, dumpiov, cnt + 1) != sizeof(hdr) + caplen) {
> +    if (writev(s->fd, &dumpiov[0], cnt + 1) != sizeof(hdr) + caplen) {
>          error_report("network dump write error - stopping dump");
>          close(s->fd);
>          s->fd = -1;