Re: [PATCH for-8.0 v2 1/3] async: Suppress GCC13 false positive in aio_bh_poll()

[Daniel Henrique Barboza]

Hi everyone,


Fedora 38 rolled out recently. I did a distro upgrade, and Fedora 38 is now 
using
GCC 13 ... and now I can't build QEMU without this patch.


I'll use it as is for now. It would be nice to fix this upstream though :D



Thanks,


Daniel


On 3/21/23 13:16, Cédric Le Goater wrote:
> From: Cédric Le Goater <clg@redhat.com>
>
> GCC13 reports an error :
>
> ../util/async.c: In function ‘aio_bh_poll’:
> include/qemu/queue.h:303:22: error: storing the address of local variable 
> ‘slice’ in ‘*ctx.bh_slice_list.sqh_last’ [-Werror=dangling-pointer=]
>    303 |     (head)->sqh_last = &(elm)->field.sqe_next;                         
>  \
>        |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
> ../util/async.c:169:5: note: in expansion of macro ‘QSIMPLEQ_INSERT_TAIL’
>    169 |     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
>        |     ^~~~~~~~~~~~~~~~~~~~
> ../util/async.c:161:17: note: ‘slice’ declared here
>    161 |     BHListSlice slice;
>        |                 ^~~~~
> ../util/async.c:161:17: note: ‘ctx’ declared here
>
> But the local variable 'slice' is removed from the global context list
> in following loop of the same routine. Add a pragma to silent GCC.
>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Cédric Le Goater <clg@redhat.com>
> ---
>   util/async.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/util/async.c b/util/async.c
> index 21016a1ac7..de9b431236 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -164,7 +164,20 @@ int aio_bh_poll(AioContext *ctx)
>   
>       /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue().  */
>       QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
> +
> +    /*
> +     * GCC13 [-Werror=dangling-pointer=] complains that the local variable
> +     * 'slice' is being stored in the global 'ctx->bh_slice_list' but the
> +     * list is emptied before this function returns.
> +     */
> +#if !defined(__clang__)
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wdangling-pointer="
> +#endif
>       QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
> +#if !defined(__clang__)
> +#pragma GCC diagnostic pop
> +#endif
>   
>       while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
>           QEMUBH *bh;