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 <c...@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 <stefa...@redhat.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Daniel P. Berrangé <berra...@redhat.com>
Signed-off-by: Cédric Le Goater <c...@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;