[PATCH] hw/intc/openpic: Avoid taking address of out-of-bounds array index

[Peter Maydell <peter.maydell@linaro.org>]

The clang sanitizer complains about the code in the EOI handling
of openpic_cpu_write_internal():

UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1 ./build/clang/qemu-system-ppc -M mac99,graphics=off -display none -kernel day15/invaders.elf
../../hw/intc/openpic.c:1034:16: runtime error: index -1 out of bounds for type 'IRQSource[264]' (aka 'struct IRQSource[264]')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../hw/intc/openpic.c:1034:16 in

This is because we do
  src = &opp->src[n_IRQ];$
when n_IRQ may be -1.  This is in practice harmless because if n_IRQ
is -1 then we don't do anything with the src pointer, but it is
undefined behaviour. (This has been present since this device
was first added to QEMU.)

Rearrange the code so we only do the array index when n_IRQ is not -1.

Cc: qemu-stable@nongnu.org
Fixes: e9df014c0b ("Implement embedded IRQ controller for PowerPC 6xx/740 & 75")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Arguable whether it's worth the stable backport or not...
---
 hw/intc/openpic.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/hw/intc/openpic.c b/hw/intc/openpic.c
index cd3d87768e0..2ead4b9ba00 100644
--- a/hw/intc/openpic.c
+++ b/hw/intc/openpic.c
@@ -1031,13 +1031,14 @@ static void openpic_cpu_write_internal(void *opaque, hwaddr addr,
         s_IRQ = IRQ_get_next(opp, &dst->servicing);
         /* Check queued interrupts. */
         n_IRQ = IRQ_get_next(opp, &dst->raised);
-        src = &opp->src[n_IRQ];
-        if (n_IRQ != -1 &&
-            (s_IRQ == -1 ||
-             IVPR_PRIORITY(src->ivpr) > dst->servicing.priority)) {
-            DPRINTF("Raise OpenPIC INT output cpu %d irq %d",
-                    idx, n_IRQ);
-            qemu_irq_raise(opp->dst[idx].irqs[OPENPIC_OUTPUT_INT]);
+        if (n_IRQ != -1) {
+            src = &opp->src[n_IRQ];
+            if (s_IRQ == -1 ||
+                IVPR_PRIORITY(src->ivpr) > dst->servicing.priority) {
+                DPRINTF("Raise OpenPIC INT output cpu %d irq %d",
+                        idx, n_IRQ);
+                qemu_irq_raise(opp->dst[idx].irqs[OPENPIC_OUTPUT_INT]);
+            }
         }
         break;
     default:
-- 
2.34.1