[PULL 6/8] linux-user/s390x: Fix unwinding from signal handlers

[Laurent Vivier <laurent@vivier.eu>]

From: Ilya Leoshkevich <iii@linux.ibm.com>

Commit 31330e6cecfd ("linux-user/s390x: Implement setup_sigtramp")
removed an unused field from rt_sigframe, disturbing offsets of other
fields and breaking unwinding from signal handlers (e.g. libgcc's
s390_fallback_frame() relies on this struct having a specific layout).
Restore the field and add a comment.

Reported-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Fixes: 31330e6cecfd ("linux-user/s390x: Implement setup_sigtramp")
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220503225157.1696774-2-iii@linux.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
---
 linux-user/s390x/signal.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/linux-user/s390x/signal.c b/linux-user/s390x/signal.c
index f47713e04a07..4979c4b01791 100644
--- a/linux-user/s390x/signal.c
+++ b/linux-user/s390x/signal.c
@@ -84,6 +84,11 @@ struct target_ucontext {
 
 typedef struct {
     uint8_t callee_used_stack[__SIGNAL_FRAMESIZE];
+    /*
+     * This field is no longer initialized by the kernel, but it's still a part
+     * of the ABI.
+     */
+    uint16_t svc_insn;
     struct target_siginfo info;
     struct target_ucontext uc;
 } rt_sigframe;
-- 
2.36.1