[PATCH] user-exec: Do not filter the signal on si_code

[Richard Henderson]

This is a workaround for a ppc64le host kernel bug.

For the test case linux-test, we have an instruction trace

IN: sig_alarm
...

IN:
0x400080ed28:  380000ac  li       r0, 0xac
0x400080ed2c:  44000002  sc

IN: __libc_nanosleep
0x1003bb4c:  7c0802a6  mflr     r0
0x1003bb50:  f8010010  std      r0, 0x10(r1)

Our signal return trampoline has, rightly, changed the guest
stack page read-only.  Which, rightly, faults on the store of
a return address into a stack frame.

Checking the host /proc/pid/maps, we see the expected state:

4000800000-4000810000 r--p 00000000 00:00 0

However, the host kernel has supplied si_code == SEGV_MAPERR,
which is obviously incorrect.

By dropping this check, we may have an extra walk of the page
tables, but this should be inexpensive.

Signed-off-by: Richard Henderson <address@hidden>
---

FWIW, filed as

  https://bugzilla.redhat.com/show_bug.cgi?id=1757189

out of habit and then

  https://bugs.centos.org/view.php?id=16499

when I remembered that the system is running Centos not RHEL.

---
 accel/tcg/user-exec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c
index 71c4bf6477..31ef091a70 100644
--- a/accel/tcg/user-exec.c
+++ b/accel/tcg/user-exec.c
@@ -143,9 +143,12 @@ static inline int handle_cpu_signal(uintptr_t pc, 
siginfo_t *info,
      * for some other kind of fault that should really be passed to the
      * guest, we'd end up in an infinite loop of retrying the faulting
      * access.
+     *
+     * XXX: At least one host kernel, ppc64le w/Centos 7 4.14.0-115.6.1,
+     * incorrectly reports SEGV_MAPERR for a STDX write to a read-only page.
+     * Therefore, do not test info->si_code.
      */
-    if (is_write && info->si_signo == SIGSEGV && info->si_code == SEGV_ACCERR 
&&
-        h2g_valid(address)) {
+    if (is_write && info->si_signo == SIGSEGV && h2g_valid(address)) {
         switch (page_unprotect(h2g(address), pc)) {
         case 0:
             /* Fault not caused by a page marked unwritable to protect
-- 
2.17.1