[Qemu-devel] [PATCH] user-exec.c: Set is_write correctly in the ARM cpu_signal_handler()

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

In the ARM implementation of cpu_signal_handler(), set is_write
correctly using the FSR value which the kernel passes us in the
error_code field of uc_mcontext. Since the WnR bit of the FSR was
only introduced in ARMv6, this means that v5 cores will continue
to behave as before this patch, but they are not really supported
as hosts for linux-user mode anyway since they do not have the
modern behaviour for unaligned accesses.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Without this linux-user won't work very well. In particular after
fork() bash will segfault, with this in the QEMU_STRACE output
immediately preceding:
 sigreturn(18,4390912,1082130608,0,0,0) = -1 errno=255 (Unknown error 255)
at least for PPC and MIPSEL guests.

 user-exec.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/user-exec.c b/user-exec.c
index 71bd6c5..336ac70 100644
--- a/user-exec.c
+++ b/user-exec.c
@@ -20,6 +20,7 @@
 #include "cpu.h"
 #include "disas/disas.h"
 #include "tcg.h"
+#include "qemu/bitops.h"
 
 #undef EAX
 #undef ECX
@@ -441,8 +442,11 @@ int cpu_signal_handler(int host_signum, void *pinfo,
 #else
     pc = uc->uc_mcontext.arm_pc;
 #endif
-    /* XXX: compute is_write */
-    is_write = 0;
+
+    /* error_code is the FSR value, in which bit 11 is WnR (assuming a v6 or
+     * later processor; on v5 we will always report this as a read).
+     */
+    is_write = extract32(uc->uc_mcontext.error_code, 11, 1);
     return handle_cpu_signal(pc, (unsigned long)info->si_addr,
                              is_write,
                              &uc->uc_sigmask, puc);
-- 
1.7.9.5