[MIPS] Leave room for frame header on stack for MIPS arch

src/trusted/service_runtime/posix/nacl_signal.c

Index: src/trusted/service_runtime/posix/nacl_signal.c
diff --git a/src/trusted/service_runtime/posix/nacl_signal.c b/src/trusted/service_runtime/posix/nacl_signal.c
index 4ce545deaf9afa0778a68d7593158ace3708b712..58abbb1984ee2a7db7217a8164aac740e1e0dc4d 100644
--- a/src/trusted/service_runtime/posix/nacl_signal.c
+++ b/src/trusted/service_runtime/posix/nacl_signal.c
@@ -271,7 +271,8 @@ static int DispatchToUntrustedHandler(struct NaClAppThread *natp,
   regs->return_addr = kReturnAddr;
   regs->a0 = context_user_addr;
   regs->prog_ctr = NaClUserToSys(nap, nap->exception_handler);
-  regs->stack_ptr = NaClUserToSys(nap, new_stack_ptr);
+  regs->stack_ptr = NaClUserToSys(nap, new_stack_ptr) - NACL_STACK_ARG_SIZE;

[Mark Seaborn, 2013/03/06 00:26:58]

The adjustment should be applied before NaClUserToSys() for more safety.

I think it would be cleaner to add this adjustment via an extra field in
NaClExceptionFrame in service_runtime/nacl_exception.h.

[petarj, 2013/03/07 14:52:48]

Done, I added the adjustment just after the alignment adjustment.
Extra field in NaClExceptionFrame would not be transparent enough, and we could
not apply the same kind of adjustment for NaClSysThreadCreate.

+  regs->t9 = regs->prog_ctr;

[Mark Seaborn, 2013/03/06 00:26:58]

Can you add a comment?  e.g. "PIC functions assume that r9 contains the
function's address on entry"

[petarj, 2013/03/07 14:52:48]

Done.

 #else
 # error Unsupported architecture
 #endif