From 5e9c4b24de8716f6286fab4336faadd831b035e7 Mon Sep 17 00:00:00 2001
From: Mark Seaborn <mrs@mythic-beasts.com>
Date: Thu, 21 Jan 2010 14:56:44 +0000
Subject: [PATCH] Debugging: Add code to print backtrace for guest on SIGSEGV

---
 service_runtime/sel_main.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/service_runtime/sel_main.c b/service_runtime/sel_main.c
index 033a69d5..873f0181 100644
--- a/service_runtime/sel_main.c
+++ b/service_runtime/sel_main.c
@@ -35,6 +35,7 @@
 #include "native_client/include/portability.h"
 
 #include <fcntl.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -171,6 +172,71 @@ static void StopForDebuggerInit (const struct NaClApp *state)
 #endif
 }
 
+static void trace_stack(struct sigcontext_struct *regs)
+{
+  fprintf(stderr, "  top_of_stack, code_addr=%lx\n", regs->eip);
+  int frame = regs->ebp;
+  int base = 0x10000000; /* TODO: don't hard code this */
+  while(frame) {
+    int ret_addr = *(int *) (base + frame + 4);
+    int next_frame = *(int *) (base + frame);
+    fprintf(stderr, "  stack_frame=%x, code_addr=%x\n", frame, ret_addr);
+    frame = next_frame;
+  }
+}
+
+static void print_regs(struct sigcontext_struct *regs)
+{
+#define PRINT_REG(name) \
+    fprintf(stderr, "  " #name "=0x%lx (%li)\n", regs->name, regs->name);
+  PRINT_REG(eip);
+  PRINT_REG(eax);
+  PRINT_REG(ebx);
+  PRINT_REG(ecx);
+  PRINT_REG(edx);
+#undef PRINT_REG
+}
+
+static int orig_gs;
+
+static void handle_signal(int arg)
+{
+  struct sigcontext_struct *regs = (void *) (&arg + 1);
+  /* Need to restore %gs even if we don't use TLS variables.
+     Stack protector value is fetched from %gs. */
+  __asm__("mov %0, %%gs" : : "r" (orig_gs));
+  /* Disable this handler. */
+  signal(SIGSEGV, SIG_DFL);
+  fprintf(stderr, "Caught signal\n");
+  print_regs(regs);
+  trace_stack(regs);
+  fprintf(stderr, "Signal handler done, exiting\n");
+  _exit(1);
+}
+
+static void set_up_signal_handler()
+{
+  stack_t st;
+  st.ss_size = 2048;
+  st.ss_sp = malloc(st.ss_size);
+  st.ss_flags = 0;
+  if(sigaltstack(&st, NULL) < 0) {
+    perror("sigaltstack");
+    exit(1);
+  }
+
+  __asm__("mov %%gs, %0" : "=r" (orig_gs));
+
+  struct sigaction act;
+  act.sa_handler = handle_signal;
+  sigemptyset(&act.sa_mask);
+  act.sa_flags = SA_ONSTACK;
+  if(sigaction(SIGSEGV, &act, NULL) < 0) {
+    perror("sigaction");
+    exit(1);
+  }
+}
+
 /*
  * Note that we cannot use the 3 arg declaration for main, since SDL
  * preprocessor defines main to SDLmain and then provides a
@@ -210,6 +276,8 @@ int main(int  ac,
   char                          *env_verbosity;
   struct GioFile                log_gio;
 
+  set_up_signal_handler();
+
   /* do expiration check first */
 #if 0
   if (NaClHasExpired()) {
-- 
2.11.4.GIT