Unix Signal Handling

Table of Contents

1 Ordinary signal handling

The handling of ordinary signals are easy:

#include <signal.h>
static void my_handler(int signum) {
  printf("received signal\n");
}

int main() {
  struct sigaction sa;
  sa.sa_handler = my_handler;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = SA_SIGINFO;
  // this segv does not work
  sigaction(SIGSEGV, &sa, NULL);
  // this sigint will work
  sigaction(SIGINT, &sa, NULL);
}

2 SIGSEGV handling

2.1 Motivation

The reason that I want to handle the SIGSEGV is that I want to get the coverage from gcov. Gcov will not report any coverage information if the program terminates by receiving some signals. Fortunately we can explicitly ask gcov to dump it by calling __gcov_flush() inside the handler. I confirmed this can work for ordinary signal handling.

// declaring the prototype of gcov
void __gcov_flush(void);

void myhanlder() {
  __gcov_flush();
}

After experiment, I found:

  1. address sanitizer cannot work with this handling. AddressSanitizer will hijact the signal, and maybe output another signal.
  2. Even if I turned off address sanitizer, and the handler function is executed, the coverage information is still not able to get. This possibly because the handler is running on a different stack.

2.2 a new stack

However, handling the SIGSEGV is challenging. The above will not work 1.

By default, when a signal is delivered, its handler is called on the same stack where the program was running. But if the signal is due to stack overflow, then attempting to execute the handler will cause a second segfault. Linux is smart enough not to send this segfault back to the same signal handler, which would prevent an infinite cascade of segfaults. Instead, in effect, the signal handler does not work.

Instead, we need to make a new stack and install the handler on that stack.

#include <signal.h>
void sigsegv_handler(int signum, siginfo_t *info, void *data) {
  printf("Received signal finally\n");
  exit(1);
}

#define SEGV_STACK_SIZE BUFSIZ

int main() {
  struct sigaction action;
  bzero(&action, sizeof(action));
  action.sa_flags = SA_SIGINFO|SA_STACK;
  action.sa_sigaction = &sigsegv_handler;
  sigaction(SIGSEGV, &action, NULL);


  stack_t segv_stack;
  segv_stack.ss_sp = valloc(SEGV_STACK_SIZE);
  segv_stack.ss_flags = 0;
  segv_stack.ss_size = SEGV_STACK_SIZE;
  sigaltstack(&segv_stack, NULL);

  char buf[10];
  char *src = "super long string";
  strcpy(buf, src);
}

2.3 libsigsegv

I also tried another library, the libsigsegv 2. I followed two of their methods, but I cannot make either work. The code lists here as a reference:

#include <signal.h>
#include <sigsegv.h>
int handler (void *fault_address, int serious) {
  printf("Handler triggered.\n");
  return 0;
}
void stackoverflow_handler (int emergency, stackoverflow_context_t scp) {
  printf("Handler received\n");
}
int main() {
  char* mystack; // don't know how to use
  sigsegv_install_handler (&handler);
  stackoverflow_install_handler (&stackoverflow_handler,
                                 mystack, SIGSTKSZ);
}

Footnotes:

Author: Hebi Li

Created: 2017-06-14 Wed 22:42

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