Why couldn't I intercept segfault signal of a child process? - linux

Question

I'm trying to monitor the child process for segment fault errors, but that doesn't work.
I always receive ABRT signal.
I see gdb can catch segment fault, so what's wrong with my code?
pid_t child;
int wstatus, signum;
struct user_regs_struct regs;
child = fork();
if (child == 0)
{
ptrace(PTRACE_TRACEME, 0, NULL, NULL);
char buf[10];
// make it always crash
strcpy (buf, "aaaaaaabbbbbbbbbbbaaaaaaaaaaaaaaaa");
printf ("Buf is %s\n", buf);
exit(0);
}
while(1)
{
wait(&wstatus);
if (WIFEXITED(wstatus) || WIFSIGNALED(wstatus))
break;
signum = WSTOPSIG(wstatus);
ptrace(PTRACE_GETREGS, child, NULL, &regs);
printf ("signal: %d, eip: 0x%08lx\n", signum, regs.eip);
ptrace(PTRACE_CONT, child, NULL, signum);
}

Answer 1

what's wrong with my code
Your code breaks out of the loop when child is signalled (WIFSIGNALED). Since you are expecting to catch a signal (most likely SIGSEGV), perhaps you shouldn't break out of the loop when the child is signalled?
I've looked at your code some more. It is not clear why your child is crashing at all. Perhaps you are building it with -fstack-protector or some such?
Here is a complete compilable test case (which you should have put into your question), that does crash (note: removed exit from child):
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
int main()
{
pid_t child;
int wstatus, signum;
struct user_regs_struct regs;
child = fork();
if (child == 0)
{
ptrace(PTRACE_TRACEME, 0, NULL, NULL);
char buf[10];
// make it always crash
strcpy (buf, "aaaaaaabbbbbbbbbbbaaaaaaaaaaaaaaaa");
printf ("Buf is %s\n", buf);
}
while(1)
{
wait(&wstatus);
if (WIFEXITED(wstatus))
break;
signum = WSTOPSIG(wstatus);
ptrace(PTRACE_GETREGS, child, NULL, &regs);
printf ("signal: %d, eip: 0x%08lx\n", signum, regs.eip);
ptrace(PTRACE_CONT, child, NULL, signum);
}
return wstatus;
}
And got infinite loop
You normally should get an infinite loop: you are resuming the child, which re-executes its current instruction, which should trigger the exact same signal again.
That's not what is happening with the above program on my system though, and I currently can't explain what I am observing:
$ ./a.out
Buf is aaaaaaabbbbbbbbbbbaaaaaaaaaaaaaaaa
signal: 159, eip: 0x08049ff4
signal: 159, eip: 0x08049ff4
...
signal: 159, eip: 0x08049ff4
*** stack smashing detected ***: ./a.out terminated
signal: 11, eip: 0xf759fb19
signal: 0, eip: 0xf759fb19
signal: 0, eip: 0xf759fb19
...