After stack is compromised, stack unwinding becomes unreliable:

[hjl@gnu-6 tmp]$ cat ssp-1.c
int main (void)
{
  int i = 0;
  char foo[255];

  /* Overflow buffer.  */
  for (i = 0; i < 400; i++)
    foo[i] = 42;

  return 1; /* fail */
}
[hjl@gnu-6 tmp]$ gcc  ssp-1.c -g -fstack-protector 
[hjl@gnu-6 tmp]$ ./a.out 
*** stack smashing detected ***: ./a.out terminated
Segmentation fault
[hjl@gnu-6 tmp]$ gdb a.out 
GNU gdb (GDB) Fedora 8.0-13.0.fc25
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Reading symbols from a.out...done.
(gdb) r
Starting program: /tmp/a.out 
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.24-9.0.fc25.x86_64
*** stack smashing detected ***: /tmp/a.out terminated

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7806c8b in ?? () from /lib64/libgcc_s.so.1
Missing separate debuginfos, use: dnf debuginfo-install libgcc-6.3.1-3.2.fc25.x86_64
(gdb) bt
#0  0x00007ffff7806c8b in ?? () from /lib64/libgcc_s.so.1
#1  0x00007ffff7808628 in _Unwind_Backtrace () from /lib64/libgcc_s.so.1
#2  0x00007ffff7b26b8f in backtrace () from /lib64/libc.so.6
#3  0x00007ffff7a2efb6 in backtrace_and_maps () from /lib64/libc.so.6
#4  0x00007ffff7a886eb in __libc_message () from /lib64/libc.so.6
#5  0x00007ffff7b2a387 in __fortify_fail () from /lib64/libc.so.6
#6  0x00007ffff7b2a350 in __stack_chk_fail () from /lib64/libc.so.6
#7  0x0000000000400592 in main () at ssp-1.c:11
(gdb)