[feffe]

In my experience debuggers handles this fine. Some archs also has a link register (jump and link) which may help finding back. This test is from x86-64 Linux.

  /*
  gcc -g -Wall -o x x.c
  gdb ./x
  (gdb) r
  (gdb) bt
  #0  0x0000000000000000 in ?? ()
  #1  0x0000555555554617 in foo () at x.c:6
  #2  0x0000555555554628 in main () at x.c:10
  (gdb) f 1
  #1  0x0000555555554617 in foo () at x.c:6
  6           bar();
  (gdb) p bar
  $1 = (void (*)(void)) 0x0
  */
  
  #include <stddef.h>
  
  void (*bar)(void) = NULL;
  
  void foo() {
      bar();
  }
  
  int main() {
      foo();
  }

[mark_undoio]

It gets worse if you have a stack smash, e.g:

  #include <string.h>
  
  void foo() {
      int array[1];
      memset(array, 0, 100); /\* Oh no, trash the stack! */
  }
  
  int main() {
      foo();
  }

This gets us:

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000000000000 in ?? ()
  (gdb) bt
  #0  0x0000000000000000 in ?? ()
  #1  0x0000000000000000 in ?? ()

But with a time travel debugger (I'm using UDB because - disclaimer - it's what I work on. `rr` would work just as well):

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000000000000 in ?? ()
  recording 10,617> backtrace
  #0  0x0000000000000000 in ?? ()
  #1  0x0000000000000000 in ?? ()

^ Because we returned to NULL we have segfaulted but the stack is also trashed due to the memset. We don't know how we got here.

  recording 10,617> reverse-stepi
  0x0000000000401146      6       }
  99% 10,616> bt
  #0  0x0000000000401146 in foo () at smash.c:6
  #1  0x0000000000000000 in ?? ()

^ We've stepped back before the return, so we can now see how we got to NULL. Still incomplete stack because it's still trashed.

  99% 10,616> reverse-step
  5           memset(array, 0, 100); /* Oh no, trash the stack! \*/
  99% 10,586> bt
  #0  foo () at smash.c:5
  #1  0x0000000000401155 in main () at smash.c:9
  99% 10,586> 

^ We've gone back before the stack smash happened, so now we get the full backtrace.

[feffe]

ASAN is pretty great with these cases as well. Spent much time looking for the needle in the haystack before ASAN came along.