Set stack size with setrlimit() and provoke a stack overflow/segfault

Question

In the given example below I try to set the stacksize to 1kb.

Why is it now possible to allocate an array of ints on the stack with size 8kb in foo()

Answer 1

I think setrlimit moves the "resource pointers" but doesn't apply the new limits until you exec a new copy of the program.

#include 
#include 
#include 
#include 

void foo(int chk) {
  unsigned ints[2048];
  ints[2047] = 42;
  printf("foo %d: %u\n", chk, ints[2047]);
}

int main(int argc, char **argv) {
  char *newarg[] = { "argv[0]", "one", "two" };
  char *newenv[] = { NULL };
  struct rlimit lim;

  newarg[0] = argv[0];
  getrlimit(RLIMIT_STACK, &lim);
  printf("lim: %d / %d\n", (int)lim.rlim_cur, (int)lim.rlim_max);
  switch (argc) {
    case 1: /* first call from command line */
      lim.rlim_cur = 65536;
      lim.rlim_max = 65536;
      if (setrlimit(RLIMIT_STACK, &lim) == -1) return EXIT_FAILURE;
      newarg[2] = NULL;
      foo(1);
      execve(argv[0], newarg, newenv);
      break;
    case 2: /* second call */
      lim.rlim_cur = 1024;
      lim.rlim_max = 1024;
      if (setrlimit(RLIMIT_STACK, &lim) == -1) return EXIT_FAILURE;
      foo(2);
      execve(argv[0], newarg, newenv);
      break;
    default: /* third call */
      foo(3);
      break;
  }
  return 0;
}

And a test run:

$ ./a.out 
lim: 8388608 / -1
foo 1: 42
lim: 65536 / 65536
foo 2: 42
Killed

Why the process gets killed before printing the limits (and before calling foo), I don't know.