1
2#include <stdio.h>
3
4
5#define COMPILER_BARRIER  __asm__ __volatile("":::"cc","memory")
6
7/* force the kernel to map in 15k below SP, so we can safely poke
8   around there. */
9__attribute__((noinline)) void make_below_sp_safe ( void )
10{
11   const int N = 15000;
12   unsigned char a[N];
13   int i;
14
15   for (i = 0; i < N; i++) {
16      a[i] = i & 0xFF;
17   }
18
19   COMPILER_BARRIER;
20
21   unsigned int r = 0;
22   for (i = 0; i < N; i++) {
23      r = (r << 1) | (r >> 31);
24      r ^= (unsigned int)a[i];
25   }
26   fprintf(stderr, "Checksum: %08x\n", r);
27}
28
29
30int main ( void )
31{
32   make_below_sp_safe();
33
34   unsigned int res;
35   __asm__ __volatile__("movl -8192(%%rsp), %0"
36                        : "=r"(res) : : "memory","cc");
37   fprintf(stderr, "Got %08x\n", res);
38   return 0;
39}
40