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. */ make_below_sp_safe(void)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 main(void)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