Elliott Hughes | a0664b9 | 2017-04-18 17:46:52 -0700 | [diff] [blame^] | 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 | |
| 30 | int 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 | } |