Valgrind detector

/**
detect valgrind on x86-64 systems
How it works is let as an exercise for the reader ;)

example :
elvanderb@elvanderb:~$ valgrind  --smc-check=all ./valgrind_detect
==18368== Memcheck, a memory error detector
==18368== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==18368== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==18368== Command: ./valgrind_detect
==18368==
Nb bytes written : 00000804
Probably monitored by valgrind
==18368==
==18368== HEAP SUMMARY:
==18368==     in use at exit: 0 bytes in 0 blocks
==18368==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==18368==
==18368== All heap blocks were freed -- no leaks are possible
==18368==
==18368== For counts of detected and suppressed errors, rerun with: -v
==18368== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
elvanderb@elvanderb:~$ ./valgrind_detect
Nb bytes written : 00000811
No valgrind detected
**/

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>

int main()
{
        uint8_t* page;
        uint32_t nb_written = 0xFFFFFFFF;

        page = mmap(NULL, 0x1000, PROT_EXEC | PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (! page)
        {
                fprintf(stderr, "mmap failed\n");
                return 1;
        }
    // 0x803 is in the middle of the cache to ensure that it'll be overwritten by the rep movsb
        page[0x803] = 0xF3; // rep prefix
        page[0x804] = 0xAA; // stosb
        __asm__("movq %1, %%rdx ;"
                "movq  %%rdx, %%rdi ;"
                "addq $0x803, %%rdx ;"
                "xor  %%ecx, %%ecx ;"
                "dec  %%ecx ;"
                "movb $0xC3, %%al ;"
                "callq *%%rdx ;"
                "movl %%ecx, %0 ;"
                : "=r"(nb_written)
                : "r"(page)
                : "%eax", "%edi", "%ecx", "%edx");
        nb_written = 0xFFFFFFFF - nb_written;
        printf("Nb bytes written : %08X\n", nb_written);
        if (nb_written <= 0x804)
        {
                printf("Probably monitored by valgrind\n");
                return 1;
        }
        printf("No valgrind detected\n");
        return 0;
}