探测指针地址是否有效

#define _GNU_SOURCE
#include <stdint.h>
#include <signal.h>
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>

#ifdef __i386__
typedef uint32_t word_t;
#define IP_REG REG_EIP
#define IP_REG_SKIP 3
#define READ_CODE __asm__ __volatile__(".byte 0x8b, 0x03\n"  /* mov (%ebx), %eax */ \
                                       ".byte 0x41\n"        /* inc %ecx */ \
                                       : "=a"(ret), "=c"(tmp) : "b"(addr), "c"(tmp));
#endif

#ifdef __x86_64__
typedef uint64_t word_t;
#define IP_REG REG_RIP
#define IP_REG_SKIP 6
#define READ_CODE __asm__ __volatile__(".byte 0x48, 0x8b, 0x03\n"  /* mov (%rbx), %rax */ \
                                       ".byte 0x48, 0xff, 0xc1\n"  /* inc %rcx */ \
                                       : "=a"(ret), "=c"(tmp) : "b"(addr), "c"(tmp));
#endif

static void segv_action(int sig, siginfo_t *info, void *ucontext) {
    (void) sig;
    (void) info;
    ucontext_t *uctx = (ucontext_t*) ucontext;
    uctx->uc_mcontext.gregs[IP_REG] += IP_REG_SKIP;
}

struct sigaction peek_sigaction = {
    .sa_sigaction = segv_action,
    .sa_flags = SA_SIGINFO,
    .sa_mask = 0,
};

word_t peek(word_t *addr, int *success) {
    word_t ret;
    int tmp, res;
    struct sigaction prev_act;

    res = sigaction(SIGSEGV, &peek_sigaction, &prev_act);
    assert(res == 0);

    tmp = 0;
    READ_CODE

    res = sigaction(SIGSEGV, &prev_act, NULL);
    assert(res == 0);

    if (success) {
        *success = tmp;
    }

    return ret;
}

int main() {
    int success;
    word_t number = 22;
    word_t value;

    number = 22;
    value = peek(&number, &success);
    printf("%d %d\n", success, value);

    value = peek(NULL, &success);
    printf("%d %d\n", success, value);

    value = peek((word_t*)0x1234, &success);
    printf("%d %d\n", success, value);

    return 0;
}