之前的一篇文章: Linux C/C++記憶體越界定位: 利用mprotect使程式在crash在第一現場。裡面沒有用malloc來申請按頁對齊的記憶體，其實也可以適配malloc，多申請2頁，取中間按照頁對齊的記憶體使用。使用malloc，可以使得程序的地址空間佈局不會相對原來發生太大變化。程式碼如下：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

struct mem_align {
    void *origin_start;  // for free
 

    void *start;         // data addr start, align page size
    void *end;           // data addr end,   align page size
    void *origin_end;
};

int malloc_align_page(size_t memsize, struct mem_align *mem)
{
    if (memsize == 0 || mem == NULL)
        return -1;

    memset(mem, 0, sizeof(*mem));
    long
 
 pagesize = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1) {
        perror("sysconf err");
        return -1;
    }

    size_t datasize = memsize + pagesize * 2;
    mem->origin_start = malloc(datasize);
    if(mem->origin_start == NULL)
        return -1;
    mem->origin_end = mem->origin_start + datasize;

    long
 
 mask = pagesize - 1;
    mem->start = (void *)((long)(mem->origin_start + pagesize) & ~mask);
    long pagenum = memsize / pagesize + 1;
    mem->end = mem->start + pagesize * pagenum;
    return 0;
}

int main()
{
    int ret;
    struct mem_align mem;

    ret = malloc_align_page(1024, &mem);
    if (ret != 0) {
        return ret;
    }

    ret = mprotect(mem.start, (size_t)(mem.end - mem.start), PROT_READ);
    if (ret == -1) {
        perror("mportect");
        return ret;
    }

    sleep(600); // 讓程序先掛起不退出，看/proc/[pid]/maps裡的地址空間對應頁的許可權
    // free(mem.origin_start);
    return 0;
}

一組執行結果如下：

[email protected]:/media/psf/Home/iLearn/learn_c/mprotect# ./a.out
malloc mem: 0xbab010 - 0xbad410
align page: 0xbac000 - 0xbad000

[email protected]:~# ps -ef|grep a.out
root      1429 32072  0 00:24 pts/11   00:00:00 ./a.out
root      1718  1153  0 00:25 pts/17   00:00:00 grep --color=auto a.out
[email protected]:~# grep bac00 /proc/1429/maps -C 3
00400000-00401000 r-xp 00000000 00:18 53150451                           /media/psf/Home/iLearn/learn_c/mprotect/a.out
00600000-00601000 r--p 00000000 00:18 53150451                           /media/psf/Home/iLearn/learn_c/mprotect/a.out
00601000-00602000 rw-p 00001000 00:18 53150451                           /media/psf/Home/iLearn/learn_c/mprotect/a.out
00bab000-00bac000 rw-p 00000000 00:00 0                                  [heap]
00bac000-00bad000 r--p 00000000 00:00 0                                  [heap]  # 這塊對記憶體頁已經被改成了只讀許可權
00bad000-00bce000 rw-p 00000000 00:00 0                                  [heap]
7f657e6b3000-7f657e86f000 r-xp 00000000 08:01 397412                     /lib/x86_64-linux-gnu/libc-2.19.so
7f657e86f000-7f657ea6e000 ---p 001bc000 08:01 397412                     /lib/x86_64-linux-gnu/libc-2.19.so