C/Linux：不同权限的双映射内存

Question

我的程序将数据指针传递给第三方插件，目的是数据应该是只读的，因此最好防止插件写入数据对象。理想情况下，如果插件尝试写入，则会出现段错误。我听说有一些方法可以双重映射内存区域，以便第二个虚拟地址范围指向相同的物理内存页面。第二个映射没有写权限，导出的指针将使用这个地址范围而不是原来的（可写的）地址范围。我宁愿不更改原始内存分配，无论它们碰巧使用 malloc 还是 mmap 或其他什么。有人能解释一下怎么做吗？

Answer 1

可以获得双重映射，但需要一些工作。

我知道如何创建这种双重映射的唯一方法是使用mmap 函数调用。对于 mmap，您需要某种文件描述符。幸运的是，Linux 允许您获取共享内存对象，因此不需要存储介质上的真实文件。

这是一个完整的示例，展示了如何创建共享内存对象，从中创建读/写和只读指针，然后进行一些基本测试：

#include <stdio.h> 
#include <fcntl.h>          
#include <unistd.h>         
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>        
#include <sys/types.h>

int main()  
{
        // Lets do this demonstration with one megabyte of memory:
        const int len = 1024*1024;

        // create shared memory object:
        int fd = shm_open("/myregion", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
        printf ("file descriptor is %d\n", fd);

        // set the size of the shared memory object:
        if (ftruncate(fd, len) == -1)
        {
            printf ("setting size failed\n");
            return 0;
        }

        // Now get two pointers. One with read-write and one with read-only.
        // These two pointers point to the same physical memory but will
        // have different virtual addresses:

        char * rw_data = mmap(0, len, PROT_READ|PROT_WRITE, MAP_SHARED, fd,0);
        char * ro_data = mmap(0, len, PROT_READ           , MAP_SHARED, fd,0);

        printf ("rw_data is mapped to address %p\n", rw_data);
        printf ("ro_data is mapped to address %p\n", ro_data);

        // ===================
        // Simple test-bench:
        // ===================

        // try writing:
        strcpy (rw_data, "hello world!");

        if (strcmp (rw_data, "hello world!") == 0)
        {
            printf ("writing to rw_data test passed\n");
        } else {
            printf ("writing to rw_data test failed\n");
        }

        // try reading from ro_data
        if (strcmp (ro_data, "hello world!") == 0)
        {
            printf ("reading from ro_data test passed\n");
        } else {
            printf ("reading from ro_data test failed\n");
        }

        printf ("now trying to write to ro_data. This should cause a segmentation fault\n");

        // trigger the segfault
        ro_data[0] = 1;

        // if the process is still alive something didn't worked.
        printf ("writing to ro_data test failed\n");
        return 0;
}

编译：gcc test.c -std=c99 -lrt

由于某种原因，我收到一条警告，指出未声明 ftruncate。不知道为什么。该代码运行良好。示例输出：

file descriptor is 3
rw_data is mapped to address 0x7f1778d60000
ro_data is mapped to address 0x7f1778385000
writing to rw_data test passed
reading from ro_data test passed
now trying to write to ro_data. This should cause a segmentation fault
Segmentation fault

我将内存释放作为练习留给读者:-)