我想编写一个信号处理程序来捕获 SIGSEGV。我保护一块内存以供读取或写入使用
char *buffer;
char *p;
char a;
int pagesize = 4096;
mprotect(buffer,pagesize,PROT_NONE)
这可以保护从缓冲区开始的页面大小字节的内存免受任何读取或写入。
其次，我尝试读取内存：
p = buffer;
a = *p
这将生成一个 SIGSEGV，并且我的处理程序将被调用。到现在为止还挺好。我的问题是，一旦调用了处理程序，我想通过执行更改内存的访问写入
mprotect(buffer,pagesize,PROT_READ);
并继续我的代码的正常运行。我不想退出该功能。在将来写入同一内存时，我想再次捕获信号并修改写权限，然后记录该事件。
这是代码：
#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
char *buffer;
int flag=0;
static void handler(int sig, siginfo_t *si, void *unused)
{
printf("Got SIGSEGV at address: 0x%lx\n",(long) si->si_addr);
printf("Implements the handler only\n");
flag=1;
//exit(EXIT_FAILURE);
}
int main(int argc, char *argv[])
{
char *p; char a;
int pagesize;
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sa.sa_sigaction = handler;
if (sigaction(SIGSEGV, &sa, NULL) == -1)
handle_error("sigaction");
pagesize=4096;
/* Allocate a buffer aligned on a page boundary;
initial protection is PROT_READ | PROT_WRITE */
buffer = memalign(pagesize, 4 * pagesize);
if (buffer == NULL)
handle_error("memalign");
printf("Start of region: 0x%lx\n", (long) buffer);
printf("Start of region: 0x%lx\n", (long) buffer+pagesize);
printf("Start of region: 0x%lx\n", (long) buffer+2*pagesize);
printf("Start of region: 0x%lx\n", (long) buffer+3*pagesize);
//if (mprotect(buffer + pagesize * 0, pagesize,PROT_NONE) == -1)
if (mprotect(buffer + pagesize * 0, pagesize,PROT_NONE) == -1)
handle_error("mprotect");
//for (p = buffer ; ; )
if(flag==0)
{
p = buffer+pagesize/2;
printf("It comes here before reading memory\n");
a = *p; //trying to read the memory
printf("It comes here after reading memory\n");
}
else
{
if (mprotect(buffer + pagesize * 0, pagesize,PROT_READ) == -1)
handle_error("mprotect");
a = *p;
printf("Now i can read the memory\n");
}
/* for (p = buffer;p<=buffer+4*pagesize ;p++ )
{
//a = *(p);
*(p) = 'a';
printf("Writing at address %p\n",p);
}*/
printf("Loop completed\n"); /* Should never happen */
exit(EXIT_SUCCESS);
}
问题是只有信号处理程序运行，捕获信号后我无法返回主函数。