Write a signal handler to catch SIGSEGV
- by Adi
Hi all,
I want to write a signal handler to catch SIGSEGV.
First , I would protect a block of memory for read or writes using
char *buffer;
char *p;
char a;
int pagesize = 4096;
" mprotect(buffer,pagesize,PROT_NONE) "
What this will do is , it will protect the memory starting from buffer till pagesize for any reads or writes.
Second , I will try to read the memory by doing something like
p = buffer;
a = *p
This will generate a SIGSEGV and i have initialized a handler for this. The handler will be called .
So far so good. Now the problem I am facing is , once the handler is called, I want to change the access write of the memory by doing
mprotect(buffer, pagesize,PROT_READ);
and continue my normal functioning of the code. I do not want to exit the function.
On future writes to the same memory, I want again catch the signal and modify the write rights and then take account of that event.
Here is the code I am trying :
#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);
}
The problem I am facing with this is ,only the signal handler is running and I am not able to return to the main function after catching the signal..
Any help in this will be greatly appreciated.
Thanks in advance
Aditya
