I want to write a signal handler to catch SIGSEGV. I protect a block of memory for read or write using
char *buffer;
char *p;
char a;
int pagesize = 4096;
mprotect(buffer,pagesize,PROT_NONE)
This protects pagesize bytes of memory starting at buffer against any reads or writes.
Second, I try to read the memory:
p = buffer;
a = *p
This will generate a SIGSEGV, and my handler will be called. So far so good. My problem is that, once the handler is called, I want to change the access write of the memory by doing
mprotect(buffer,pagesize,PROT_READ);
and continue normal functioning of my code. I do not want to exit the function. On future writes to the same memory, I want to catch the signal again and modify the write rights and then record that event.
Here is the code:
#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 is that only the signal handler runs and I can't return to the main function after catching the signal.
gcc, at a minimum use:-Wall -Wextra -pedanticI also use:-Wconversion -std=gnu99) The compiler will tell you: 1) parameterargcunused 2) parameterargvunused (suggest using main() signature of:int main( void )3) local variablepused in theelsecode block without being initialized. 4) parameterunusedunused, suggest: add statement:(void)unused;as first line in that function. 5) local variableaset but not used. – Linguiformprintf()in a signal handler! The functionwrite()would be ok to use, but best to not do any I/O in a signal handler, just set a flag and let the main line of code be checking that flag – Linguiformpagesizeis declared as anint, but it should be declared as asize_t– Linguiformsigparameter should be compared to SIGSEGV, as there are other signals, and such a comparison would removed the compiler message about a unusedsigparameter – Linguiformmemalign()is obsolete. the code should be using:posix_memalign()which also requires the definition of_POSIX_C_SOURCEwith a value greater/equal to 200112L – Linguiformgetpagesize()rather than hardcoding the value4096. – Linguiform