A note about mprotect (2)

Overview

Note that I didn't really understand the regular mprotect as a result of strace

What is mprotect (2)?

mprotect (2) is a system call that controls memory area permissions.

#include <sys/mman.h>

int mprotect(const void *addr, size_t len, int prot);

Pass the pointer, size, and bitwise OR.

flag

Currently, protection bits that can be combined with the following ORs

Flag name	Overview
PROT_NONE	No protection at all
PROT_READ	Page is readable
PROT_WRITE	Page is writable
PROT_EXEC	Page is executable

error

If successful, mprotect () returns 0. In case of an error, -1 is returned and errno is set appropriately.

errno	Overview
EACCES	Unable to set specified access to memory
EINVAL	addr is not a valid pointer or is not a multiple of the system page size
ENOMEM	Could not allocate structure inside kernel
ENOMEM	[addr, addr+len-1]The address in the range is invalid as the process's address space, or the address in the range points to one or more pages that are not mapped.

Example of use

A sample program that generates SIGSEGV by writing to the memory in the READ_ONLY area.

SIGSEGV itself executes the processing handled using sigaction.

`mprotect.c`


#include <unistd.h>
#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)

static char *buffer;

static void handler(int sig, siginfo_t *si, void *unused)
{
    printf("Got SIGSEGV at address: 0x%lx\n",
        (long) si->si_addr);
    exit(EXIT_FAILURE);
}

int main(int argc, char **argv)
{
    char *p;
    int pagesize;
    

    //Changes in signal behavior
    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 = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1)
        handle_error("sysconf");

    //Allocate aligned memory
    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL)
        handle_error("memalign");

    printf("Start of region:        0x%lx\n", (long) buffer);

    //Control memory area permissions
    if (mprotect(buffer + pagesize * 2, pagesize, PROT_READ) == -1)
        handle_error("mprotect");

    for (p = buffer ; ; )
        *(p++) = 'a';

    printf("Loop completed\n");     /* Should never happen */
    exit(EXIT_SUCCESS);
}