mprotect

example1：

#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)

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;
    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 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);

    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);
}

  
example2：

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/mman.h>
#include <execinfo.h>


#define PAGE_SIZE    4096 //getpagesize()
//#define GCC_ALIGN


#ifdef GCC_ALIGN
struct t {
    char a;
} __attribute__((aligned(PAGE_SIZE))) ta;
#else
struct t {
    char padding[PAGE_SIZE-1];
    char a;
} ta;
#endif

char *p;


void dump_stack(void)
{
    int j, nptrs;
#define SIZE 100
    void *buffer[SIZE];
    char **strings;

    nptrs = backtrace(buffer, SIZE);
    printf("backtrace() returned %d addresses\n", nptrs);

    /* The call backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO)
      would produce similar output to the following: */

    strings = backtrace_symbols(buffer, nptrs);
    if (strings == NULL) {
       perror("backtrace_symbols");
     exit(EXIT_FAILURE);
    }

    for (j = 0; j < nptrs; j++)
       printf("%s\n", strings[j]);

    free(strings);
}

void segv_fault(int sig)
{
    printf("memory access!\n");
    dump_stack();
    mprotect(p, PAGE_SIZE, PROT_READ|PROT_WRITE);
}

main()
{
    signal(SIGSEGV, segv_fault);

#ifdef GCC_ALIGN
    p = (char *)&(ta.a);
#else
    p = (char *)((unsigned long)&(ta.a) & ~(PAGE_SIZE - 1));
#endif

    printf("p@%p\n", p);

    if (mprotect(p, PAGE_SIZE, PROT_READ))
    {
        printf("mprotect error!\n");
    }

    printf("write buffer\n");
    ta.a = 1;
}