test_segv.c

#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sched.h>

void print_maps(void *addr) {
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "grep %lx /proc/%d/maps", 
             ((unsigned long)addr) & ~0xFFF, getpid());
    system(cmd);
}

int main() {
    size_t page_size = sysconf(_SC_PAGESIZE);
    volatile char *page = mmap(NULL, page_size, PROT_READ | PROT_WRITE, 
                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    
    if (page == MAP_FAILED) {
        perror("mmap"); return 1;
    }
    
    printf("Page mapped at: %p\n", (void*)page);
    memset((char*)page, 'a', page_size);
    
    printf("Before mprotect:\n");
    print_maps((void*)page);
    
    if (mprotect((char*)page, page_size, PROT_READ) == -1) {
        perror("mprotect"); return 1;
    }

    printf("\nAfter mprotect:\n");
    print_maps((void*)page);
    
    printf("Before write: %c\n", page[0]);
    printf("\nAttempting write...\n");
    page[0] = 'x';
    printf("After write: %c\n", page[0]);

    return 0;
}

test_segv_simple.c

#include <sys/mman.h>
#include <stdio.h>


int main() {
    int *ptr = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
                    MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
    *ptr = 42;  // Write should work

    mprotect(ptr, 4096, PROT_READ);  // Make read-only

    fprintf(stderr, "Trying write after mprotect(PROT_READ)...\n");
    *ptr = 99;  // This should SEGFAULT

    fprintf(stderr, "BUG: Write succeeded! *ptr = %d\n", *ptr);
    fprintf(stderr, "\n\n\n\n");
    fflush(stderr);
    return 0;
}