CODEWITHSUNDEEP
cmultithreadingmprotect
Chaoran
Chaoran

Reputation: 321

Calling mprotect on the main thread's stack always fails

I attempt to call mprotect on the main thread's stack, but it always fails. Every time I try, it yields an ENOMEM error. And I have checked in /proc/[pid]/maps the whole stack is mapped in the address space. However, I can successfully call mprotect on a child thread's stack from the main stack. There must be something special about the main thread's stack that prevents itself from being changed by mprotect. But I can't find any document for this. Anyone has an idea?

#define _GNU_SOURCE
#include <unistd.h>
#include <sys/mman.h>
#include <assert.h>
#include <pthread.h>
#include <stdio.h>

void * addr;
size_t size;

void * thread(void * tls) {
    sleep(1);
    if (mprotect(addr, size, PROT_NONE) == -1) perror(NULL);
    return NULL;
}

int main(int argc, const char *argv[]) {
    pthread_attr_t attr;
    pthread_getattr_np(pthread_self(), &attr);
    pthread_attr_getstack(&attr, &addr, &size);

    pthread_t th;
    pthread_create(&th, NULL, thread, NULL);
    sleep(2);
    pthread_join(th, NULL);
    return 0;
}

Upvotes: 2

Views: 413

Answers (2)

Sigcont
Sigcont

Reputation: 715

I have used this code to protect meory. It's working fine with your code. Call the method like this->

39     pthread_attr_getstack(&attr, &addr, &size);
40     __enable_execute_stack(&addr);

and compile : gcc your_code.c __enable_execute_stack_code.h -lpthread -fbuilding-libgcc

Upvotes: 0

Sigcont
Sigcont

Reputation: 715

You trying to mprotect the pages which are not mapped. If you check the source of pthread_attr_getstack, you will find that there is no mmap.Stack pages are mapped on demand. this might help you

Upvotes: 3

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