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Manual Reference Pages  - MPROTECT (2)

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mprotect - control allowable accesses to a region of memory

CONTENTS

Synopsis
Description
Errors
Example

#include <sys/mman.h> 


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

The function mprotect specifies the desired protection for the memory page(s) containing part or all of the interval [addr,addr+len-1]. If an access is disallowed by the protection given it, the program receives a SIGSEGV.

prot is a bitwise-or of the following values:

PROT_NONE	The memory cannot be accessed at all.
PROT_READ	The memory can be read.
PROT_WRITE	The memory can be written to.
PROT_EXEC	The memory can contain executing code.

The new protection replaces any existing protection. For example, if the memory had previously been marked PROT_READ, and mprotect is then called with prot PROT_WRITE, it will no longer be readable.

On success, mprotect returns zero. On error, -1 is returned, and errno is set appropriately.

EINVAL	addr is not a valid pointer, or not a multiple of PAGESIZE.
EFAULT	The memory cannot be accessed.
EACCES	The memory cannot be given the specified access. This can happen, for example, if you mmap(2) a file to which you have read-only access, then ask mprotect to mark it PROT_WRITE.
ENOMEM	Internal kernel structures could not be allocated.

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

#include <limits.h>    /* for PAGESIZE */
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif

int
main(void)
{
    char *p;
    char c;

    /* Allocate a buffer; it will have the default
       protection of PROT_READ|PROT_WRITE. */
    p = malloc(1024+PAGESIZE-1);
    if (!p) {
        perror("Couldn’t malloc(1024)");
        exit(errno);
    }

    /* Align to a multiple of PAGESIZE, assumed to be a power of two */
    p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));

    c = p[666];         /* Read; ok */
    p[666] = 42;        /* Write; ok */

    /* Mark the buffer read-only. */
    if (mprotect(p, 1024, PROT_READ)) {
        perror("Couldn’t mprotect");
        exit(errno);
    }

    c = p[666];         /* Read; ok */
    p[666] = 42;        /* Write; program dies on SIGSEGV */

    exit(0);
}

SVr4, POSIX.1b (formerly POSIX.4). SVr4 defines an additional error code EAGAIN. The SVr4 error conditions don’t map neatly onto Linux’s. POSIX.1b says that mprotect can be used only on regions of memory obtained from mmap(2).

mmap(2)

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