mprotect() - Unix, Linux System Call

NAME

mprotect - control allowable accesses to a region of memory

SYNOPSIS

#include <sys/mman.h> 

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

DESCRIPTION

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:

Tag	Description
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.

RETURN VALUE

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

ERRORS

Tag	Description
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.
EFAULT	The memory cannot be accessed.
EINVAL	addr is not a valid pointer, or not a multiple of PAGESIZE.
ENOMEM	Internal kernel structures could not be allocated. Or: addresses in the range [addr, addr+len] are invalid for the address space of the process, or specify one or more pages that are not mapped.

EXAMPLE

#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);
}

CONFORMING TO

SVr4, POSIX.1-2001. POSIX says that mprotect() can be used only on regions of memory obtained from mmap(2).

NOTES

On Linux it is always legal to call mprotect() on any address in a process’ address space (except for the kernel vsyscall area). In particular it can be used to change existing code mappings to be writable.

Whether PROT_EXEC has any effect different from PROT_READ is architecture and kernel version dependent.