mprotect

Sommaire

NAME

mprotect - set protection on a region of memory

SYNOPSIS


#include <sys/mman.h>



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

DESCRIPTION

mprotect() changes protection for the calling process's memory page(s) containing any part of the address range in the interval [addr, addr+len-1]. addr must be aligned to a page boundary.

If the calling process tries to access memory in a manner that violates the protection, then the kernel generates a SIGSEGV signal for the process.

prot is either PROT_NONE or a bitwise-or of the other values in the following list:

PROT_NONE
The memory cannot be accessed at all.
PROT_READ
The memory can be read.
PROT_WRITE
The memory can be modified.
PROT_EXEC
The memory can be executed.

RETURN VALUE

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

ERRORS

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 the system page size.
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.

CONFORMING TO

SVr4, POSIX.1-2001. POSIX says that the behavior of mprotect() is unspecified if it is applied to a region of memory that was not obtained via mmap(2).

NOTES

On Linux it is always legal to call mprotect() on any address in a process's 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. On some hardware architectures (e.g., x86), PROT_WRITE implies PROT_READ.

POSIX.1-2001 says that an implementation may permit access other than that specified in prot, but at a minimum can only allow write access if PROT_WRITE has been set, and must not allow any access if PROT_NONE has been set.

EXAMPLE

The program below allocates four pages of memory, makes the third of these pages read-only, and then executes a loop that walks upwards through the allocated region modifying bytes.

An example of what we might see when running the program is the following:


$ ./a.out

Start of region:        0x804c000

Got SIGSEGV at address: 0x804e000





#include <unistd.h>

#include <signal.h>

#include <stdio.h>

#include <malloc.h>

#include <stdlib.h>

#include <errno.h>

#include <sys/mman.h>



#define die(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)

        die("sigaction");



    pagesize = sysconf(_SC_PAGE_SIZE);

    if (pagesize == -1)

        die("sysconf");



    /* Allocate a buffer aligned on a page boundary;

       initial protection is PROT_READ | PROT_WRITE */



    buffer = memalign(pagesize, 4 * pagesize);

    if (buffer == NULL)

        die("memalign");



    printf("Start of region:        0x%lx\n", (long) buffer);



    if (mprotect(buffer + pagesize * 2, pagesize,

                PROT_NONE) == -1)

        die("mprotect");



    for (p = buffer ; ; )

        *(p++) = 'a';



    printf("Loop completed\n");     /* Should never happen */

    exit(EXIT_SUCCESS);

}

SEE ALSO

mmap(2), sysconf(3)