mprotect (2)

#include <sys/mman.h>

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

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.

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

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.

EINVAL

addr is not a valid pointer, or not a multiple of the system page size.

ENOMEM

Internal kernel structures could not be allocated.

ENOMEM

Addresses in the range [addr, addr+len-1] are invalid for the address space of the process, or specify one or more pages that are not mapped. (Before kernel 2.4.19, the error EFAULT was incorrectly produced for these cases.)

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).

On Linux it is always permissible 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., i386), 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 allow write access only if PROT_WRITE has been set, and must not allow any access if PROT_NONE has been set.

The program below allocates four pages of memory, makes the third of these pages read-only, and then executes a loop that walks upward 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

Program source

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

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

static 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)
        handle_error("sigaction");

    pagesize = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1)
        handle_error("sysconf");

    /* Allocate a buffer aligned on a page boundary;
       initial protection is PROT_READ | PROT_WRITE */

    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL)
        handle_error("memalign");

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

    if (mprotect(buffer + pagesize * 2, pagesize,
                PROT_READ) == -1)
        handle_error("mprotect");

    for (p = buffer ; ; )
        *(p++) = 'a';

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

mmap(2), sysconf(3)

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