mprotect, pkey_mprotect - set protection on a region of memory

#include <sys/mman.h>
int mprotect(void addr[.len], size_t len, int prot);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
int pkey_mprotect(void addr[.len], size_t len, int prot, int pkey);

mprotect() changes the access protections for the calling process's memory pages 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 protections, then the kernel generates a SIGSEGV signal for the process.

prot is a combination of the following access flags: 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.

PROT_SEM (since Linux 2.5.7)

The memory can be used for atomic operations. This flag was introduced as part of the futex(2) implementation (in order to guarantee the ability to perform atomic operations required by commands such as FUTEX_WAIT), but is not currently used in on any architecture.

PROT_SAO (since Linux 2.6.26)

The memory should have strong access ordering. This feature is specific to the PowerPC architecture (version 2.06 of the architecture specification adds the SAO CPU feature, and it is available on POWER 7 or PowerPC A2, for example).

Additionally (since Linux 2.6.0), prot can have one of the following flags set:

PROT_GROWSUP

Apply the protection mode up to the end of a mapping that grows upwards. (Such mappings are created for the stack area on architectures—for example, HP-PARISC—that have an upwardly growing stack.)

PROT_GROWSDOWN

Apply the protection mode down to the beginning of a mapping that grows downward (which should be a stack segment or a segment mapped with the MAP_GROWSDOWN flag set).

Like mprotect(), pkey_mprotect() changes the protection on the pages specified by addr and len. The pkey argument specifies the protection key (see pkeys(7)) to assign to the memory. The protection key must be allocated with pkey_alloc(2) before it is passed to pkey_mprotect(). For an example of the use of this system call, see pkeys(7).

On success, mprotect() and pkey_mprotect() return zero. On error, these system calls return -1, and errno is set to indicate the error.

The program below demonstrates the use of mprotect(). The program 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 <malloc.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.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)
{
    /* Note: calling printf() from a signal handler is not safe
       (and should not be done in production programs), since
       printf() is not async-signal-safe; see signal-safety(7).
       Nevertheless, we use printf() here as a simple way of
       showing that the handler was called. */
    printf("Got SIGSEGV at address: %p\n", si->si_addr);
    exit(EXIT_FAILURE);
}
int
main(void)
{
    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:        %p\n", buffer);
    if (mprotect(buffer + pagesize * 2, pagesize,
                 PROT_READ) == -1)
        handle_error("mprotect");
    for (char *p = buffer ; ; )
        *(p++) = 'a';
    printf("Loop completed\n");     /* Should never happen */
    exit(EXIT_SUCCESS);
}

mmap(2), sysconf(3), pkeys(7)