DESCRIPTION

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:

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

RETURN VALUE

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

ERRORS

VERSIONS

pkey_mprotect() first appeared in Linux 4.9; library support was added in glibc 2.27.

CONFORMING TO

mprotect(): POSIX.1-2001, POSIX.1-2008, SVr4. 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).

pkey_mprotect() is a nonportable Linux extension.

NOTES

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 depends on processor architecture, kernel version, and process state. If READ_IMPLIES_EXEC is set in the process's personality flags (see personality(2)), specifying PROT_READ will implicitly add PROT_EXEC.

On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ.

POSIX.1 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.

Applications should be careful when mixing use of mprotect() and pkey_mprotect(). On x86, when mprotect() is used with prot set to PROT_EXEC a pkey may be allocated and set on the memory implicitly by the kernel, but only when the pkey was 0 previously.

On systems that do not support protection keys in hardware, pkey_mprotect() may still be used, but pkey must be set to −1. When called this way, the operation of pkey_mprotect() is equivalent to mprotect().

EXAMPLES

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:

#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)
{
    /* 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(int argc, char *argv[])
{
    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);
}

SEE ALSO

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