.\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>

.\" and Copyright (C) 1995 Michael Shields <shields@tembel.org>.

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and author of this work.

.\" %%%LICENSE_END

.\"

.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified 1997-05-31 by Andries Brouwer <aeb@cwi.nl>

.\" Modified 2003-08-24 by Andries Brouwer <aeb@cwi.nl>

.\" Modified 2004-08-16 by Andi Kleen <ak@muc.de>

.\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and

.\" a much improved example program.

.\"

.TH MPROTECT 2 2018-02-02 "Linux" "Linux Programmer's Manual"

.SH NAME

mprotect, pkey_mprotect \- set protection on a region of memory

.SH SYNOPSIS

.nf

.B #include <sys/mman.h>

.PP

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

.BI "int pkey_mprotect(void *" addr ", size_t " len ", int " prot ", int " pkey ");

.fi

.SH DESCRIPTION

.BR mprotect ()

changes the access protections for the calling process's memory pages

containing any part of the address range in the

interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1].

.I addr

must be aligned to a page boundary.

.PP

If the calling process tries to access memory in a manner

that violates the protections, then the kernel generates a

.B SIGSEGV

signal for the process.

.PP

.I prot

is a combination of the following access flags:

.B PROT_NONE

or a bitwise-or of the other values in the following list:

.TP 1.1i

.B PROT_NONE

The memory cannot be accessed at all.

.TP

.B PROT_READ

The memory can be read.

.TP

.B PROT_WRITE

The memory can be modified.

.TP

.B PROT_EXEC

The memory can be executed.

.TP

.BR PROT_SEM " (since Linux 2.5.7)"

The memory can be used for atomic operations.

This flag was introduced as part of the

.BR futex (2)

implementation (in order to guarantee the ability to perform atomic

operations required by commands such as

.BR FUTEX_WAIT ),

but is not currently used in on any architecture.

.TP

.BR PROT_SAO " (since Linux 2.6.26)"

.\" commit aba46c5027cb59d98052231b36efcbbde9c77a1d

.\" commit ef3d3246a0d06be622867d21af25f997aeeb105f

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).

.PP

Additionally (since Linux 2.6.0),

.I prot

can have one of the following flags set:

.TP 1.1i

.\" mm/mmap.c:

.\"	vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |

.\"			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

.\" And calc_vm_flag_bits converts only GROWSDOWN/DENYWRITE/LOCKED.

.B 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\(emfor example, HP-PARISC\(emthat

have an upwardly growing stack.)

.\" The VMA is one that was marked with VM_GROWSUP by the kernel

.\" when the stack was created. Note that (unlike VM_GROWSDOWN),

.\" there is no mmap() flag (analogous to MAP_GROWSDOWN) for

.\" creating a VMA that is marked VM_GROWSUP.

.TP

.B 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

.B MAP_GROWSDOWN

flag set).

.PP

Like

.BR mprotect (),

.BR pkey_mprotect ()

changes the protection on the pages specified by

.IR addr

and

.IR len .

The

.I pkey

argument specifies the protection key (see

.BR pkeys (7))

to assign to the memory.

The protection key must be allocated with

.BR pkey_alloc (2)

before it is passed to

.BR pkey_mprotect ().

For an example of the use of this system call, see

.BR pkeys (7).

.SH RETURN VALUE

On success,

.BR mprotect ()

and

.BR pkey_mprotect ()

return zero.

On error, these system calls return \-1, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EACCES

The memory cannot be given the specified access.

This can happen, for example, if you

.BR mmap (2)

a file to which you have read-only access, then ask

.BR mprotect ()

to mark it

.BR PROT_WRITE .

.TP

.B EINVAL

\fIaddr\fP is not a valid pointer,

or not a multiple of the system page size.

.TP

.BR EINVAL

.RB ( pkey_mprotect ())

\fIpkey\fP has not been allocated with

.BR pkey_alloc (2)

.TP

.BR EINVAL

Both

.BR PROT_GROWSUP

and

.BR PROT_GROWSDOWN

were specified in

.IR prot .

.TP

.BR EINVAL

Invalid flags specified in

.IR prot .

.TP

.BR EINVAL

(PowerPC architecture)

.B PROT_SAO

was specified in

.IR prot ,

but SAO hardware feature is not available.

.TP

.B ENOMEM

Internal kernel structures could not be allocated.

.TP

.B ENOMEM

Addresses in the range

.RI [ addr ,

.IR 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

.BR EFAULT

was incorrectly produced for these cases.)

.TP

.B ENOMEM

Changing the protection of a memory region would result in the total number of

mappings with distinct attributes (e.g., read versus read/write protection)

exceeding the allowed maximum.

.\" I.e., the number of VMAs would exceed the 64 kB maximum

(For example, making the protection of a range

.BR PROT_READ

in the middle of a region currently protected as

.BR PROT_READ|PROT_WRITE

would result in three mappings:

two read/write mappings at each end and a read-only mapping in the middle.)

.SH VERSIONS

.BR pkey_mprotect ()

first appeared in Linux 4.9;

library support was added in glibc 2.27.

.SH CONFORMING TO

.BR mprotect ():

POSIX.1-2001, POSIX.1-2008, SVr4.

.\" SVr4 defines an additional error

.\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.

POSIX says that the behavior of

.BR mprotect ()

is unspecified if it is applied to a region of memory that

was not obtained via

.BR mmap (2).

.PP

.BR pkey_mprotect ()

is a nonportable Linux extension.

.SH NOTES

On Linux, it is always permissible to call

.BR 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.

.PP

Whether

.B PROT_EXEC

has any effect different from

.B PROT_READ

depends on processor architecture, kernel version, and process state.

If

.B READ_IMPLIES_EXEC

is set in the process's personality flags (see

.BR personality (2)),

specifying

.B PROT_READ

will implicitly add

.BR PROT_EXEC.

.PP

On some hardware architectures (e.g., i386),

.B PROT_WRITE

implies

.BR PROT_READ .

.PP

POSIX.1 says that an implementation may permit access

other than that specified in

.IR prot ,

but at a minimum can allow write access only if

.B PROT_WRITE

has been set, and must not allow any access if

.B PROT_NONE

has been set.

.PP

Applications should be careful when mixing use of

.BR mprotect ()

and

.BR pkey_mprotect ().

On x86, when

.BR mprotect ()

is used with

.IR prot

set to

.B PROT_EXEC

a pkey is may be allocated and set on the memory implicitly

by the kernel, but only when the pkey was 0 previously.

.PP

On systems that do not support protection keys in hardware,

.BR pkey_mprotect ()

may still be used, but

.IR pkey

must be set to 0.

When called this way, the operation of

.BR pkey_mprotect ()

is equivalent to

.BR mprotect ().

.SH EXAMPLE

.\" sigaction.2 refers to this example

.PP

The program below demonstrates the use of

.BR 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.

.PP

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

following:

.PP

.in +4n

.EX

.RB "$" " ./a.out"

Start of region:        0x804c000

Got SIGSEGV at address: 0x804e000

.EE

.in

.SS Program source

\&

.EX

#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: 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++) = \(aqa\(aq;

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

    exit(EXIT_SUCCESS);

}

.EE

.SH SEE ALSO

.BR mmap (2),

.BR sysconf (3),

.BR pkeys (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.