mprotect — set protection on a region of memory
#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_NONEThe memory cannot be accessed at all.
PROT_READThe memory can be read.
PROT_WRITEThe memory can be modified.
PROT_EXECThe memory can be executed.
On success, mprotect()
returns zero. On error, −1 is returned, and
errno is set appropriately.
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.
addr is not
a valid pointer, or not a multiple of the system page
size.
Internal kernel structures could not be allocated.
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.)
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. (For example, making the
protection of a range PROT_READ in the middle of a region
currently protected as PROT_READ|PROT_WRITE
would result in three mappings: two read/write mappings
at each end and a read-only mapping in the middle.)
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).
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.
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
#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);
}
This page is part of release 4.07 of the Linux 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/.
|
Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com> and Copyright (C) 1995 Michael Shields <shieldstembel.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 <esrthyrsus.com> Modified 1997-05-31 by Andries Brouwer <aebcwi.nl> Modified 2003-08-24 by Andries Brouwer <aebcwi.nl> Modified 2004-08-16 by Andi Kleen <akmuc.de> 2007-06-02, mtk: Fairly substantial rewrites and additions, and a much improved example program. FIXME The following protection flags need documenting: PROT_SEM PROT_GROWSDOWN PROT_GROWSUP PROT_SAO (PowerPC) |