mprotect(2) System Calls Manual mprotect(2)
NAME
mprotect, pkey_mprotect - set protection on a region of memory
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
mprotect() changes the access protections for the calling process's mem-
ory 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 bit-
wise 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 in-
troduced 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 archi-
tecture specification adds the SAO CPU feature, and it is avail-
able 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 archi-
tectures—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).
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
EACCES The memory cannot be given the specified access. This can hap-
pen, 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.
EINVAL (pkey_mprotect()) pkey has not been allocated with pkey_alloc(2)
EINVAL Both PROT_GROWSUP and PROT_GROWSDOWN were specified in prot.
EINVAL Invalid flags specified in prot.
EINVAL (PowerPC architecture) PROT_SAO was specified in prot, but SAO
hardware feature is not available.
ENOMEM Internal kernel structures could not be allocated.
ENOMEM Addresses in the range [addr, addr+len-1] are invalid for the ad-
dress space of the process, or specify one or more pages that are
not mapped. (Before Linux 2.4.19, the error EFAULT was incor-
rectly produced for these cases.)
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.
(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.)
VERSIONS
POSIX says that the behavior of mprotect() is unspecified if it is ap-
plied 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_IM-
PLIES_EXEC is set in the process's personality flags (see personal-
ity(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_mpro-
tect() may still be used, but pkey must be set to -1. When called this
way, the operation of pkey_mprotect() is equivalent to mprotect().
STANDARDS
mprotect()
POSIX.1-2008.
pkey_mprotect()
Linux.
HISTORY
mprotect()
POSIX.1-2001, SVr4.
pkey_mprotect()
Linux 4.9, glibc 2.27.
NOTES
EXAMPLES
The program below demonstrates the use of mprotect(). The program allo-
cates 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 follow-
ing:
$ ./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);
}
SEE ALSO
mmap(2), sysconf(3), pkeys(7)
Linux man-pages 6.9.1 2024-06-15 mprotect(2)
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