mprotect, pkey_mprotect - set protection on a region of memory
Standard C library (
libc,
-lc)
#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.
- EACCES
- 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.
- 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 address space of the
process, or specify one or more pages that are not mapped. (Before Linux
2.4.19, the error EFAULT was incorrectly 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.)
pkey_mprotect() first appeared in Linux 4.9; library support was added in
glibc 2.27.
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.
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().
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
#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)