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mprotect (2)
MPROTECT(2) Linux Programmer's Manual MPROTECT(2)
NAME
mprotect - set protection on a region of memory
SYNOPSIS
#include <sys/mman.h>
int mprotect(const void *addr, size_t len, int prot);
DESCRIPTION
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 pro-
cess.
prot is either 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.
RETURN VALUE
On success, mprotect() returns zero. On error, -1 is returned, and
errno is set appropriately.
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.
EFAULT The memory cannot be accessed.
EINVAL addr is not a valid pointer, or not a multiple of the system
page size.
ENOMEM Internal kernel structures could not be allocated. Or:
addresses in the range [addr, addr+len] are invalid for the
address space of the process, or specify one or more pages that
are not mapped.
CONFORMING TO
SVr4, POSIX.1-2001. 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).
NOTES
On Linux it is always legal to call mprotect() on any address in a pro-
cess's address space (except for the kernel vsyscall area). In partic-
ular it can be used to change existing code mappings to be writable.
Whether PROT_EXEC has any effect different from PROT_READ is architec-
ture and kernel version dependent. On some hardware architectures
(e.g., x86), PROT_WRITE implies PROT_READ.
POSIX.1-2001 says that an implementation may permit access other than
that specified in prot, but at a minimum can only allow write access if
PROT_WRITE has been set, and must not allow any access if PROT_NONE has
been set.
EXAMPLE
The program below allocates four pages of memory, makes the third of
these pages read-only, and then executes a loop that walks upwards
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
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>
#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
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)
die("sigaction");
pagesize = sysconf(_SC_PAGE_SIZE);
if (pagesize == -1)
die("sysconf");
/* Allocate a buffer aligned on a page boundary;
initial protection is PROT_READ | PROT_WRITE */
buffer = memalign(pagesize, 4 * pagesize);
if (buffer == NULL)
die("memalign");
printf("Start of region: 0x%lx\n", (long) buffer);
if (mprotect(buffer + pagesize * 2, pagesize,
PROT_NONE) == -1)
die("mprotect");
for (p = buffer ; ; )
*(p++) = 'a';
printf("Loop completed\n"); /* Should never happen */
exit(EXIT_SUCCESS);
}
SEE ALSO
mmap(2), sysconf(3)
Linux 2007-06-02 MPROTECT(2)
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