MPROTECT(2) | Linux Programmer's Manual | MPROTECT(2) |
NAME
mprotect - set protection on a region of memorySYNOPSIS
#include<sys/mman.h>
int mprotect(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.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 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.
- 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 kernel 2.4.19, the error EFAULT was incorrectly produced for these cases.)
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 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.EXAMPLE
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.$ ./a.out
Start of region: 0x804c000
Got SIGSEGV at address: 0x804e000
Program source
#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)
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);
}
SEE ALSO
mmap(2), sysconf(3)COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.2012-08-14 | Linux |