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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  gener-
       ates a SIGSEGV signal for the process.

       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] 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.

       Whether PROT_EXEC has any effect different from PROT_READ is architecture- and  kernel  version-dependent.   On
       some hardware architectures (e.g., i386), 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 following:

           $ ./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_NONE) == -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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-06                       MPROTECT(2)