NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

MPROTECT(2)               Linux Programmer's Manual              MPROTECT(2)

NAME         top

       mprotect, pkey_mprotect - set protection on a region of memory

SYNOPSIS         top

       #include <sys/mman.h>

       int mprotect(void *addr, size_t len, int prot);
       int pkey_mprotect(void *addr, size_t len, int prot, int pkey);

DESCRIPTION         top

       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).

RETURN VALUE         top

       On success, mprotect() and pkey_mprotect() return zero.  On error,
       these system calls return -1, and errno is set appropriately.

ERRORS         top

       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 kernel 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.)

VERSIONS         top

       pkey_mprotect() first appeared in Linux 4.9.  Glibc support is not
       yet available.

CONFORMING TO         top

       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.

NOTES         top

       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 is 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 0.  When
       called this way, the operation of pkey_mprotect() is equivalent to
       mprotect().

EXAMPLE         top

       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

   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)

       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: 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         top

       mmap(2), sysconf(3), pkeys(7)

COLOPHON         top

       This page is part of release 4.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2017-09-15                      MPROTECT(2)

Pages that refer to this page: madvise(2)mmap(2)pkey_alloc(2)prctl(2)remap_file_pages(2)seccomp(2)sigaction(2)subpage_prot(2)syscalls(2)pthread_attr_setguardsize(3)pthread_attr_setstack(3)systemd.exec(5)pkeys(7)shm_overview(7)