mprotect(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | 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);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/mman.h>

       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 to indicate
       the error.

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; library support was
       added in glibc 2.27.

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 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().

EXAMPLES         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: %p\n", si->si_addr);
           exit(EXIT_FAILURE);
       }

       int
       main(int argc, char *argv[])
       {
           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);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                    MPROTECT(2)

Pages that refer to this page: clone(2)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)