Manpages - mprotect.2

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
EXAMPLES
- Program source
SEE ALSO
COLOPHON

NAME

mprotect, pkey_mprotect - set protection on a region of memory

SYNOPSIS

  #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

*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

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

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.
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

*pkey_mprotect*() first appeared in Linux 4.9; library support was added in glibc 2.27.

CONFORMING TO

*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

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

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);
  }

COLOPHON

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