MPROTECT(2)               (2020-11-01)                MPROTECT(2)

          mprotect, pkey_mprotect - set protection on a region of

          #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

          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 follow-
          ing list:

               The memory cannot be accessed at all.

               The memory can be read.

               The memory can be modified.

               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) implementa-
               tion (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 archi-

          PROT_SAO (since Linux 2.6.26)
               The memory should have strong access ordering.  This

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               feature is specific to the PowerPC architecture (ver-
               sion 2.06 of the architecture specification adds the
               SAO CPU feature, and it is available on POWER 7 or Pow-
               erPC A2, for example).

          Additionally (since Linux 2.6.0), prot can have one of the
          following flags set:

               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.)

               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

          Like mprotect(), pkey_mprotect() changes the protection on
          the pages specified by addr and len. The pkey argument spec-
          ifies 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).

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

               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.

               addr is not a valid pointer, or not a multiple of the
               system page size.

               (pkey_mprotect()) pkey has not been allocated with

               Both PROT_GROWSUP and PROT_GROWSDOWN were specified in


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               Invalid flags specified in prot.

               (PowerPC architecture) PROT_SAO was specified in prot,
               but SAO hardware feature is not available.

               Internal kernel structures could not be allocated.

               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

               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.)

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

          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

          pkey_mprotect() is a nonportable Linux extension.

          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 pro-
          cess 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.

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          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().

          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. */

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              printf("Got SIGSEGV at address: %p\n", si->si_addr);

          main(int argc, char *argv[])
              int pagesize;
              struct sigaction sa;

              sa.sa_flags = SA_SIGINFO;
              sa.sa_sigaction = handler;
              if (sigaction(SIGSEGV, &sa, NULL) == -1)

              pagesize = sysconf(_SC_PAGE_SIZE);
              if (pagesize == -1)

              /* Allocate a buffer aligned on a page boundary;
                 initial protection is PROT_READ | PROT_WRITE */

              buffer = memalign(pagesize, 4 * pagesize);
              if (buffer == NULL)

              printf("Start of region:        %p\n", buffer);

              if (mprotect(buffer + pagesize * 2, pagesize,
                          PROT_READ) == -1)

              for (char *p = buffer ; ; )
                  *(p++) = aqaaq;

              printf("Loop completed\n");     /* Should never happen */

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

          This page is part of release 5.10 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

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