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

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);
       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 archi-
                  tecture 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_mpro-
       tect().  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 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.

       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 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

       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
",
                   (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
", (long) buffer);

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

           for (p = buffer ; ; )
               *(p++) = 'a';

           printf("Loop completed
");     /* Should never happen */
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This page is part of release 4.15 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                                                               2018-02-02                                                         MPROTECT(2)