mprotect

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 cur-
                  rently 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 appropriately.

ERRORS
       EACCES The memory cannot be given the specified access.  This can  hap-
              pen,  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 per-
       sonality(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_mpro-
       tect() 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 follow-
       ing:

           $ ./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
       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)
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