Ubuntu Manpage: mprotect, pkey_mprotect - set protection on a region of memory

NAME

       mprotect, pkey_mprotect - set protection on a region of memory

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/mman.h>

       int mprotect(void addr[.len], size_t len, int prot);

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

       int pkey_mprotect(void addr[.len], 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  Linux  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

       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).

       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().

STANDARDS

       mprotect()
              POSIX.1-2008.

       pkey_mprotect()
              Linux.

HISTORY

       mprotect()
              POSIX.1-2001, SVr4.

       pkey_mprotect()
              Linux 4.9, glibc 2.27.

NOTES

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 <malloc.h>
       #include <signal.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/mman.h>
       #include <unistd.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(void)
       {
           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);
       }