# mprotect

## NAME



           mprotect, pkey_mprotect - set protection on a region of memory
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/mman.h>**
    
           **int mprotect(** size_t size;
                        **void** _addr_**[**_size_**], size_t** _size_**, int** _prot_**);**
    
           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#include <sys/mman.h>**
    
           **int pkey_mprotect(** size_t size;
                        **void** _addr_**[**_size_**], size_t** _size_**, 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_ +_size_ -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 _size_.  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_ +_size_ -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.
    


## 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);
           }
    


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