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


     mprotect, pkey_mprotect - set protection on a region of memory


     #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);


     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
     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:
                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 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).


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


     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.)


     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 mmap(2).
     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
     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
     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().


     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-
         $ ./a.out Start of region:        0x804c000 Got SIGSEGV at address:
 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;
         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:        0x%lx\n", (long) buffer);
         if (mprotect(buffer + pagesize * 2, pagesize,
                     PROT_READ) == -1)
         for (p = buffer ; ; )
             *(p++) = 'a';
         printf("Loop completed\n");     /* Should never happen */
         exit(EXIT_SUCCESS); }


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


     This  page  is  part of release 4.16 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

Linux 2018-02-02 MPROTECT(2)

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