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       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 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:

                  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 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.)


       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 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

       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().


       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\n",
                   (long) si->si_addr);

       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 */


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


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