1MPROTECT(2)                Linux Programmer's Manual               MPROTECT(2)
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NAME

6       mprotect, pkey_mprotect - set protection on a region of memory
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SYNOPSIS

9       #include <sys/mman.h>
10
11       int mprotect(void *addr, size_t len, int prot);
12       int pkey_mprotect(void *addr, size_t len, int prot, int pkey);
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DESCRIPTION

15       mprotect()  changes  the  access  protections for the calling process's
16       memory pages containing any part of the address range in  the  interval
17       [addr, addr+len-1].  addr must be aligned to a page boundary.
18
19       If the calling process tries to access memory in a manner that violates
20       the protections, then the kernel generates a  SIGSEGV  signal  for  the
21       process.
22
23       prot  is  a  combination  of the following access flags: PROT_NONE or a
24       bitwise-or of the other values in the following list:
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26       PROT_NONE  The memory cannot be accessed at all.
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28       PROT_READ  The memory can be read.
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30       PROT_WRITE The memory can be modified.
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32       PROT_EXEC  The memory can be executed.
33
34       PROT_SEM (since Linux 2.5.7)
35                  The memory can be used for atomic operations.  This flag was
36                  introduced  as part of the futex(2) implementation (in order
37                  to  guarantee  the  ability  to  perform  atomic  operations
38                  required  by  commands  such as FUTEX_WAIT), but is not cur‐
39                  rently used in on any architecture.
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41       PROT_SAO (since Linux 2.6.26)
42                  The memory should have strong access ordering.  This feature
43                  is specific to the PowerPC architecture (version 2.06 of the
44                  architecture specification adds the SAO CPU feature, and  it
45                  is available on POWER 7 or PowerPC A2, for example).
46
47       Additionally  (since  Linux  2.6.0), prot can have one of the following
48       flags set:
49
50       PROT_GROWSUP
51                  Apply the protection mode up to the end of  a  mapping  that
52                  grows  upwards.   (Such  mappings  are created for the stack
53                  area on architectures—for example,  HP-PARISC—that  have  an
54                  upwardly growing stack.)
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56       PROT_GROWSDOWN
57                  Apply the protection mode down to the beginning of a mapping
58                  that grows downward (which should be a stack  segment  or  a
59                  segment mapped with the MAP_GROWSDOWN flag set).
60
61       Like  mprotect(),  pkey_mprotect()  changes the protection on the pages
62       specified by addr and len.  The pkey argument specifies the  protection
63       key (see pkeys(7)) to assign to the memory.  The protection key must be
64       allocated with pkey_alloc(2) before it is  passed  to  pkey_mprotect().
65       For an example of the use of this system call, see pkeys(7).
66

RETURN VALUE

68       On  success,  mprotect()  and  pkey_mprotect()  return zero.  On error,
69       these system calls return -1, and errno is set appropriately.
70

ERRORS

72       EACCES The memory cannot be given the specified access.  This can  hap‐
73              pen,  for example, if you mmap(2) a file to which you have read-
74              only access, then ask mprotect() to mark it PROT_WRITE.
75
76       EINVAL addr is not a valid pointer, or not a  multiple  of  the  system
77              page size.
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79       EINVAL (pkey_mprotect()) pkey has not been allocated with pkey_alloc(2)
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81       EINVAL Both PROT_GROWSUP and PROT_GROWSDOWN were specified in prot.
82
83       EINVAL Invalid flags specified in prot.
84
85       EINVAL (PowerPC  architecture)  PROT_SAO was specified in prot, but SAO
86              hardware feature is not available.
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88       ENOMEM Internal kernel structures could not be allocated.
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90       ENOMEM Addresses in the range [addr, addr+len-1] are  invalid  for  the
91              address  space of the process, or specify one or more pages that
92              are not mapped.  (Before kernel 2.4.19,  the  error  EFAULT  was
93              incorrectly produced for these cases.)
94
95       ENOMEM Changing  the  protection of a memory region would result in the
96              total number of mappings with distinct  attributes  (e.g.,  read
97              versus  read/write  protection)  exceeding  the allowed maximum.
98              (For example, making the protection of a range PROT_READ in  the
99              middle  of  a region currently protected as PROT_READ|PROT_WRITE
100              would result in three mappings: two read/write mappings at  each
101              end and a read-only mapping in the middle.)
102

VERSIONS

104       pkey_mprotect()  first appeared in Linux 4.9; library support was added
105       in glibc 2.27.
106

CONFORMING TO

108       mprotect(): POSIX.1-2001, POSIX.1-2008,  SVr4.   POSIX  says  that  the
109       behavior  of  mprotect() is unspecified if it is applied to a region of
110       memory that was not obtained via mmap(2).
111
112       pkey_mprotect() is a nonportable Linux extension.
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NOTES

115       On Linux, it is always permissible to call mprotect() on any address in
116       a  process's  address  space (except for the kernel vsyscall area).  In
117       particular, it can be used to  change  existing  code  mappings  to  be
118       writable.
119
120       Whether  PROT_EXEC  has  any effect different from PROT_READ depends on
121       processor  architecture,  kernel  version,  and  process   state.    If
122       READ_IMPLIES_EXEC  is  set in the process's personality flags (see per‐
123       sonality(2)), specifying PROT_READ will implicitly add PROT_EXEC.
124
125       On  some  hardware  architectures  (e.g.,  i386),  PROT_WRITE   implies
126       PROT_READ.
127
128       POSIX.1  says  that an implementation may permit access other than that
129       specified in prot, but at a minimum can  allow  write  access  only  if
130       PROT_WRITE has been set, and must not allow any access if PROT_NONE has
131       been set.
132
133       Applications should be  careful  when  mixing  use  of  mprotect()  and
134       pkey_mprotect().   On  x86,  when  mprotect()  is used with prot set to
135       PROT_EXEC a pkey is may be allocated and set on the  memory  implicitly
136       by the kernel, but only when the pkey was 0 previously.
137
138       On  systems that do not support protection keys in hardware, pkey_mpro‐
139       tect() may still be used, but pkey must be set to 0.  When called  this
140       way, the operation of pkey_mprotect() is equivalent to mprotect().
141

EXAMPLE

143       The  program  below  demonstrates  the  use of mprotect().  The program
144       allocates four pages of memory, makes the third of  these  pages  read-
145       only,  and then executes a loop that walks upward through the allocated
146       region modifying bytes.
147
148       An example of what we might see when running the program is the follow‐
149       ing:
150
151           $ ./a.out
152           Start of region:        0x804c000
153           Got SIGSEGV at address: 0x804e000
154
155   Program source
156
157       #include <unistd.h>
158       #include <signal.h>
159       #include <stdio.h>
160       #include <malloc.h>
161       #include <stdlib.h>
162       #include <errno.h>
163       #include <sys/mman.h>
164
165       #define handle_error(msg) \
166           do { perror(msg); exit(EXIT_FAILURE); } while (0)
167
168       static char *buffer;
169
170       static void
171       handler(int sig, siginfo_t *si, void *unused)
172       {
173           /* Note: calling printf() from a signal handler is not safe
174              (and should not be done in production programs), since
175              printf() is not async-signal-safe; see signal-safety(7).
176              Nevertheless, we use printf() here as a simple way of
177              showing that the handler was called. */
178
179           printf("Got SIGSEGV at address: 0x%lx\n",
180                   (long) si->si_addr);
181           exit(EXIT_FAILURE);
182       }
183
184       int
185       main(int argc, char *argv[])
186       {
187           char *p;
188           int pagesize;
189           struct sigaction sa;
190
191           sa.sa_flags = SA_SIGINFO;
192           sigemptyset(&sa.sa_mask);
193           sa.sa_sigaction = handler;
194           if (sigaction(SIGSEGV, &sa, NULL) == -1)
195               handle_error("sigaction");
196
197           pagesize = sysconf(_SC_PAGE_SIZE);
198           if (pagesize == -1)
199               handle_error("sysconf");
200
201           /* Allocate a buffer aligned on a page boundary;
202              initial protection is PROT_READ | PROT_WRITE */
203
204           buffer = memalign(pagesize, 4 * pagesize);
205           if (buffer == NULL)
206               handle_error("memalign");
207
208           printf("Start of region:        0x%lx\n", (long) buffer);
209
210           if (mprotect(buffer + pagesize * 2, pagesize,
211                       PROT_READ) == -1)
212               handle_error("mprotect");
213
214           for (p = buffer ; ; )
215               *(p++) = 'a';
216
217           printf("Loop completed\n");     /* Should never happen */
218           exit(EXIT_SUCCESS);
219       }
220

SEE ALSO

222       mmap(2), sysconf(3), pkeys(7)
223

COLOPHON

225       This  page  is  part of release 4.15 of the Linux man-pages project.  A
226       description of the project, information about reporting bugs,  and  the
227       latest     version     of     this    page,    can    be    found    at
228       https://www.kernel.org/doc/man-pages/.
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232Linux                             2018-02-02                       MPROTECT(2)
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