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1 .\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
2 .\" and Copyright (C) 1995 Michael Shields <shields@tembel.org>.
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14 .\" Since the Linux kernel and libraries are constantly changing, this
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19 .\" which is licensed free of charge, as they might when working
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25 .\"
26 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
27 .\" Modified 1997-05-31 by Andries Brouwer <aeb@cwi.nl>
28 .\" Modified 2003-08-24 by Andries Brouwer <aeb@cwi.nl>
29 .\" Modified 2004-08-16 by Andi Kleen <ak@muc.de>
30 .\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and
31 .\" a much improved example program.
32 .\"
33 .TH MPROTECT 2 2021-03-22 "Linux" "Linux Programmer's Manual"
34 .SH NAME
35 mprotect, pkey_mprotect \- set protection on a region of memory
36 .SH SYNOPSIS
37 .nf
38 .B #include <sys/mman.h>
39 .PP
40 .BI "int mprotect(void *" addr ", size_t " len ", int " prot );
41 .PP
42 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
43 .B #include <sys/mman.h>
44 .PP
45 .BI "int pkey_mprotect(void *" addr ", size_t " len ", int " prot ", int " pkey ");"
46 .fi
47 .SH DESCRIPTION
48 .BR mprotect ()
49 changes the access protections for the calling process's memory pages
50 containing any part of the address range in the
51 interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1].
52 .I addr
53 must be aligned to a page boundary.
54 .PP
55 If the calling process tries to access memory in a manner
56 that violates the protections, then the kernel generates a
57 .B SIGSEGV
58 signal for the process.
59 .PP
60 .I prot
61 is a combination of the following access flags:
62 .B PROT_NONE
63 or a bitwise-or of the other values in the following list:
64 .TP
65 .B PROT_NONE
66 The memory cannot be accessed at all.
67 .TP
68 .B PROT_READ
69 The memory can be read.
70 .TP
71 .B PROT_WRITE
72 The memory can be modified.
73 .TP
74 .B PROT_EXEC
75 The memory can be executed.
76 .TP
77 .BR PROT_SEM " (since Linux 2.5.7)"
78 The memory can be used for atomic operations.
79 This flag was introduced as part of the
80 .BR futex (2)
81 implementation (in order to guarantee the ability to perform atomic
82 operations required by commands such as
83 .BR FUTEX_WAIT ),
84 but is not currently used in on any architecture.
85 .TP
86 .BR PROT_SAO " (since Linux 2.6.26)"
87 .\" commit aba46c5027cb59d98052231b36efcbbde9c77a1d
88 .\" commit ef3d3246a0d06be622867d21af25f997aeeb105f
89 The memory should have strong access ordering.
90 This feature is specific to
91 the PowerPC architecture
92 (version 2.06 of the architecture specification adds the SAO CPU feature,
93 and it is available on POWER 7 or PowerPC A2, for example).
94 .PP
95 Additionally (since Linux 2.6.0),
96 .I prot
97 can have one of the following flags set:
98 .TP
99 .\" mm/mmap.c:
100 .\" vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
101 .\" mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
102 .\" And calc_vm_flag_bits converts only GROWSDOWN/DENYWRITE/LOCKED.
103 .B PROT_GROWSUP
104 Apply the protection mode up to the end of a mapping
105 that grows upwards.
106 (Such mappings are created for the stack area on
107 architectures\(emfor example, HP-PARISC\(emthat
108 have an upwardly growing stack.)
109 .\" The VMA is one that was marked with VM_GROWSUP by the kernel
110 .\" when the stack was created. Note that (unlike VM_GROWSDOWN),
111 .\" there is no mmap() flag (analogous to MAP_GROWSDOWN) for
112 .\" creating a VMA that is marked VM_GROWSUP.
113 .TP
114 .B PROT_GROWSDOWN
115 Apply the protection mode down to the beginning of a mapping
116 that grows downward
117 (which should be a stack segment or a segment mapped with the
118 .B MAP_GROWSDOWN
119 flag set).
120 .PP
121 Like
122 .BR mprotect (),
123 .BR pkey_mprotect ()
124 changes the protection on the pages specified by
125 .IR addr
126 and
127 .IR len .
128 The
129 .I pkey
130 argument specifies the protection key (see
131 .BR pkeys (7))
132 to assign to the memory.
133 The protection key must be allocated with
134 .BR pkey_alloc (2)
135 before it is passed to
136 .BR pkey_mprotect ().
137 For an example of the use of this system call, see
138 .BR pkeys (7).
139 .SH RETURN VALUE
140 On success,
141 .BR mprotect ()
142 and
143 .BR pkey_mprotect ()
144 return zero.
145 On error, these system calls return \-1, and
146 .I errno
147 is set to indicate the error.
148 .SH ERRORS
149 .TP
150 .B EACCES
151 The memory cannot be given the specified access.
152 This can happen, for example, if you
153 .BR mmap (2)
154 a file to which you have read-only access, then ask
155 .BR mprotect ()
156 to mark it
157 .BR PROT_WRITE .
158 .TP
159 .B EINVAL
160 \fIaddr\fP is not a valid pointer,
161 or not a multiple of the system page size.
162 .TP
163 .BR EINVAL
164 .RB ( pkey_mprotect ())
165 \fIpkey\fP has not been allocated with
166 .BR pkey_alloc (2)
167 .TP
168 .BR EINVAL
169 Both
170 .BR PROT_GROWSUP
171 and
172 .BR PROT_GROWSDOWN
173 were specified in
174 .IR prot .
175 .TP
176 .BR EINVAL
177 Invalid flags specified in
178 .IR prot .
179 .TP
180 .BR EINVAL
181 (PowerPC architecture)
182 .B PROT_SAO
183 was specified in
184 .IR prot ,
185 but SAO hardware feature is not available.
186 .TP
187 .B ENOMEM
188 Internal kernel structures could not be allocated.
189 .TP
190 .B ENOMEM
191 Addresses in the range
192 .RI [ addr ,
193 .IR addr + len \-1]
194 are invalid for the address space of the process,
195 or specify one or more pages that are not mapped.
196 (Before kernel 2.4.19, the error
197 .BR EFAULT
198 was incorrectly produced for these cases.)
199 .TP
200 .B ENOMEM
201 Changing the protection of a memory region would result in the total number of
202 mappings with distinct attributes (e.g., read versus read/write protection)
203 exceeding the allowed maximum.
204 .\" I.e., the number of VMAs would exceed the 64 kB maximum
205 (For example, making the protection of a range
206 .BR PROT_READ
207 in the middle of a region currently protected as
208 .BR PROT_READ|PROT_WRITE
209 would result in three mappings:
210 two read/write mappings at each end and a read-only mapping in the middle.)
211 .SH VERSIONS
212 .BR pkey_mprotect ()
213 first appeared in Linux 4.9;
214 library support was added in glibc 2.27.
215 .SH CONFORMING TO
216 .BR mprotect ():
217 POSIX.1-2001, POSIX.1-2008, SVr4.
218 .\" SVr4 defines an additional error
219 .\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
220 POSIX says that the behavior of
221 .BR mprotect ()
222 is unspecified if it is applied to a region of memory that
223 was not obtained via
224 .BR mmap (2).
225 .PP
226 .BR pkey_mprotect ()
227 is a nonportable Linux extension.
228 .SH NOTES
229 On Linux, it is always permissible to call
230 .BR mprotect ()
231 on any address in a process's address space (except for the
232 kernel vsyscall area).
233 In particular, it can be used
234 to change existing code mappings to be writable.
235 .PP
236 Whether
237 .B PROT_EXEC
238 has any effect different from
239 .B PROT_READ
240 depends on processor architecture, kernel version, and process state.
241 If
242 .B READ_IMPLIES_EXEC
243 is set in the process's personality flags (see
244 .BR personality (2)),
245 specifying
246 .B PROT_READ
247 will implicitly add
248 .BR PROT_EXEC .
249 .PP
250 On some hardware architectures (e.g., i386),
251 .B PROT_WRITE
252 implies
253 .BR PROT_READ .
254 .PP
255 POSIX.1 says that an implementation may permit access
256 other than that specified in
257 .IR prot ,
258 but at a minimum can allow write access only if
259 .B PROT_WRITE
260 has been set, and must not allow any access if
261 .B PROT_NONE
262 has been set.
263 .PP
264 Applications should be careful when mixing use of
265 .BR mprotect ()
266 and
267 .BR pkey_mprotect ().
268 On x86, when
269 .BR mprotect ()
270 is used with
271 .IR prot
272 set to
273 .B PROT_EXEC
274 a pkey may be allocated and set on the memory implicitly
275 by the kernel, but only when the pkey was 0 previously.
276 .PP
277 On systems that do not support protection keys in hardware,
278 .BR pkey_mprotect ()
279 may still be used, but
280 .IR pkey
281 must be set to \-1.
282 When called this way, the operation of
283 .BR pkey_mprotect ()
284 is equivalent to
285 .BR mprotect ().
286 .SH EXAMPLES
287 .\" sigaction.2 refers to this example
288 The program below demonstrates the use of
289 .BR mprotect ().
290 The program allocates four pages of memory, makes the third
291 of these pages read-only, and then executes a loop that walks upward
292 through the allocated region modifying bytes.
293 .PP
294 An example of what we might see when running the program is the
295 following:
296 .PP
297 .in +4n
298 .EX
299 .RB "$" " ./a.out"
300 Start of region: 0x804c000
301 Got SIGSEGV at address: 0x804e000
302 .EE
303 .in
304 .SS Program source
305 \&
306 .EX
307 #include <unistd.h>
308 #include <signal.h>
309 #include <stdio.h>
310 #include <malloc.h>
311 #include <stdlib.h>
312 #include <errno.h>
313 #include <sys/mman.h>
315 #define handle_error(msg) \e
316 do { perror(msg); exit(EXIT_FAILURE); } while (0)
318 static char *buffer;
320 static void
321 handler(int sig, siginfo_t *si, void *unused)
322 {
323 /* Note: calling printf() from a signal handler is not safe
324 (and should not be done in production programs), since
325 printf() is not async\-signal\-safe; see signal\-safety(7).
326 Nevertheless, we use printf() here as a simple way of
327 showing that the handler was called. */
329 printf("Got SIGSEGV at address: %p\en", si\->si_addr);
330 exit(EXIT_FAILURE);
331 }
333 int
334 main(int argc, char *argv[])
335 {
336 int pagesize;
337 struct sigaction sa;
339 sa.sa_flags = SA_SIGINFO;
340 sigemptyset(&sa.sa_mask);
341 sa.sa_sigaction = handler;
342 if (sigaction(SIGSEGV, &sa, NULL) == \-1)
343 handle_error("sigaction");
345 pagesize = sysconf(_SC_PAGE_SIZE);
346 if (pagesize == \-1)
347 handle_error("sysconf");
349 /* Allocate a buffer aligned on a page boundary;
350 initial protection is PROT_READ | PROT_WRITE. */
352 buffer = memalign(pagesize, 4 * pagesize);
353 if (buffer == NULL)
354 handle_error("memalign");
356 printf("Start of region: %p\en", buffer);
358 if (mprotect(buffer + pagesize * 2, pagesize,
359 PROT_READ) == \-1)
360 handle_error("mprotect");
362 for (char *p = buffer ; ; )
363 *(p++) = \(aqa\(aq;
365 printf("Loop completed\en"); /* Should never happen */
366 exit(EXIT_SUCCESS);
367 }
368 .EE
369 .SH SEE ALSO
370 .BR mmap (2),
371 .BR sysconf (3),
372 .BR pkeys (7)