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c13182ef | 1 | .\" -*- nroff -*- |
c11b1abf | 2 | .\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com> |
2a5e0dcd | 3 | .\" and Copyright (C) 1995 Michael Shields <shields@tembel.org>. |
fea681da MK |
4 | .\" |
5 | .\" Permission is granted to make and distribute verbatim copies of this | |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and author of this work. | |
24 | .\" | |
25 | .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com> | |
26 | .\" Modified 1997-05-31 by Andries Brouwer <aeb@cwi.nl> | |
27 | .\" Modified 2003-08-24 by Andries Brouwer <aeb@cwi.nl> | |
28 | .\" Modified 2004-08-16 by Andi Kleen <ak@muc.de> | |
2a5e0dcd MK |
29 | .\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and |
30 | .\" a much improved example program. | |
fea681da | 31 | .\" |
2a5e0dcd | 32 | .TH MPROTECT 2 2007-06-02 "Linux" "Linux Programmer's Manual" |
fea681da | 33 | .SH NAME |
2a5e0dcd | 34 | mprotect \- set protection on a region of memory |
fea681da MK |
35 | .SH SYNOPSIS |
36 | .nf | |
37 | .B #include <sys/mman.h> | |
38 | .sp | |
495846d9 | 39 | .BI "int mprotect(const void *" addr ", size_t *" len ", int " prot "); |
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40 | .fi |
41 | .SH DESCRIPTION | |
e511ffb6 | 42 | .BR mprotect () |
2a5e0dcd | 43 | changes protection for the calling process's memory page(s) |
988db661 | 44 | containing any part of the address range in the |
657e762d | 45 | interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1]. |
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46 | .I addr |
47 | must be aligned to a page boundary. | |
48 | ||
49 | If the calling process tries to access memory in a manner | |
988db661 | 50 | that violates the protection, then the kernel generates a |
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51 | .B SIGSEGV |
52 | signal for the process. | |
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53 | .PP |
54 | .I prot | |
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55 | is either |
56 | .B PROT_NONE | |
57 | or a bitwise-or of the other values in the following list: | |
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58 | .TP 1.1i |
59 | .B PROT_NONE | |
60 | The memory cannot be accessed at all. | |
61 | .TP | |
62 | .B PROT_READ | |
63 | The memory can be read. | |
64 | .TP | |
65 | .B PROT_WRITE | |
2a5e0dcd | 66 | The memory can be modified. |
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67 | .TP |
68 | .B PROT_EXEC | |
fc15ae54 | 69 | The memory can be executed. |
e7d3b070 | 70 | .\" FIXME |
2b83e9fe | 71 | .\" Document PROT_GROWSUP and PROT_GROWSDOWN |
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72 | .SH "RETURN VALUE" |
73 | On success, | |
e511ffb6 | 74 | .BR mprotect () |
c13182ef MK |
75 | returns zero. |
76 | On error, \-1 is returned, and | |
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77 | .I errno |
78 | is set appropriately. | |
79 | .SH ERRORS | |
80 | .TP | |
81 | .B EACCES | |
c13182ef MK |
82 | The memory cannot be given the specified access. |
83 | This can happen, for example, if you | |
fea681da MK |
84 | .BR mmap (2) |
85 | a file to which you have read-only access, then ask | |
e511ffb6 | 86 | .BR mprotect () |
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87 | to mark it |
88 | .BR PROT_WRITE . | |
89 | .TP | |
90 | .B EFAULT | |
91 | The memory cannot be accessed. | |
92 | .TP | |
93 | .B EINVAL | |
a8d55537 | 94 | \fIaddr\fP is not a valid pointer, |
2a5e0dcd | 95 | or not a multiple of the system page size. |
3a928a50 | 96 | .\" Or: both PROT_GROWSUP and PROT_GROWSDOWN were specified in 'prot'. |
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97 | .TP |
98 | .B ENOMEM | |
c13182ef | 99 | Internal kernel structures could not be allocated. |
cfadad46 | 100 | Or: addresses in the range |
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101 | .RI [ addr , |
102 | .IR addr + len ] | |
103 | are invalid for the address space of the process, | |
104 | or specify one or more pages that are not mapped. | |
2b2581ee MK |
105 | .SH "CONFORMING TO" |
106 | SVr4, POSIX.1-2001. | |
107 | .\" SVr4 defines an additional error | |
108 | .\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's. | |
d9bfdb9c | 109 | POSIX says that the behavior of |
2b2581ee | 110 | .BR mprotect () |
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111 | is unspecified if it is applied to a region of memory that |
112 | was not obtained via | |
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113 | .BR mmap (2). |
114 | .SH NOTES | |
115 | On Linux it is always legal to call | |
116 | .BR mprotect () | |
fc7ba057 | 117 | on any address in a process's address space (except for the |
2b2581ee MK |
118 | kernel vsyscall area). |
119 | In particular it can be used | |
120 | to change existing code mappings to be writable. | |
121 | ||
122 | Whether | |
123 | .B PROT_EXEC | |
124 | has any effect different from | |
125 | .B PROT_READ | |
126 | is architecture and kernel version dependent. | |
34ccb744 | 127 | On some hardware architectures (e.g., i386), |
0daa9e92 | 128 | .B PROT_WRITE |
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129 | implies |
130 | .BR PROT_READ . | |
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131 | |
132 | POSIX.1-2001 says that an implementation may permit access | |
133 | other than that specified in | |
134 | .IR prot , | |
135 | but at a minimum can only allow write access if | |
136 | .B PROT_WRITE | |
137 | has been set, and must not allow any access if | |
138 | .B PROT_NONE | |
139 | has been set. | |
fea681da | 140 | .SH EXAMPLE |
2720c2ed | 141 | .\" sigaction.2 refers to this example |
f20d7d8e | 142 | .PP |
2a5e0dcd | 143 | The program below allocates four pages of memory, makes the third |
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144 | of these pages read-only, and then executes a loop that walks upwards |
145 | through the allocated region modifying bytes. | |
146 | ||
147 | An example of what we might see when running the program is the | |
148 | following: | |
149 | ||
088a639b | 150 | .in +4n |
fea681da | 151 | .nf |
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152 | $ ./a.out |
153 | Start of region: 0x804c000 | |
154 | Got SIGSEGV at address: 0x804e000 | |
155 | .fi | |
156 | .in | |
157 | .nf | |
158 | ||
159 | #include <unistd.h> | |
160 | #include <signal.h> | |
fea681da | 161 | #include <stdio.h> |
f20d7d8e | 162 | #include <malloc.h> |
fea681da MK |
163 | #include <stdlib.h> |
164 | #include <errno.h> | |
165 | #include <sys/mman.h> | |
166 | ||
6a578b88 MK |
167 | #define handle_error(msg) \\ |
168 | do { perror(msg); exit(EXIT_FAILURE); } while (0) | |
d3b5ab82 | 169 | |
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170 | char *buffer; |
171 | ||
172 | static void | |
173 | handler(int sig, siginfo_t *si, void *unused) | |
174 | { | |
988db661 | 175 | printf("Got SIGSEGV at address: 0x%lx\\n", |
29059a65 | 176 | (long) si\->si_addr); |
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177 | exit(EXIT_FAILURE); |
178 | } | |
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179 | |
180 | int | |
f20d7d8e | 181 | main(int argc, char *argv[]) |
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182 | { |
183 | char *p; | |
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184 | int pagesize; |
185 | struct sigaction sa; | |
186 | ||
187 | sa.sa_flags = SA_SIGINFO; | |
188 | sigemptyset(&sa.sa_mask); | |
189 | sa.sa_sigaction = handler; | |
29059a65 | 190 | if (sigaction(SIGSEGV, &sa, NULL) == \-1) |
6a578b88 | 191 | handle_error("sigaction"); |
fea681da | 192 | |
f20d7d8e | 193 | pagesize = sysconf(_SC_PAGE_SIZE); |
29059a65 | 194 | if (pagesize == \-1) |
6a578b88 | 195 | handle_error("sysconf"); |
fea681da | 196 | |
988db661 | 197 | /* Allocate a buffer aligned on a page boundary; |
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198 | initial protection is PROT_READ | PROT_WRITE */ |
199 | ||
200 | buffer = memalign(pagesize, 4 * pagesize); | |
d3b5ab82 | 201 | if (buffer == NULL) |
6a578b88 | 202 | handle_error("memalign"); |
fea681da | 203 | |
f20d7d8e | 204 | printf("Start of region: 0x%lx\\n", (long) buffer); |
fea681da | 205 | |
988db661 | 206 | if (mprotect(buffer + pagesize * 2, pagesize, |
29059a65 | 207 | PROT_NONE) == \-1) |
6a578b88 | 208 | handle_error("mprotect"); |
fea681da | 209 | |
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210 | for (p = buffer ; ; ) |
211 | *(p++) = 'a'; | |
fea681da | 212 | |
f20d7d8e | 213 | printf("Loop completed\\n"); /* Should never happen */ |
646f46f0 | 214 | exit(EXIT_SUCCESS); |
fea681da MK |
215 | } |
216 | .fi | |
fea681da | 217 | .SH "SEE ALSO" |
2a5e0dcd MK |
218 | .BR mmap (2), |
219 | .BR sysconf (3) |