man2/mprotect.2

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   2 .\" Copyright (C) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
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  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>
  29 .\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and
  30 .\" a much improved example program.
  31 .\"
  32 .TH MPROTECT 2 2007-06-02 "Linux" "Linux Programmer's Manual"
  33 .SH NAME
  34 mprotect \- set protection on a region of memory
  35 .SH SYNOPSIS
  36 .nf
  37 .B #include <sys/mman.h>
  38 .sp
  39 \fBint mprotect(const void *\fIaddr\fB, size_t \fIlen\fB, int \fIprot\fB);
  40 .fi
  41 .SH DESCRIPTION
  42 .BR mprotect ()
  43 changes protection for the calling process's memory page(s)
  44 containing any part of the address range in the
  45 interval [\fIaddr\fP,\fIaddr\fP+\fIlen\fP\-1].
  46 .I addr
  47 must be aligned to a page boundary.
  48
  49 If the calling process tries to access memory in a manner
  50 that violates the protection, then the kernel generates a
  51 .B SIGSEGV
  52 signal for the process.
  53 .PP
  54 .I prot
  55 is either
  56 .B PROT_NONE
  57 or a bitwise-or of the other values in the following list:
  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
  66 The memory can be modified.
  67 .TP
  68 .B PROT_EXEC
  69 The memory can contain be executed.
  70 .\" FIXME
  71 .\" Document MAP_GROWSUP and MAP_GROWSDOWN
  72 .SH "RETURN VALUE"
  73 On success,
  74 .BR mprotect ()
  75 returns zero.
  76 On error, \-1 is returned, and
  77 .I errno
  78 is set appropriately.
  79 .SH ERRORS
  80 .TP
  81 .B EACCES
  82 The memory cannot be given the specified access.
  83 This can happen, for example, if you
  84 .BR mmap (2)
  85 a file to which you have read-only access, then ask
  86 .BR mprotect ()
  87 to mark it
  88 .BR PROT_WRITE .
  89 .TP
  90 .B EFAULT
  91 The memory cannot be accessed.
  92 .TP
  93 .B EINVAL
  94 \fIaddr\fR is not a valid pointer,
  95 or not a multiple of the system page size.
  96 .TP
  97 .B ENOMEM
  98 Internal kernel structures could not be allocated.
  99 Or: addresses  in the range
 100 .RI [ addr ,
 101 .IR addr + len ]
 102 are invalid for the address space of the process,
 103 or specify one or more pages that are not mapped.
 104 .SH "CONFORMING TO"
 105 SVr4, POSIX.1-2001.
 106 .\" SVr4 defines an additional error
 107 .\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
 108 POSIX says that the behavior of
 109 .BR mprotect ()
 110 is unspecified if it is applied to a region of memory that
 111 was not obtained via
 112 .BR mmap (2).
 113 .SH NOTES
 114 On Linux it is always legal to call
 115 .BR mprotect ()
 116 on any address in a process' address space (except for the
 117 kernel vsyscall area).
 118 In particular it can be used
 119 to change existing code mappings to be writable.
 120
 121 Whether
 122 .B PROT_EXEC
 123 has any effect different from
 124 .B PROT_READ
 125 is architecture and kernel version dependent.
 126
 127 POSIX.1-2001 says that an implementation may permit access
 128 other than that specified in
 129 .IR prot ,
 130 but at a minimum can only allow write access if
 131 .B PROT_WRITE
 132 has been set, and must not allow any access if
 133 .B PROT_NONE
 134 has been set.
 135 .SH EXAMPLE
 136 .\" sigaction.2 refers to this example
 137 .PP
 138 The program below allocates four pages of memory, makes the third
 139 of these pages read-only, and then executes a loop that walks upwards
 140 through the allocated region modifying bytes.
 141
 142 An example of what we might see when running the program is the
 143 following:
 144
 145 .in +0.5i
 146 .nf
 147 $ ./a.out
 148 Start of region:        0x804c000
 149 Got SIGSEGV at address: 0x804e000
 150 .fi
 151 .in
 152 .nf
 153
 154 #include <unistd.h>
 155 #include <signal.h>
 156 #include <stdio.h>
 157 #include <malloc.h>
 158 #include <stdlib.h>
 159 #include <errno.h>
 160 #include <sys/mman.h>
 161
 162 #define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
 163
 164 char *buffer;
 165
 166 static void
 167 handler(int sig, siginfo_t *si, void *unused)
 168 {
 169     printf("Got SIGSEGV at address: 0x%lx\\n",
 170             (long) si->si_addr);
 171     exit(EXIT_FAILURE);
 172 }
 173
 174 int
 175 main(int argc, char *argv[])
 176 {
 177     char *p;
 178     int pagesize;
 179     struct sigaction sa;
 180
 181     sa.sa_flags = SA_SIGINFO;
 182     sigemptyset(&sa.sa_mask);
 183     sa.sa_sigaction = handler;
 184     if (sigaction(SIGSEGV, &sa, NULL) == -1)
 185         die("sigaction");
 186
 187     pagesize = sysconf(_SC_PAGE_SIZE);
 188     if (pagesize == -1)
 189         die("sysconf");
 190
 191     /* Allocate a buffer aligned on a page boundary;
 192        initial protection is PROT_READ | PROT_WRITE */
 193
 194     buffer = memalign(pagesize, 4 * pagesize);
 195     if (buffer == NULL)
 196         die("memalign");
 197
 198     printf("Start of region:        0x%lx\\n", (long) buffer);
 199
 200     if (mprotect(buffer + pagesize * 2, pagesize,
 201                 PROT_NONE) == -1)
 202         die("mprotect");
 203
 204     for (p = buffer ; ; )
 205         *(p++) = 'a';
 206
 207     printf("Loop completed\\n");     /* Should never happen */
 208     exit(EXIT_SUCCESS);
 209 }
 210 .fi
 211 .SH "SEE ALSO"
 212 .BR mmap (2),
 213 .BR sysconf (3)