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 be executed.
  70 .\" FIXME
  71 .\" Document PROT_GROWSUP and PROT_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\fP is not a valid pointer,
  95 or not a multiple of the system page size.
  96 .\" Or: both PROT_GROWSUP and PROT_GROWSDOWN were specified in 'prot'.
  97 .TP
  98 .B ENOMEM
  99 Internal kernel structures could not be allocated.
 100 Or: addresses  in the range
 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.
 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.
 109 POSIX says that the behavior of
 110 .BR mprotect ()
 111 is unspecified if it is applied to a region of memory that
 112 was not obtained via
 113 .BR mmap (2).
 114 .SH NOTES
 115 On Linux it is always legal to call
 116 .BR mprotect ()
 117 on any address in a process's address space (except for the
 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.
 127
 128 POSIX.1-2001 says that an implementation may permit access
 129 other than that specified in
 130 .IR prot ,
 131 but at a minimum can only allow write access if
 132 .B PROT_WRITE
 133 has been set, and must not allow any access if
 134 .B PROT_NONE
 135 has been set.
 136 .SH EXAMPLE
 137 .\" sigaction.2 refers to this example
 138 .PP
 139 The program below allocates four pages of memory, makes the third
 140 of these pages read-only, and then executes a loop that walks upwards
 141 through the allocated region modifying bytes.
 142
 143 An example of what we might see when running the program is the
 144 following:
 145
 146 .in +0.5i
 147 .nf
 148 $ ./a.out
 149 Start of region:        0x804c000
 150 Got SIGSEGV at address: 0x804e000
 151 .fi
 152 .in
 153 .nf
 154
 155 #include <unistd.h>
 156 #include <signal.h>
 157 #include <stdio.h>
 158 #include <malloc.h>
 159 #include <stdlib.h>
 160 #include <errno.h>
 161 #include <sys/mman.h>
 162
 163 #define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
 164
 165 char *buffer;
 166
 167 static void
 168 handler(int sig, siginfo_t *si, void *unused)
 169 {
 170     printf("Got SIGSEGV at address: 0x%lx\\n",
 171             (long) si\->si_addr);
 172     exit(EXIT_FAILURE);
 173 }
 174
 175 int
 176 main(int argc, char *argv[])
 177 {
 178     char *p;
 179     int pagesize;
 180     struct sigaction sa;
 181
 182     sa.sa_flags = SA_SIGINFO;
 183     sigemptyset(&sa.sa_mask);
 184     sa.sa_sigaction = handler;
 185     if (sigaction(SIGSEGV, &sa, NULL) == \-1)
 186         die("sigaction");
 187
 188     pagesize = sysconf(_SC_PAGE_SIZE);
 189     if (pagesize == \-1)
 190         die("sysconf");
 191
 192     /* Allocate a buffer aligned on a page boundary;
 193        initial protection is PROT_READ | PROT_WRITE */
 194
 195     buffer = memalign(pagesize, 4 * pagesize);
 196     if (buffer == NULL)
 197         die("memalign");
 198
 199     printf("Start of region:        0x%lx\\n", (long) buffer);
 200
 201     if (mprotect(buffer + pagesize * 2, pagesize,
 202                 PROT_NONE) == \-1)
 203         die("mprotect");
 204
 205     for (p = buffer ; ; )
 206         *(p++) = 'a';
 207
 208     printf("Loop completed\\n");     /* Should never happen */
 209     exit(EXIT_SUCCESS);
 210 }
 211 .fi
 212 .SH "SEE ALSO"
 213 .BR mmap (2),
 214 .BR sysconf (3)