man2/mprotect.2

   1 .\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" and Copyright (C) 1995 Michael Shields <shields@tembel.org>.
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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 2019-03-06 "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
  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 1.1i
  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 1.1i
  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 appropriately.
 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 is 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 EXAMPLE
 287 .\" sigaction.2 refers to this example
 288 .PP
 289 The program below demonstrates the use of
 290 .BR mprotect ().
 291 The program allocates four pages of memory, makes the third
 292 of these pages read-only, and then executes a loop that walks upward
 293 through the allocated region modifying bytes.
 294 .PP
 295 An example of what we might see when running the program is the
 296 following:
 297 .PP
 298 .in +4n
 299 .EX
 300 .RB "$" " ./a.out"
 301 Start of region:        0x804c000
 302 Got SIGSEGV at address: 0x804e000
 303 .EE
 304 .in
 305 .SS Program source
 306 \&
 307 .EX
 308 #include <unistd.h>
 309 #include <signal.h>
 310 #include <stdio.h>
 311 #include <malloc.h>
 312 #include <stdlib.h>
 313 #include <errno.h>
 314 #include <sys/mman.h>
 315
 316 #define handle_error(msg) \e
 317     do { perror(msg); exit(EXIT_FAILURE); } while (0)
 318
 319 static char *buffer;
 320
 321 static void
 322 handler(int sig, siginfo_t *si, void *unused)
 323 {
 324     /* Note: calling printf() from a signal handler is not safe
 325        (and should not be done in production programs), since
 326        printf() is not async\-signal\-safe; see signal-safety(7).
 327        Nevertheless, we use printf() here as a simple way of
 328        showing that the handler was called. */
 329
 330     printf("Got SIGSEGV at address: 0x%lx\en",
 331             (long) si\->si_addr);
 332     exit(EXIT_FAILURE);
 333 }
 334
 335 int
 336 main(int argc, char *argv[])
 337 {
 338     char *p;
 339     int pagesize;
 340     struct sigaction sa;
 341
 342     sa.sa_flags = SA_SIGINFO;
 343     sigemptyset(&sa.sa_mask);
 344     sa.sa_sigaction = handler;
 345     if (sigaction(SIGSEGV, &sa, NULL) == \-1)
 346         handle_error("sigaction");
 347
 348     pagesize = sysconf(_SC_PAGE_SIZE);
 349     if (pagesize == \-1)
 350         handle_error("sysconf");
 351
 352     /* Allocate a buffer aligned on a page boundary;
 353        initial protection is PROT_READ | PROT_WRITE */
 354
 355     buffer = memalign(pagesize, 4 * pagesize);
 356     if (buffer == NULL)
 357         handle_error("memalign");
 358
 359     printf("Start of region:        0x%lx\en", (long) buffer);
 360
 361     if (mprotect(buffer + pagesize * 2, pagesize,
 362                 PROT_READ) == \-1)
 363         handle_error("mprotect");
 364
 365     for (p = buffer ; ; )
 366         *(p++) = \(aqa\(aq;
 367
 368     printf("Loop completed\en");     /* Should never happen */
 369     exit(EXIT_SUCCESS);
 370 }
 371 .EE
 372 .SH SEE ALSO
 373 .BR mmap (2),
 374 .BR sysconf (3),
 375 .BR pkeys (7)