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 .BI "int pkey_mprotect(void *" addr ", size_t " len ", int " prot ", int " pkey ");
  42 .fi
  43 .SH DESCRIPTION
  44 .BR mprotect ()
  45 changes the access protections for the calling process's memory pages
  46 containing any part of the address range in the
  47 interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1].
  48 .I addr
  49 must be aligned to a page boundary.
  50 .PP
  51 If the calling process tries to access memory in a manner
  52 that violates the protections, then the kernel generates a
  53 .B SIGSEGV
  54 signal for the process.
  55 .PP
  56 .I prot
  57 is a combination of the following access flags:
  58 .B PROT_NONE
  59 or a bitwise-or of the other values in the following list:
  60 .TP 1.1i
  61 .B PROT_NONE
  62 The memory cannot be accessed at all.
  63 .TP
  64 .B PROT_READ
  65 The memory can be read.
  66 .TP
  67 .B PROT_WRITE
  68 The memory can be modified.
  69 .TP
  70 .B PROT_EXEC
  71 The memory can be executed.
  72 .TP
  73 .BR PROT_SEM " (since Linux 2.5.7)"
  74 The memory can be used for atomic operations.
  75 This flag was introduced as part of the
  76 .BR futex (2)
  77 implementation (in order to guarantee the ability to perform atomic
  78 operations required by commands such as
  79 .BR FUTEX_WAIT ),
  80 but is not currently used in on any architecture.
  81 .TP
  82 .BR PROT_SAO " (since Linux 2.6.26)"
  83 .\" commit aba46c5027cb59d98052231b36efcbbde9c77a1d
  84 .\" commit ef3d3246a0d06be622867d21af25f997aeeb105f
  85 The memory should have strong access ordering.
  86 This feature is specific to
  87 the PowerPC architecture
  88 (version 2.06 of the architecture specification adds the SAO CPU feature,
  89 and it is available on POWER 7 or PowerPC A2, for example).
  90 .PP
  91 Additionally (since Linux 2.6.0),
  92 .I prot
  93 can have one of the following flags set:
  94 .TP 1.1i
  95 .\" mm/mmap.c:
  96 .\"     vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
  97 .\"                     mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
  98 .\" And calc_vm_flag_bits converts only GROWSDOWN/DENYWRITE/LOCKED.
  99 .B PROT_GROWSUP
 100 Apply the protection mode up to the end of a mapping
 101 that grows upwards.
 102 (Such mappings are created for the stack area on
 103 architectures\(emfor example, HP-PARISC\(emthat
 104 have an upwardly growing stack.)
 105 .\" The VMA is one that was marked with VM_GROWSUP by the kernel
 106 .\" when the stack was created. Note that (unlike VM_GROWSDOWN),
 107 .\" there is no mmap() flag (analogous to MAP_GROWSDOWN) for
 108 .\" creating a VMA that is marked VM_GROWSUP.
 109 .TP
 110 .B PROT_GROWSDOWN
 111 Apply the protection mode down to the beginning of a mapping
 112 that grows downward
 113 (which should be a stack segment or a segment mapped with the
 114 .B MAP_GROWSDOWN
 115 flag set).
 116 .PP
 117 Like
 118 .BR mprotect (),
 119 .BR pkey_mprotect ()
 120 changes the protection on the pages specified by
 121 .IR addr
 122 and
 123 .IR len .
 124 The
 125 .I pkey
 126 argument specifies the protection key (see
 127 .BR pkeys (7))
 128 to assign to the memory.
 129 The protection key must be allocated with
 130 .BR pkey_alloc (2)
 131 before it is passed to
 132 .BR pkey_mprotect ().
 133 For an example of the use of this system call, see
 134 .BR pkeys (7).
 135 .SH RETURN VALUE
 136 On success,
 137 .BR mprotect ()
 138 and
 139 .BR pkey_mprotect ()
 140 return zero.
 141 On error, these system calls return \-1, and
 142 .I errno
 143 is set appropriately.
 144 .SH ERRORS
 145 .TP
 146 .B EACCES
 147 The memory cannot be given the specified access.
 148 This can happen, for example, if you
 149 .BR mmap (2)
 150 a file to which you have read-only access, then ask
 151 .BR mprotect ()
 152 to mark it
 153 .BR PROT_WRITE .
 154 .TP
 155 .B EINVAL
 156 \fIaddr\fP is not a valid pointer,
 157 or not a multiple of the system page size.
 158 .TP
 159 .BR EINVAL
 160 .RB ( pkey_mprotect ())
 161 \fIpkey\fP has not been allocated with
 162 .BR pkey_alloc (2)
 163 .TP
 164 .BR EINVAL
 165 Both
 166 .BR PROT_GROWSUP
 167 and
 168 .BR PROT_GROWSDOWN
 169 were specified in
 170 .IR prot .
 171 .TP
 172 .BR EINVAL
 173 Invalid flags specified in
 174 .IR prot .
 175 .TP
 176 .BR EINVAL
 177 (PowerPC architecture)
 178 .B PROT_SAO
 179 was specified in
 180 .IR prot ,
 181 but SAO hardware feature is not available.
 182 .TP
 183 .B ENOMEM
 184 Internal kernel structures could not be allocated.
 185 .TP
 186 .B ENOMEM
 187 Addresses in the range
 188 .RI [ addr ,
 189 .IR addr + len \-1]
 190 are invalid for the address space of the process,
 191 or specify one or more pages that are not mapped.
 192 (Before kernel 2.4.19, the error
 193 .BR EFAULT
 194 was incorrectly produced for these cases.)
 195 .TP
 196 .B ENOMEM
 197 Changing the protection of a memory region would result in the total number of
 198 mappings with distinct attributes (e.g., read versus read/write protection)
 199 exceeding the allowed maximum.
 200 .\" I.e., the number of VMAs would exceed the 64 kB maximum
 201 (For example, making the protection of a range
 202 .BR PROT_READ
 203 in the middle of a region currently protected as
 204 .BR PROT_READ|PROT_WRITE
 205 would result in three mappings:
 206 two read/write mappings at each end and a read-only mapping in the middle.)
 207 .SH VERSIONS
 208 .BR pkey_mprotect ()
 209 first appeared in Linux 4.9;
 210 library support was added in glibc 2.27.
 211 .SH CONFORMING TO
 212 .BR mprotect ():
 213 POSIX.1-2001, POSIX.1-2008, SVr4.
 214 .\" SVr4 defines an additional error
 215 .\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's.
 216 POSIX says that the behavior of
 217 .BR mprotect ()
 218 is unspecified if it is applied to a region of memory that
 219 was not obtained via
 220 .BR mmap (2).
 221 .PP
 222 .BR pkey_mprotect ()
 223 is a nonportable Linux extension.
 224 .SH NOTES
 225 On Linux, it is always permissible to call
 226 .BR mprotect ()
 227 on any address in a process's address space (except for the
 228 kernel vsyscall area).
 229 In particular, it can be used
 230 to change existing code mappings to be writable.
 231 .PP
 232 Whether
 233 .B PROT_EXEC
 234 has any effect different from
 235 .B PROT_READ
 236 depends on processor architecture, kernel version, and process state.
 237 If
 238 .B READ_IMPLIES_EXEC
 239 is set in the process's personality flags (see
 240 .BR personality (2)),
 241 specifying
 242 .B PROT_READ
 243 will implicitly add
 244 .BR PROT_EXEC .
 245 .PP
 246 On some hardware architectures (e.g., i386),
 247 .B PROT_WRITE
 248 implies
 249 .BR PROT_READ .
 250 .PP
 251 POSIX.1 says that an implementation may permit access
 252 other than that specified in
 253 .IR prot ,
 254 but at a minimum can allow write access only if
 255 .B PROT_WRITE
 256 has been set, and must not allow any access if
 257 .B PROT_NONE
 258 has been set.
 259 .PP
 260 Applications should be careful when mixing use of
 261 .BR mprotect ()
 262 and
 263 .BR pkey_mprotect ().
 264 On x86, when
 265 .BR mprotect ()
 266 is used with
 267 .IR prot
 268 set to
 269 .B PROT_EXEC
 270 a pkey is may be allocated and set on the memory implicitly
 271 by the kernel, but only when the pkey was 0 previously.
 272 .PP
 273 On systems that do not support protection keys in hardware,
 274 .BR pkey_mprotect ()
 275 may still be used, but
 276 .IR pkey
 277 must be set to -1.
 278 When called this way, the operation of
 279 .BR pkey_mprotect ()
 280 is equivalent to
 281 .BR mprotect ().
 282 .SH EXAMPLE
 283 .\" sigaction.2 refers to this example
 284 .PP
 285 The program below demonstrates the use of
 286 .BR mprotect ().
 287 The program allocates four pages of memory, makes the third
 288 of these pages read-only, and then executes a loop that walks upward
 289 through the allocated region modifying bytes.
 290 .PP
 291 An example of what we might see when running the program is the
 292 following:
 293 .PP
 294 .in +4n
 295 .EX
 296 .RB "$" " ./a.out"
 297 Start of region:        0x804c000
 298 Got SIGSEGV at address: 0x804e000
 299 .EE
 300 .in
 301 .SS Program source
 302 \&
 303 .EX
 304 #include <unistd.h>
 305 #include <signal.h>
 306 #include <stdio.h>
 307 #include <malloc.h>
 308 #include <stdlib.h>
 309 #include <errno.h>
 310 #include <sys/mman.h>
 311
 312 #define handle_error(msg) \e
 313     do { perror(msg); exit(EXIT_FAILURE); } while (0)
 314
 315 static char *buffer;
 316
 317 static void
 318 handler(int sig, siginfo_t *si, void *unused)
 319 {
 320     /* Note: calling printf() from a signal handler is not safe
 321        (and should not be done in production programs), since
 322        printf() is not async\-signal\-safe; see signal-safety(7).
 323        Nevertheless, we use printf() here as a simple way of
 324        showing that the handler was called. */
 325
 326     printf("Got SIGSEGV at address: 0x%lx\en",
 327             (long) si\->si_addr);
 328     exit(EXIT_FAILURE);
 329 }
 330
 331 int
 332 main(int argc, char *argv[])
 333 {
 334     char *p;
 335     int pagesize;
 336     struct sigaction sa;
 337
 338     sa.sa_flags = SA_SIGINFO;
 339     sigemptyset(&sa.sa_mask);
 340     sa.sa_sigaction = handler;
 341     if (sigaction(SIGSEGV, &sa, NULL) == \-1)
 342         handle_error("sigaction");
 343
 344     pagesize = sysconf(_SC_PAGE_SIZE);
 345     if (pagesize == \-1)
 346         handle_error("sysconf");
 347
 348     /* Allocate a buffer aligned on a page boundary;
 349        initial protection is PROT_READ | PROT_WRITE */
 350
 351     buffer = memalign(pagesize, 4 * pagesize);
 352     if (buffer == NULL)
 353         handle_error("memalign");
 354
 355     printf("Start of region:        0x%lx\en", (long) buffer);
 356
 357     if (mprotect(buffer + pagesize * 2, pagesize,
 358                 PROT_READ) == \-1)
 359         handle_error("mprotect");
 360
 361     for (p = buffer ; ; )
 362         *(p++) = \(aqa\(aq;
 363
 364     printf("Loop completed\en");     /* Should never happen */
 365     exit(EXIT_SUCCESS);
 366 }
 367 .EE
 368 .SH SEE ALSO
 369 .BR mmap (2),
 370 .BR sysconf (3),
 371 .BR pkeys (7)