.\" Copyright (C) 2007 Michael Kerrisk .\" and Copyright (C) 1995 Michael Shields . .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Modified 1996-10-22 by Eric S. Raymond .\" Modified 1997-05-31 by Andries Brouwer .\" Modified 2003-08-24 by Andries Brouwer .\" Modified 2004-08-16 by Andi Kleen .\" 2007-06-02, mtk: Fairly substantial rewrites and additions, and .\" a much improved example program. .\" .TH mprotect 2 2023-03-30 "Linux man-pages 6.04" .SH NAME mprotect, pkey_mprotect \- set protection on a region of memory .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .PP .BI "int mprotect(void " addr [. len "], size_t " len ", int " prot ); .PP .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .PP .BI "int pkey_mprotect(void " addr [. len "], size_t " len ", int " prot ", int " pkey ");" .fi .SH DESCRIPTION .BR mprotect () changes the access protections for the calling process's memory pages containing any part of the address range in the interval [\fIaddr\fP,\ \fIaddr\fP+\fIlen\fP\-1]. .I addr must be aligned to a page boundary. .PP If the calling process tries to access memory in a manner that violates the protections, then the kernel generates a .B SIGSEGV signal for the process. .PP .I prot is a combination of the following access flags: .B PROT_NONE or a bitwise-or of the other values in the following list: .TP .B PROT_NONE The memory cannot be accessed at all. .TP .B PROT_READ The memory can be read. .TP .B PROT_WRITE The memory can be modified. .TP .B PROT_EXEC The memory can be executed. .TP .BR PROT_SEM " (since Linux 2.5.7)" The memory can be used for atomic operations. This flag was introduced as part of the .BR futex (2) implementation (in order to guarantee the ability to perform atomic operations required by commands such as .BR FUTEX_WAIT ), but is not currently used in on any architecture. .TP .BR PROT_SAO " (since Linux 2.6.26)" .\" commit aba46c5027cb59d98052231b36efcbbde9c77a1d .\" commit ef3d3246a0d06be622867d21af25f997aeeb105f The memory should have strong access ordering. This feature is specific to the PowerPC architecture (version 2.06 of the architecture specification adds the SAO CPU feature, and it is available on POWER 7 or PowerPC A2, for example). .PP Additionally (since Linux 2.6.0), .I prot can have one of the following flags set: .TP .\" mm/mmap.c: .\" vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) | .\" mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC; .\" And calc_vm_flag_bits converts only GROWSDOWN/DENYWRITE/LOCKED. .B PROT_GROWSUP Apply the protection mode up to the end of a mapping that grows upwards. (Such mappings are created for the stack area on architectures\[em]for example, HP-PARISC\[em]that have an upwardly growing stack.) .\" The VMA is one that was marked with VM_GROWSUP by the kernel .\" when the stack was created. Note that (unlike VM_GROWSDOWN), .\" there is no mmap() flag (analogous to MAP_GROWSDOWN) for .\" creating a VMA that is marked VM_GROWSUP. .TP .B PROT_GROWSDOWN Apply the protection mode down to the beginning of a mapping that grows downward (which should be a stack segment or a segment mapped with the .B MAP_GROWSDOWN flag set). .PP Like .BR mprotect (), .BR pkey_mprotect () changes the protection on the pages specified by .I addr and .IR len . The .I pkey argument specifies the protection key (see .BR pkeys (7)) to assign to the memory. The protection key must be allocated with .BR pkey_alloc (2) before it is passed to .BR pkey_mprotect (). For an example of the use of this system call, see .BR pkeys (7). .SH RETURN VALUE On success, .BR mprotect () and .BR pkey_mprotect () return zero. On error, these system calls return \-1, and .I errno is set to indicate the error. .SH ERRORS .TP .B EACCES The memory cannot be given the specified access. This can happen, for example, if you .BR mmap (2) a file to which you have read-only access, then ask .BR mprotect () to mark it .BR PROT_WRITE . .TP .B EINVAL \fIaddr\fP is not a valid pointer, or not a multiple of the system page size. .TP .B EINVAL .RB ( pkey_mprotect ()) \fIpkey\fP has not been allocated with .BR pkey_alloc (2) .TP .B EINVAL Both .B PROT_GROWSUP and .B PROT_GROWSDOWN were specified in .IR prot . .TP .B EINVAL Invalid flags specified in .IR prot . .TP .B EINVAL (PowerPC architecture) .B PROT_SAO was specified in .IR prot , but SAO hardware feature is not available. .TP .B ENOMEM Internal kernel structures could not be allocated. .TP .B ENOMEM Addresses in the range .RI [ addr , .IR addr + len \-1] are invalid for the address space of the process, or specify one or more pages that are not mapped. (Before Linux 2.4.19, the error .B EFAULT was incorrectly produced for these cases.) .TP .B ENOMEM Changing the protection of a memory region would result in the total number of mappings with distinct attributes (e.g., read versus read/write protection) exceeding the allowed maximum. .\" I.e., the number of VMAs would exceed the 64 kB maximum (For example, making the protection of a range .B PROT_READ in the middle of a region currently protected as .B PROT_READ|PROT_WRITE would result in three mappings: two read/write mappings at each end and a read-only mapping in the middle.) .SH VERSIONS .\" SVr4 defines an additional error .\" code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's. POSIX says that the behavior of .BR mprotect () is unspecified if it is applied to a region of memory that was not obtained via .BR mmap (2). .PP On Linux, it is always permissible to call .BR mprotect () on any address in a process's address space (except for the kernel vsyscall area). In particular, it can be used to change existing code mappings to be writable. .PP Whether .B PROT_EXEC has any effect different from .B PROT_READ depends on processor architecture, kernel version, and process state. If .B READ_IMPLIES_EXEC is set in the process's personality flags (see .BR personality (2)), specifying .B PROT_READ will implicitly add .BR PROT_EXEC . .PP On some hardware architectures (e.g., i386), .B PROT_WRITE implies .BR PROT_READ . .PP POSIX.1 says that an implementation may permit access other than that specified in .IR prot , but at a minimum can allow write access only if .B PROT_WRITE has been set, and must not allow any access if .B PROT_NONE has been set. .PP Applications should be careful when mixing use of .BR mprotect () and .BR pkey_mprotect (). On x86, when .BR mprotect () is used with .I prot set to .B PROT_EXEC a pkey may be allocated and set on the memory implicitly by the kernel, but only when the pkey was 0 previously. .PP On systems that do not support protection keys in hardware, .BR pkey_mprotect () may still be used, but .I pkey must be set to \-1. When called this way, the operation of .BR pkey_mprotect () is equivalent to .BR mprotect (). .SH STANDARDS .TP .BR mprotect () POSIX.1-2008. .TP .BR pkey_mprotect () Linux. .SH HISTORY .TP .BR mprotect () POSIX.1-2001, SVr4. .TP .BR pkey_mprotect () Linux 4.9, glibc 2.27. .SH NOTES .SH EXAMPLES .\" sigaction.2 refers to this example The program below demonstrates the use of .BR mprotect (). The program allocates four pages of memory, makes the third of these pages read-only, and then executes a loop that walks upward through the allocated region modifying bytes. .PP An example of what we might see when running the program is the following: .PP .in +4n .EX .RB "$" " ./a.out" Start of region: 0x804c000 Got SIGSEGV at address: 0x804e000 .EE .in .SS Program source \& .\" SRC BEGIN (mprotect.c) .EX #include #include #include #include #include #include #define handle_error(msg) \e do { perror(msg); exit(EXIT_FAILURE); } while (0) static char *buffer; static void handler(int sig, siginfo_t *si, void *unused) { /* Note: calling printf() from a signal handler is not safe (and should not be done in production programs), since printf() is not async\-signal\-safe; see signal\-safety(7). Nevertheless, we use printf() here as a simple way of showing that the handler was called. */ printf("Got SIGSEGV at address: %p\en", si\->si_addr); exit(EXIT_FAILURE); } int main(void) { int pagesize; struct sigaction sa; sa.sa_flags = SA_SIGINFO; sigemptyset(&sa.sa_mask); sa.sa_sigaction = handler; if (sigaction(SIGSEGV, &sa, NULL) == \-1) handle_error("sigaction"); pagesize = sysconf(_SC_PAGE_SIZE); if (pagesize == \-1) handle_error("sysconf"); /* Allocate a buffer aligned on a page boundary; initial protection is PROT_READ | PROT_WRITE. */ buffer = memalign(pagesize, 4 * pagesize); if (buffer == NULL) handle_error("memalign"); printf("Start of region: %p\en", buffer); if (mprotect(buffer + pagesize * 2, pagesize, PROT_READ) == \-1) handle_error("mprotect"); for (char *p = buffer ; ; ) *(p++) = \[aq]a\[aq]; printf("Loop completed\en"); /* Should never happen */ exit(EXIT_SUCCESS); } .EE .\" SRC END .SH SEE ALSO .BR mmap (2), .BR sysconf (3), .BR pkeys (7)