SIMPLE SOLUTIONS

MPROTECT(2) - man page online | system calls

Set protection on a region of memory.

Chapter
2015-07-23
MPROTECT(2)                         Linux Programmer's Manual                         MPROTECT(2)

NAME mprotect - set protection on a region of memory
SYNOPSIS #include <sys/mman.h> int mprotect(void *addr, size_t len, int prot);
DESCRIPTION mprotect() changes protection for the calling process's memory page(s) containing any part of the address range in the interval [addr, addr+len-1]. addr must be aligned to a page boundary. If the calling process tries to access memory in a manner that violates the protection, then the kernel generates a SIGSEGV signal for the process. prot is either PROT_NONE or a bitwise-or of the other values in the following list: PROT_NONE The memory cannot be accessed at all. PROT_READ The memory can be read. PROT_WRITE The memory can be modified. PROT_EXEC The memory can be executed.
RETURN VALUE On success, mprotect() returns zero. On error, -1 is returned, and errno is set appropri‐ ately.
ERRORS EACCES The memory cannot be given the specified access. This can happen, for example, if you mmap(2) a file to which you have read-only access, then ask mprotect() to mark it PROT_WRITE. EINVAL addr is not a valid pointer, or not a multiple of the system page size. ENOMEM Internal kernel structures could not be allocated. ENOMEM Addresses in the range [addr, addr+len-1] are invalid for the address space of the process, or specify one or more pages that are not mapped. (Before kernel 2.4.19, the error EFAULT was incorrectly produced for these cases.) ENOMEM Changing the protection of a memory region would result in the total number of map‐ pings with distinct attributes (e.g., read versus read/write protection) exceeding the allowed maximum. (For example, making the protection of a range PROT_READ in the middle of a region currently protected as PROT_READ|PROT_WRITE would result in three mappings: two read/write mappings at each end and a read-only mapping in the middle.)
CONFORMING TO POSIX.1-2001, POSIX.1-2008, SVr4. POSIX says that the behavior of mprotect() is unspeci‐ fied if it is applied to a region of memory that was not obtained via mmap(2).
NOTES On Linux it is always permissible to call 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. Whether PROT_EXEC has any effect different from PROT_READ depends on processor architec‐ ture, kernel version, and process state. If READ_IMPLIES_EXEC is set in the process's personality flags (see personality(2)), specifying PROT_READ will implicitly add PROT_EXEC. On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ. POSIX.1 says that an implementation may permit access other than that specified in prot, but at a minimum can allow write access only if PROT_WRITE has been set, and must not allow any access if PROT_NONE has been set.
EXAMPLE The program below 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. An example of what we might see when running the program is the following: $ ./a.out Start of region: 0x804c000 Got SIGSEGV at address: 0x804e000 Program source #include <unistd.h> #include <signal.h> #include <stdio.h> #include <malloc.h> #include <stdlib.h> #include <errno.h> #include <sys/mman.h> #define handle_error(msg) \ do { perror(msg); exit(EXIT_FAILURE); } while (0) static char *buffer; static void handler(int sig, siginfo_t *si, void *unused) { printf("Got SIGSEGV at address: 0x%lx\n", (long) si->si_addr); exit(EXIT_FAILURE); } int main(int argc, char *argv[]) { char *p; 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: 0x%lx\n", (long) buffer); if (mprotect(buffer + pagesize * 2, pagesize, PROT_READ) == -1) handle_error("mprotect"); for (p = buffer ; ; ) *(p++) = 'a'; printf("Loop completed\n"); /* Should never happen */ exit(EXIT_SUCCESS); }
SEE ALSO mmap(2), sysconf(3)
COLOPHON This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2015-07-23 MPROTECT(2)
This manual Reference Other manuals
mprotect(2) referred by clsync(1) | duma(3) | libefence(3) | madvise(2) | madvise(2freebsd) | mincore(2freebsd) | minherit(2freebsd) | mmap(2) | mmap(2freebsd) | mprotect(2freebsd) | msync(2freebsd) | munmap(2freebsd) | posix_madvise(2freebsd) | prctl(2) | pthread_attr_setguardsize(3) | pthread_attr_setstack(3) | remap_file_pages(2) | seccomp(2) | shm_overview(7) | sigaction(2)
refer to mmap(2) | personality(2) | sysconf(3)