#include <sys/mman.h> int mprotect(void *addr, size_t len, int prot); int pkey_mprotect(void *addr, size_t len, int prot, int pkey);
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:
Like mprotect(), pkey_mprotect() changes the protection on the pages specified by addr and len. The pkey argument specifies the protection key (see pkeys(7)) to assign to the memory. The protection key must be allocated with pkey_alloc(2) before it is passed to pkey_mprotect(). For an example of the use of this system call, see pkeys(7).
pkey_mprotect() is a nonportable Linux extension.
Whether PROT_EXEC has any effect different from PROT_READ depends on processor architecture, 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.
Applications should be careful when mixing use of mprotect() and pkey_mprotect(). On x86, when mprotect() is used with prot set to PROT_EXEC a pkey is may be allocated and set on the memory implicitly by the kernel, but only when the pkey was 0 previously.
On systems that do not support protection keys in hardware, pkey_mprotect() may still be used, but pkey must be set to 0. When called this way, the operation of pkey_mprotect() is equivalent to mprotect().
The program below demonstrates the use of 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.
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
#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); }