Manpage of MPROTECT

MPROTECT

Section: Linux Programmer's Manual (2)
Updated: 2015-07-23
Index
 

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 [addraddr+len-1]. addrmust 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 SIGSEGVsignal for the process.

protis either PROT_NONEor 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 errnois set appropriately.  

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 EFAULTwas incorrectly produced for these cases.)
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. (For example, making the protection of a range PROT_READin the middle of a region currently protected as PROT_READ|PROT_WRITEwould 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 unspecified 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_EXEChas any effect different from PROT_READdepends on processor architecture, kernel version, and process state. If READ_IMPLIES_EXECis set in the process's personality flags (see personality(2)), specifying PROT_READwill implicitly add PROT_EXEC.

On some hardware architectures (e.g., i386), PROT_WRITEimplies 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_WRITEhas been set, and must not allow any access if PROT_NONEhas 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.outStart 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++) = aqaaq;

    printf("Loop completed\n");     /* Should never happen */
    exit(EXIT_SUCCESS);
}
 

SEE ALSO

mmap(2), sysconf(3)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
EXAMPLE
Program source
SEE ALSO

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Time: 22:27:43 GMT, June 20, 2016