mprotect: ovládání povolených přístupů do oblasti paměti

Originální popis anglicky: mprotect - control allowable accesses to a region of memory

Návod, kniha: Linux Programmer's Manual

STRUČNĚ

#include <sys/mman.h>
 
int mprotect(const void *addr, size_t len, int prot);

POPIS / INSTRUKCE

The function mprotect specifies the desired protection for the memory page(s) containing part or all of the interval [ addr,addr+len-1]. If an access is disallowed by the protection given it, the program receives a SIGSEGV. prot is a bitwise-or of the following values:

PROT_NONE

The memory cannot be accessed at all.

PROT_READ

The memory can be read.

PROT_WRITE

The memory can be written to.

PROT_EXEC

The memory can contain executing code.

The new protection replaces any existing protection. For example, if the memory had previously been marked PROT_READ, and mprotect is then called with prot PROT_WRITE, it will no longer be readable.

NÁVRATOVÁ HODNOTA

On success, mprotect returns zero. On error, -1 is returned, and errno is set appropriately.

CHYBY / ERRORY

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.

EFAULT

The memory cannot be accessed.

EINVAL

addr is not a valid pointer, or not a multiple of PAGESIZE.

ENOMEM

Internal kernel structures could not be allocated.

EXAMPLE

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

#include <limits.h>    /* for PAGESIZE */
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif

int
main(void)
{
    char *p;
    char c;

    /* Allocate a buffer; it will have the default
       protection of PROT_READ|PROT_WRITE. */
    p = malloc(1024+PAGESIZE-1);
    if (!p) {
        perror("Couldn't malloc(1024)");
        exit(errno);
    }

    /* Align to a multiple of PAGESIZE, assumed to be a power of two */
    p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));

    c = p[666];         /* Read; ok */
    p[666] = 42;        /* Write; ok */

    /* Mark the buffer read-only. */
    if (mprotect(p, 1024, PROT_READ)) {
        perror("Couldn't mprotect");
        exit(errno);
    }

    c = p[666];         /* Read; ok */
    p[666] = 42;        /* Write; program dies on SIGSEGV */

    exit(0);
}

ODPOVÍDAJÍCÍ

SVr4, POSIX.1b (formerly POSIX.4). SVr4 defines an additional error code EAGAIN. The SVr4 error conditions don't map neatly onto Linux's. POSIX says that mprotect can be used only on regions of memory obtained from mmap(2).

NOTES

On Linux it is always legal to call mprotect on any address in a process' 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 is architecture and kernel version dependent.

SOUVISEJÍCÍ

mmap(2)