mprotect

Sommaire

NAME

mprotect - control allowable accesses to a region of memory

SYNOPSIS

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

DESCRIPTION

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.

RETURN VALUE

On success, mprotect() returns zero. On error, -1 is returned, and errno is 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.
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. Or: addresses in the range [addr, addr+len] are invalid for the address space of the process, or specify one or more pages that are not mapped.

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);
 }

CONFORMING TO

SVr4, POSIX.1-2001. 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.

SEE ALSO

mmap(2)