mprotect — control allowable accesses to a region of memory
#include <sys/mman.h>
int
mprotect( |
const void * | addr, |
| size_t | len, | |
| int | prot); |
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_NONEThe memory cannot be accessed at all.
PROT_READThe memory can be read.
PROT_WRITEThe memory can be written to.
PROT_EXECThe 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.
On success, mprotect()
returns zero. On error, −1 is returned, and
errno is set appropriately.
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.
The memory cannot be accessed.
addr is not
a valid pointer, or not a multiple of PAGESIZE.
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.
#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);
}
SVr4, POSIX.1-2001. POSIX says that mprotect() can be used only on regions of
memory obtained from mmap(2).
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.
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