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GNU/Linux |
CentOS 5.4 |
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mprotect(2) |
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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:
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PROT_NONE |
The memory cannot be accessed at all. |
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PROT_READ |
The memory can be read. |
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PROT_WRITE |
The memory can be written to. |
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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.
On success, mprotect() returns zero. On error, −1 is returned, and errno is set appropriately.
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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. | ||
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EFAULT |
The memory cannot be accessed. | ||
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EINVAL |
addr is not a valid pointer, or not a multiple of PAGESIZE. | ||
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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. |
#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.
mmap(2)
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mprotect(2) | |