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mprotect() - Unix, Linux System Call

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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:

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.


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


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.


#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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