Clearly, it was a mistake to post that code. I had no idea so many
people would point out the bleeding obvious.
Here is a more elaborate version--that does the same thing, but more
lines of code. In it malloc'd memory is mprotect'd. The program
generates a SIGSEGV, a page fault.
----------------
#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);
}
Arjan van de Ven wrote:
But I can't mprotect the 66th page I malloc. And mprotect fails SILENTLY!
I'm not convinced it does that.. not until the bugs are out of the
code.... since right now it mprotects the wrong stuff, which sometimes
overlaps with what you malloced, sometimes not.
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