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SubjectRe: Understanding Linux addr space, malloc, and heap
From
Date
On Fri, 2005-10-21 at 11:11 -0400, Vincent W. Freeh wrote:
> Arjan van de Ven wrote:
> > On Fri, 2005-10-21 at 09:45 -0400, Vincent W. Freeh wrote:
> >
> >>Thanks for your quick response. It basically confirmed that I observed
> >>what I thought I did. However, I am no closer to solving my problem. I
> >>cannot mprotect data that I malloc beyond the first 65 pages.
> >
> >
> > you can't mprotect malloc() memory period ..
>
> Actually, I can and do. Simple program at end.
>
> >
> >> Why is
> >>that? Can that be fixed? Second, why does mprotect silently fail? I
> >>could live with it failing--but I cannot deal with a call the "works"
> >>but doesn't work.
> >
> >
> > need more info :)
> >
>
> I call mprotect and it return 0--meaning it succeeded. But the
> permissions on the page remain rw. So it fails to change the
> permissions, but doesn't give any indication of this.
>
> Thanks,
> vince.
>
> ------------------
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/mman.h>
> #include <unistd.h>
>
> int main(int argc, char *argv[])
> {
> void *p;
> int pgsize = getpagesize();
>
> p = malloc(1024);
> mprotect((void*)((unsigned)p & ~(pgsize-1)), 1024, PROT_NONE);
> printf("\t*p = %d\n", *(int *)p);
> return 0;
> }

This program is completely screwed. Read the mprotect man page in
particular the examples section has an example of how to do your program
correctly.

Best regards,

Anton
--
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

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