Re: Understanding Linux addr space, malloc, and heap

[Arjan van de Ven <arjan@infradead.org>]

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.

Ok I meant in the "while adhering to the standard" :)


> 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 has a bug, the 1024 is wrong... what if your "p" point actually
spans 2 pages? 

but to have "some effect" even for malloc-falling-back-to-mmap..
just there's a bunch of collateral damage since you mprotect more than
just the memory you got from malloc. mprotect works on page size.. so if
p spans 2 pages (why wouldn't it ;) you mprotect either the wrong memory
(as in your example) or too much (eg both pages)...