Re: Bug Report: Dereferencing a bad pointer

[Richard B. Johnson]

On Thu, 8 Nov 2001, David Chandler wrote:

> Benjamin LaHaise wrote:
> 
> > On Wed, Nov 07, 2001 at 06:23:13PM -0500, David Chandler wrote:
> > > The following one-line C program, when compiled by gcc 2.96 without
> > > optimization, should produce a SIGSEGV segmentation fault (on a machine
> > > with 3 or less gigabytes of virtual memory, at least):
> > >
> > >         int main() { int k  = *(int *)0xc0000000; }
> > >

This may not necessarily produce a seg-fault! If this virtual
address is mapped within the current process (.bss .stack, etc.),
It's perfectly all right to write to it although you probably
broke malloc() by doing it. The actual value of the number in
the pointer depends upon PAGE_OFFSET and other kernel variables.
If you change the kernel, this number may change. It has nothing
to do with the size of virtual address space, really.

Script started on Thu Nov  8 10:44:03 2001
# cat >xxx.c
#include <stdio.h>
int bss;
int data = 0x100;
const char cons[]="X";

main()
{
   int stack;

   printf("main() = %p\n", main);
   printf("stack  = %p\n", &stack);
   printf("const  = %p\n", cons);
   printf("  data = %p\n", &data);
   printf("   bss = %p\n", &bss);
   return 0;

}

# gcc -o xxx xxx.c
# ./xxx
main() = 0x80484cc
stack  = 0xbffff6fc
const  = 0x8048584
  data = 0x80495d4
   bss = 0x80496b8
# exit
exit

Script done on Thu Nov  8 10:44:27 2001

All this stuff you "own". You can write to most all of it because
the kernel has allocated it for you. Whether or not 'const' is
really read-only is "implementation dependent".

In your case, it looks as though you scribbled over the top of
your user stack, in some harmless place.

You cannot presume that a program that doesn't seg-fault is
memory-error free. Protection is in pages, not bytes, and you
already own a lot of address-space that you may think that
you don't. FYI, if you allocate a lot of memory using malloc(),
it sets the break address to acquire more memory. Then if you
free that memory, it does not necessarily give back the memory.

You may be able to write to freed memory without a seg-fault.
However, subsequent calls to malloc() may fail because you have
ticked-off malloc() and it's gonna get even. 

Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

    I was going to compile a list of innovations that could be
    attributed to Microsoft. Once I realized that Ctrl-Alt-Del
    was handled in the BIOS, I found that there aren't any.