Re: [x86] Access off the bottom of stack causes a segfault?

[Davide Libenzi <davidel@xmailserver.org>]

On Tue, 14 Oct 2003, Chris Lattner wrote:

> > > Generated code:
> > >         .intel_syntax
> > > ...
> > > main:
> > >         mov DWORD PTR [%ESP - 16004], %EBP    # Save EBP to stack
> >                          ^^^^^^^^^^^^
> >
> > Yes, this is the problem (even Windows does that IIRC).
>
> Ok, I realize what's going on here.  The question is, why does the linux
> kernel consider this to be a bug?  Where (in the X86 specs) is it
> documented that it's illegal to access off the bottom of the stack?
>
> My compiler does a nice leaf function optimization where it does not even
> bother to adjust the stack for leaf functions, which eliminates the adds
> and subtracts entirely from these (common) functions.  This completely
> invalidates the optimization.
>
> Since I'm going to have to live with lots of preexisting kernels, it looks
> like I'm going to have to disable it entirely, which is disappointing.
> I'm still curious though why this is thought to be illegal.

I don't know, I didn't code it :) I discovered it a few months ago while I
was doing some portable code to measure the maximum stack growth of our
software. I was doing something like:

	memset(%esp - SAFE, 'S', SIZE);

and it was puking on me on both Linux and Windows. An alloca+memset solved
the problem in both environments.



- Davide