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

[Richard B. Johnson]

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.
>
> -Chris

Any interrupt causes the return address to be pushed onto the
stack. This will overwrite any data you've put there. In principle,
in user-mode, you can write below the stack-pointer because an
interrupt uses the kernel stack. However, your data will still
get corrupted by signal delivery, etc. So as to not corrupt your
user-mode stack, the kernel calls your signal code, just like
a nested call. This means the new return address will be below
the non-signal user-mode stack-pointer value. This will surely
corrupt anything you have written below the stack-pointer.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.


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