Re: i386 ABI and the stack

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

On Fri, 2006-06-23 at 22:43 -0400, Albert Cahalan wrote:
> On 6/23/06, Linus Torvalds <torvalds@osdl.org> wrote:
> > On Fri, 23 Jun 2006, H. Peter Anvin wrote:
> > > >
> > > > The x86-64 ABI has a 128-byte(*) zone that is safe from signals etc, so you
> > > > can use a small amount of stack below the stackpointer safely. Not so on
> > > > x86.
> > >
> > > Adding a small redzone like this to i386 would be easy, though -- just drop
> > > the stack pointer by that much when creating a signal frame.  128 bytes isn't
> > > enough to interfere with libraries.
> >
> > However, any binaries created with that in mind would be
> > buggy-by-definition on older kernels, so I don't think it's worth it.
> 
> Since gcc-2.96 would access 256 bytes below the stack pointer
> (according to the valgrind man page), the kernel needs to allow
> for this in signal handlers anyway.

only a handful buggy editions of that compiler did in a few corner
cases. And they were really buggy, and they were corner cases. Nobody
should be using a compiler like that; and nobody is expected to compile
software with a broken version of that compiler (iirc the window in
which it was broken was really small). There is a limit to userspace
brokenness that the kernel should work around. This imo is on the other
side of the line.