Re: [PATCH v3 45/51] x86: remove 64-byte gap at end of irq stack

[Josh Poimboeuf]

On Sun, Aug 14, 2016 at 08:50:57AM -0400, Brian Gerst wrote:
> On Sun, Aug 14, 2016 at 3:52 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Fri, Aug 12, 2016 at 7:29 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >> There has been a 64-byte gap at the end of the irq stack for at least 12
> >> years.  It predates git history, and I can't find any good reason for
> >> it.  Remove it.  What's the worst that could happen?
> >
> > I can't think of any reason this would matter.
> >
> > For that matter, do you have any idea why irq_stack_union is a union
> > or why we insist on sticking it at %gs:0?  Sure, the *canary* needs to
> > live at a fixed offset (because GCC is daft, sigh), but I don't see
> > what that has to do with the rest of the IRQ stack.
> >
> > --Andy
> 
> Because the IRQ stack requires page alignment so it was convenient to
> put it at the start of the per-cpu area.  I don't think at the time I
> wrote this there was specific support for page-aligned objects in
> per-cpu memory.  Since stacks grow down, it was tolerable to reserve a
> few bytes at the bottom for the canary.

Hm.  Sounds like another good opportunity for a cleanup (though it's
well outside the scope of this patch set).

> What would be great is if we could leverage the new GCC plugin tools
> to reimplement stack protector in a manner that is more compatible
> with the kernel environment.  It would make the stack canary a true
> per-cpu variable instead of the hard-coded TLS-based location it is
> now.  That would make 64-bit be able to use normal delta per-cpu
> offsets instead of zero-based, and would allow 32-bit to always do
> lazy GS.
> 
> --
> Brian Gerst

-- 
Josh