On Sun, Aug 14, 2016 at 08:50:57AM -0400, Brian Gerst wrote: > On Sun, Aug 14, 2016 at 3:52 AM, Andy Lutomirski <l...@amacapital.net> wrote: > > On Fri, Aug 12, 2016 at 7:29 AM, Josh Poimboeuf <jpoim...@redhat.com> 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
