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. 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
