Re: Why is irq_stack_union a union?

From: Andy Lutomirski
Date: Tue Sep 01 2015 - 17:42:53 EST

On Mon, Aug 31, 2015 at 8:55 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Mon, Aug 31, 2015 at 6:01 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mon, Aug 31, 2015 at 2:47 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>> On Mon, Aug 31, 2015 at 5:00 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>> Why not just a struct? Also, why is this all tangled up in gsbase initialization?
>>> It has to do with the fact that the GCC stackprotector is hardcoded to
>>> look for the canary at %gs:40. Since we also use %gs for the percpu
>>> segment, we have to make sure that the canary is placed at the start
>>> of the percpu section. Overlaying it onto the bottom of the IRQ stack
>>> and was the most convenient way to do it, with a side benefit that
>>> overflowing the stack will trip the canary.
>> Would a struct not make more sense, then?
> Why would it? A union shows the overlay better.

I guess you're thinking of it as an overlay and I'm thinking of it as
the layout being the IRQ stack directly above the stack canary.


> --
> Brian Gerst

Andy Lutomirski
AMA Capital Management, LLC
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