Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack

[Linus Torvalds]

On Wed, Jun 16, 2021 at 1:00 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> And even for debugging, I think it would be both easier and cheaper to
> just add a magic word to the entry stack instead.

IOW, just add a

      unsigned long magic;

to "struct switch_stack", and then make the stack switch code push that value.

That would be cheap enough to be just unconditional, but you could
make it depend on a debug config option too, of course.

It helps if 'xyz' is some constant that is easyish to generate. It
might not be a constant - maybe it could be the address of that
'magic' field itself, so you'd just generate it with

    stq $r,($r)

and it would be equally easy to just validate at lookup for that WARN_ON_ONCE():

    WARN_ON_ONCE(switch_stack->magic != (unsigned long)&switch_stack->magic);

or whatever.

It's for debugging, not security. So it doesn't have to be some kind
of super-great magic number, just something easy to generate and check
(that isn't a common value like "0" that trivially exist on the stack
anyway).

             Linus