Re: objtool - what if I want to clobber rbp?

From: Jason A. Donenfeld
Date: Wed Nov 22 2017 - 05:23:53 EST

Hi Josh,

On Wed, Nov 22, 2017 at 4:31 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> - Make your feature conflict with CONFIG_FRAME_POINTER on x86_64. The
> ORC unwinder is now the default anyway for 4.15, and we renamed the
> configs, so most people will be actively switching to ORC.
> BTW, since CONFIG_FRAME_POINTER is no longer the default and is becoming
> deprecated, there has been some talk of disabling objtool with
> CONFIG_FRAME_POINTER. That would make your life easier. However I
> think we're not quite ready for that, so it might be a few more release
> cycles before that happens.

Fortunately my code won't be ready to be merged for a few release
cycles, so thanks for letting me know about future plans. I think my
patch set and these changes will synchronize nicely.

> - Add some ifdefs so your code only uses %rbp as a scratch register when
> - If one of the registers is used less often than the others, you could
> spill it to the stack. I know you said you need all the registers,
> but I'd be willing to review the code for ideas, if that would help.
> Sometimes it helps to get fresh eyes on the problem. We were able to
> fix this problem with all the existing crypto code without affecting
> performance measurably. We had to get creative with a few of those.

That actually could be interesting, if you're curious about looking at it.