linux-kernel - Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for __arch

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Date:	Tue, 21 Jul 2015 17:13:12 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Michal Marek <mmarek@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, live-patching@...r.kernel.org,
	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for __arch_hweight*()

On Jul 18, 2015 9:13 PM, "Borislav Petkov" <bp@...en8.de> wrote:
>
> On Sat, Jul 18, 2015 at 10:57:14AM -0500, Josh Poimboeuf wrote:
> > Currently, when stackvalidate sees an ALTERNATIVE, it assumes that
> > either code path is possible, so it follows both paths in parallel.
> >
> > If I understand right, you're proposing that stackvalidate should only
> > follow the POPCNT path and never follow the !POPCNT path?
>
> Actually, you don't even need to follow the POPCNT case either because
> it is a single instruction - no stack operations there.
>
> So yeah, either that or special-case the case where the original insn is
> CALL and the replacement is a POPCNT and ignore those CALL locations.
>
> The advantage is that the burden is put on the tool and not by adding
> markers to kernel code paths.
>
> > In general, I agree, and I like the original patch much better.  IMO, it
> > achieved the goal of keeping the kernel code clean, while fixing the
> > frame pointer bug.
>
> And I think that in that case, adding that rSP dependency is too much
> because even though it fixes the "bug", it is very very unlikely any
> stack trace will have __sw_hweight* in it for reasons pointed out
> earlier and also because those functions can't fail and they get
> integral types as args which can't fail when deref-fing either. And even
> if they do, they don't call any other functions so rIP pointing to them
> is already enough.

Enough for oopses, perhaps, but maybe not enough for perf.

It sounds like you want CFI unwinding :)

--Andy
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