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

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Date:	Sun, 19 Jul 2015 06:12:35 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, x86@...nel.org,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for
 __arch_hweight*()

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.

So even if we're not 100% correct wrt stack traces in this case, I think
that's ok.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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