Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: David Laight <David.Laight@aculab.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"mark.rutland@arm.com" <mark.rutland@arm.com>,
	"dvyukov@google.com" <dvyukov@google.com>,
	"seanjc@google.com" <seanjc@google.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"mbenes@suse.cz" <mbenes@suse.cz>,
	"llvm@lists.linux.dev" <llvm@lists.linux.dev>,
	"linux-toolchains@vger.kernel.org"
	<linux-toolchains@vger.kernel.org>,
	live-patching@vger.kernel.org
Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage
Date: Fri, 12 Nov 2021 10:33:36 +0100	[thread overview]
Message-ID: <YY408BW0phe9I1/o@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20211112015003.pefl656m3zmir6ov@treble>

On Thu, Nov 11, 2021 at 05:50:03PM -0800, Josh Poimboeuf wrote:

> Hm, I think there is actually a livepatch problem here.

I suspected as much, because I couldn't find any code dealing with it
when I looked in a hurry.. :/

> Some ideas to fix:

> c) Update the reliable stacktrace code to mark the stack unreliable if
>    it has a function with ".cold" in the name?

Why not simply match func.cold as func in the transition thing? Then
func won't get patched as long as it (or it's .cold part) is in use.
This seems like the natural thing to do.

If there are enough .cold functions, always reporting stacktrace as
unreliable will make progress hard, even though it might be perfectly
safe.

> e) Disable .cold optimization?

Yeah, lets not do that. That'll have me lobbying to kill KLP again
because it generates crap code.