linux-kernel - Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation

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Message-ID: <20150610150412.GR19417@two.firstfloor.org>
Date:	Wed, 10 Jun 2015 17:04:12 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation

> > > - duplicate the destination code inside the function
> > > - convert the jump to a call
> > 
> > That all won't work for a lot of cases.
> 
> Hm, could you give an example?

Just a standard *_user exception handler.

> 
> Well, I don't see how that's really a logical conclusion.  

What's special about assembler code?

> But we're
> probably being too vague here... Do you have any examples where you
> really need to jump outside of a callable function?

It's not needed, but it's an optimization to optimize icache usage.
It is optional (-freorder-blocks-and-partition)

In this case gcc splits the function into two (hot and cold)

It's actually a nice optimization and it would be sad from stopping
the kernel from using it.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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