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

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Date:	Wed, 10 Jun 2015 09:32:09 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
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>,
	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

On Wed, Jun 10, 2015 at 04:11:04PM +0200, Andi Kleen wrote:
> > In most cases there are ways to keep the optimizations.  For example:
> > 
> > - grow the function bounds to keep the jump internal
> 
> So you mean moving it after the ret? That still means icache bloat.

No, in most cases it just means changing the ELF annotations.  See patch
9 for an example.

> > - 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?

> > Also note that these rules only affect _callable_ functions, so the
> > entry code and other non-function asm code can still be a pile of
> > spaghetti (though I think Andy is working on improving that).
> 
> Thank you for your kind words.

Don't like spaghetti? :-)

> > > In fact even gcc with the right options can generate code that violates
> > > this. Standard Linux constructions, such as exception handling,
> > > also violate this.
> > >
> > > If your tool needs that your tool is broken.
> > 
> > This tool only validates asm code, so I don't see how whatever gcc does
> > is relevant.
> 
> Whoever needs it would need it everywhere, right? If it's not needed
> for gcc then it shouldn't be needed for assembler code either.

Well, I don't see how that's really a logical conclusion.  But we're
probably being too vague here... Do you have any examples where you
really need to jump outside of a callable function?

If we ignore C++, then 99% of the time, C functions are self-contained.
The only exception I can think of is for switch statements, which
sometimes have an external jump table.

-- 
Josh
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