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

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Message-ID: <20150610153155.GB29724@treble.redhat.com>
Date:	Wed, 10 Jun 2015 10:31:55 -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 05:04:12PM +0200, Andi Kleen wrote:
> > > > - 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.

I'm afraid I don't follow.  Exception handlers don't work via jump
instructions, but rather via CPU exceptions.

Or are you talking about something else?

> > Well, I don't see how that's really a logical conclusion.  
> 
> What's special about assembler code?

Ok, I'll bite:

- it's human-generated
- it's much simpler
- it doesn't have stack metadata by default
- it has far fewer constraints

But I don't see this line of discussion going anywhere without any real
examples of why you need external jumps in asm functions...

> > 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.

Sorry if I wasn't clear, I was trying to ask for examples in kernel asm
code.

Are you suggesting that we implement this gcc optimization in kernel asm
code?

-- 
Josh
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