linux-kernel - Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in aesni-intel

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Message-ID: <20150720180006.GD28075@treble.redhat.com>
Date:	Mon, 20 Jul 2015 13:00:06 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, X86 ML <x86@...nel.org>,
	live-patching@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in
 aesni-intel_asm.S

On Mon, Jul 20, 2015 at 07:21:24PM +0200, Ingo Molnar wrote:
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Mon, Jul 20, 2015 at 09:56:11AM +0200, Ingo Molnar wrote:
> > I should point out that there are still a few cases where the more granular 
> > FRAME/ENDFRAME and ENTRY/ENDPROC macros would still be needed.
> > 
> > For example, if the function ends with a jump instead of a ret.  If the
> > jump is a sibling call, the code would look like:
> > 
> > FUNCTION_ENTRY(func)
> > 	...
> > 	ENDFRAME
> > 	jmp another_func
> > ENDPROC(func)
> > 
> > 
> > Or if it's a jump within the function to an internal ret:
> > 
> > FUNCTION_ENTRY(func)
> > 	...
> > 1:	...
> > 	ENDFRAME
> > 	ret
> > 2:	...
> > 	jmp 1b
> > ENDPROC(func)
> > 
> > 
> > Or if it jumps to some shared code before returning:
> > 
> > FUNCTION_ENTRY(func_1)
> > 	...
> > 	jmp common_return
> > ENDPROC(func_1)
> > 
> > FUNCTION_ENTRY(func_2)
> > 	...
> > 	jmp common_return
> > ENDPROC(func_2)
> > 
> > common_return:
> > 	...
> > 	ENDFRAME
> > 	ret
> > 
> > 
> > So in some cases we'd still need the more granular macros, unless we
> > decided to make special macros for these cases as well.
> 
> Ok, I see how the naming scheme I proposed won't work with all that very well, but 
> I'd still suggest using consistently named patterns.
> 
> Let me suggest yet another approach. How about open-coding something like this:
> 
>  FUNCTION_START(func)
> 
> 	push_bp
> 	mov_sp_bp
> 
> 	...
> 
> 	pop_bp
> 	ret
> 
>  FUNCTION_END(func)
> 
> This is just two easy things:
> 
>  - a redefine of the FUNCTION_ENTRY and ENDPROC names
> 
>  - the introduction of three quasi-mnemonics: push_bp, mov_sp_bp, pop_bp - which 
>    all look very similar to a real frame setup sequence, except that we can easily 
>    make them go away in the !CONFIG_FRAME_POINTERS case.
> 
> The advantage of this approach would be:
> 
>  - it looks pretty 'natural' and very close to how the real disassembly looks
>    like in CONFIG_FRAME_POINTERS=y kernels. So while it's not as compact as some 
>    of the other variants, it's close to what the real instruction sequence looks 
>    like and that is a positive quality in itself.
> 
>  - it also makes it apparent 'on sight' that it's probably a bug to have
>    unbalanced push/pop sequences in a regular function, to any reasonably alert 
>    assembly coder.
> 
>  - if we ever unsupport framepointer kernels in the (far far) future, we can get
>    rid of all lines with those 3 mnemonics and be done with it.
> 
>  - it's finegrained enough so that we can express all the special function/tail
>    variants you listed above.
> 
> What do you think?

I agree that the edge cases make FUNCTION_ENTRY and FUNCTION_RETURN less
attractive.  Slowly we are circling around to where we started :-)

Personally, I prefer FRAME/ENDFRAME instead of push_bp/mov_sp_bp/pop_bp,
because it more communicates *what* it's doing rather than how.  IMO,
it's easier to grok with a quick glance.

> I'd still keep existing frame setup functionality and names and only use these in 
> fixes, new code and new annotations - and do a full rename and cleanup once the 
> dust has settled.

That sounds good.

-- 
Josh
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