linux-kernel - Re: [PATCH v5 02/10] x86: Compile-time asm code validation

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Date:	Wed, 10 Jun 2015 12:53:20 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Michal Marek <mmarek@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	live-patching@...r.kernel.org, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 02/10] x86: Compile-time asm code validation

On Wed, Jun 10, 2015 at 10:21:36AM -0700, Andy Lutomirski wrote:
> On Jun 10, 2015 5:07 AM, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
> >
> > Add a new CONFIG_ASM_VALIDATION option which adds an asmvalidate host
> > tool which runs on every compiled .S file.  Its goal is to enforce sane
> > rules on all asm code, so that stack debug metadata (frame/back chain
> > pointers and/or DWARF CFI metadata) can be made reliable.
> >
> > It enforces the following rules:
> >
> > 1. Each callable function must be annotated with the ELF STT_FUNC type.
> >    This is typically done using the ENTRY/ENDPROC macros.  If
> >    asmvalidate finds a return instruction outside of a function, it
> >    flags an error, since that usually indicates callable code which
> >    should be annotated accordingly.
> >
> > 2. Each callable function must never leave its own bounds (i.e. with a
> >    jump to outside the function) except when returning.
> 
> Won't that break with sibling/tail calls?

Yes, asmvalidate will flag a warning for tail calls.

> GCC can generate those, and the ia32_ptregs_common label is an example
> of such a thing.
> 
> I'd rather have the script understand tail calls and possibly require
> that ia32_ptregs_common have a dummy frame pointer save in front
> before the label if needed.

Why do you prefer tail calls there?  See patch 3 for how I handled that
for ia32_ptregs_common (I duplicated the code with macros).

I think adding support for tail calls in the tooling would be tricky.
So I'm just trying to figure out if there's a good reason to keep them.

-- 
Josh
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