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

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Date:	Wed, 10 Jun 2015 11:18:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Vojtech Pavlik <vojtech@...e.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	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 11:16 AM, Vojtech Pavlik <vojtech@...e.com> wrote:
> 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?  GCC can generate those, and
>> the ia32_ptregs_common label is an example of such a thing.
>
> It only validates hand-written assembly, so how it could?
>

It's a hand-written tail call.

--Andy
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