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

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Date:	Wed, 10 Jun 2015 10:21:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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 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.

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.

--Andy
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