linux-kernel - Re: [PATCH v10 03/20] x86/stackvalidate: Compile-time stack validation

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Message-ID: <20150821075449.GA10443@gmail.com>
Date:	Fri, 21 Aug 2015 09:54:49 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, 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>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	live-patching@...r.kernel.org
Subject: Re: [PATCH v10 03/20] x86/stackvalidate: Compile-time stack
 validation


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> +Why do we need stack validation?
> +--------------------------------
> +
> +Here are some of the benefits of validating stack metadata:
> +
> +a) More reliable stack traces for frame pointer enabled kernels
> +
> +   Frame pointers are used for debugging purposes.  They allow runtime
> +   code and debug tools to be able to walk the stack to determine the
> +   chain of function call sites that led to the currently executing
> +   code.
> +
> +   For some architectures, frame pointers are enabled by
> +   CONFIG_FRAME_POINTER.  For some other architectures they may be
> +   required by the ABI (sometimes referred to as "backchain pointers").
> +
> +   For C code, gcc automatically generates instructions for setting up
> +   frame pointers when the -fno-omit-frame-pointer option is used.
> +
> +   But for asm code, the frame setup instructions have to be written by
> +   hand, which most people don't do.  So the end result is that
> +   CONFIG_FRAME_POINTER is honored for C code but not for most asm code.
> +
> +   For stack traces based on frame pointers to be reliable, all
> +   functions which call other functions must first create a stack frame
> +   and update the frame pointer.  If a first function doesn't properly
> +   create a stack frame before calling a second function, the *caller*
> +   of the first function will be skipped on the stack trace.
> +
> +   The benefit of stackvalidate here is that it ensures that *all*
> +   functions honor CONFIG_FRAME_POINTER.  As a result, no functions will
> +   ever [*] be skipped on a stack trace.
> +
> +   [*] unless an interrupt or exception has occurred at the very
> +       beginning of a function before the stack frame has been created,
> +       or at the very end of the function after the stack frame has been
> +       destroyed.  This is an inherent limitation of frame pointers.

What this section does not point out is the actual effects of missing frame 
pointer annotations. I.e. how about quoting a before/after stack backtrace to 
demonstrate it?

Thanks,

	Ingo
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