linux-kernel - Re: [PATCH] ARM: unwinder: Handle Stackoverflow in unwind_exec

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Date:	Fri, 22 Nov 2013 19:37:13 +0000
From:	Dave Martin <Dave.Martin@....com>
To:	Anurag Aggarwal <anurag19aggarwal@...il.com>
Cc:	naveen.sel@...sung.com,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	narendra.m1@...sung.com, nico@...aro.org,
	Anurag Aggarwal <a.anurag@...sung.com>,
	Catalin Marinas <catalin.marinas@....com>, will.deacon@....com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	cpgs@...sung.com, naveenkrishna.ch@...il.com,
	rajat.suri@...sung.com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, mohammad.a2@...sung.com
Subject: Re: [PATCH] ARM: unwinder: Handle Stackoverflow in unwind_exec_insn

On Sat, Nov 09, 2013 at 12:28:57PM +0530, Anurag Aggarwal wrote:
> Thanks for your input Dave,
> 
> I think there is another way to avoid the stack overflow and reduce
> the number of checks also,
> 
> Stack overflow will cause a problem only when we are backtracking the
> last set of registers.
> i.e when the difference between current SP and top of stack is less
> than or equal to number of registers

Apologies, it looks like I failed to respond to this earlier...


Although that will usually be correct, there is no rule in the ABI to
guarantee it.

> we can create two unwind_exec_insn, one without checks and one with checks.
> 
> then we call the correct function from unwind_frame depending on the
> difference of SP and top of stack.
> 
> This will reduce the amount of checks every time we read a set of
> registers from stack

That sounds like it might duplicate a lot of code, to optimise based on
assumptions that may not always be true, for what really should not be a
hot path in the kernel.

If you can find a tidy way of doing it, it would be certainly worth
reviewing, but I still think it would be simpler just to do a simple
bounds check for every word read from the stack -- it should be
impossible for that to go wrong, even if some of the bounds checks
are not stictly required.

Cheers
---Dave
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