linux-kernel - Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

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Date:   Tue, 23 Mar 2021 17:09:25 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     broonie@...nel.org, jpoimboe@...hat.com, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a
 stack trace unreliable

On Tue, Mar 23, 2021 at 11:53:04AM -0500, Madhavan T. Venkataraman wrote:
> On 3/23/21 11:48 AM, Mark Rutland wrote:
> > On Tue, Mar 23, 2021 at 10:26:50AM -0500, Madhavan T. Venkataraman wrote:
> >> So, my next question is - can we define a practical limit for the
> >> nesting so that any nesting beyond that is fatal? The reason I ask is
> >> - if there is a max, then we can allocate an array of stack frames out
> >> of band for the special frames so they are not part of the stack and
> >> will not likely get corrupted.

> >> Also, we don't have to do any special detection. If the number of out
> >> of band frames used is one or more then we have exceptions and the
> >> stack trace is unreliable.
> > 
> > What is expected to protect against?
> 
> It is not a protection thing. I just wanted a reliable way to tell that there
> is an exception without having to unwind the stack up to the exception frame.
> That is all.

I see.

Given that's an optimization, we can consider doing something like that
that after we have the functional bits in place, where we'll be in a
position to see whether this is even a measureable concern in practice.

I suspect that longer-term we'll end up trying to use metadata to unwind
across exception boundaries, since it's possible to get blocked within
those for long periods (e.g. for a uaccess fault), and the larger scale
optimization for patching is to not block the patch.

Thanks,
Mark.