linux-kernel - Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

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Message-ID: <20210413110255.GB5586@sirena.org.uk>
Date:   Tue, 13 Apr 2021 12:02:55 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....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,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability
 checks

On Mon, Apr 12, 2021 at 02:55:35PM -0500, Madhavan T. Venkataraman wrote:

> 
> OK. Just so I am clear on the whole picture, let me state my understanding so far.
> Correct me if I am wrong.

> 1. We are hoping that we can convert a significant number of SYM_CODE functions to
>    SYM_FUNC functions by providing them with a proper FP prolog and epilog so that
>    we can get objtool coverage for them. These don't need any blacklisting.

I wouldn't expect to be converting lots of SYM_CODE to SYM_FUNC.  I'd
expect the overwhelming majority of SYM_CODE to be SYM_CODE because it's
required to be non standard due to some external interface - things like
the exception vectors, ftrace, and stuff around suspend/hibernate.  A
quick grep seems to confirm this.

> 3. We are going to assume that the reliable unwinder is only for livepatch purposes
>    and will only be invoked on a task that is not currently running. The task either

The reliable unwinder can also be invoked on itself.

> 4. So, the only functions that will need blacklisting are the remaining SYM_CODE functions
>    that might give up the CPU voluntarily. At this point, I am not even sure how
>    many of these will exist. One hopes that all of these would have ended up as
>    SYM_FUNC functions in (1).

There's stuff like ret_from_fork there.

> I suggest we do (3) first. Then, review the assembly functions to do (1). Then, review the
> remaining ones to see which ones must be blacklisted, if any.

I'm not clear what the concrete steps you're planning to do first are
there - your 3 seems like a statement of assumptions.  For flagging
functions I do think it'd be safer to default to assuming that all
SYM_CODE functions can't be unwound reliably rather than only explicitly
listing ones that cause problems.

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