linux-kernel - Re: Tracing text poke / kernel self-modifying code (Was: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion)

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Message-ID: <20190829114602.GR2369@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Aug 2019 13:46:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Nadav Amit <nadav.amit@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        songliubraving@...com
Subject: Re: Tracing text poke / kernel self-modifying code (Was: Re: [RFC v2
 0/6] x86: dynamic indirect branch promotion)

On Thu, Aug 29, 2019 at 12:40:56PM +0300, Adrian Hunter wrote:
> Can you expand on "and ensure the poke_handler preserves the existing
> control flow"?  Whatever the INT3-handler does will be traced normally so
> long as it does not itself execute self-modified code.

My thinking was that the code shouldn't change semantics before emitting
the RECORD_TEXT_POKE; but you're right in that that doesn't seem to
matter much.

Either we run the old code or INT3 does 'something'.  Then we get
RECORD_TEXT_POKE and finish the poke.  Which tells that the moment INT3
stops the new text is in place.

I suppose that works too, and requires less changes.