linux-kernel - Re: [PATCH v3 22/22] x86/int3: Ensure that poke_int3

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Date:   Thu, 20 Feb 2020 17:22:14 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andy Lutomirski <luto@...nel.org>, tony.luck@...el.com,
        Frederic Weisbecker <frederic@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH v3 22/22] x86/int3: Ensure that poke_int3_handler() is not sanitized

On Thu, Feb 20, 2020 at 1:06 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Feb 20, 2020 at 11:37:32AM +0100, Dmitry Vyukov wrote:
> > On Wed, Feb 19, 2020 at 6:20 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Wed, Feb 19, 2020 at 05:30:25PM +0100, Peter Zijlstra wrote:
> > >
> > > > By inlining everything in poke_int3_handler() (except bsearch :/) we can
> > > > mark the whole function off limits to everything and call it a day. That
> > > > simplicity has been the guiding principle so far.
> > > >
> > > > Alternatively we can provide an __always_inline variant of bsearch().
> > >
> > > This reduces the __no_sanitize usage to just the exception entry
> > > (do_int3) and the critical function: poke_int3_handler().
> > >
> > > Is this more acceptible?
> >
> > Let's say it's more acceptable.
> >
> > Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
>
> Thanks, I'll go make it happen.
>
> > I guess there is no ideal solution here.
> >
> > Just a straw man proposal: expected number of elements is large enough
> > to make bsearch profitable, right? I see 1 is a common case, but the
> > other case has multiple entries.
>
> Latency was the consideration; the linear search would dramatically
> increase the runtime of the exception.
>
> The current limit is 256 entries and we're hitting that quite often.
>
> (we can trivially increase, but nobody has been able to show significant
> benefits for that -- as of yet)

I see. Thanks for explaining. Just wanted to check because inlining a
linear search would free us from all these unpleasant problems.