linux-kernel - Re: [PATCH] kcsan: Use tracing-safe version of prandom

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Date:   Wed, 26 Aug 2020 14:21:30 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kcsan: Use tracing-safe version of prandom

On Wed, Aug 26, 2020 at 02:17:57PM +0200, Marco Elver wrote:
> On Fri, 21 Aug 2020 at 14:31, Marco Elver <elver@...gle.com> wrote:
> > In the core runtime, we must minimize any calls to external library
> > functions to avoid any kind of recursion. This can happen even though
> > instrumentation is disabled for called functions, but tracing is
> > enabled.
> >
> > Most recently, prandom_u32() added a tracepoint, which can cause
> > problems for KCSAN even if the rcuidle variant is used. For example:
> >         kcsan -> prandom_u32() -> trace_prandom_u32_rcuidle ->
> >         srcu_read_lock_notrace -> __srcu_read_lock -> kcsan ...
> >
> > While we could disable KCSAN in kcsan_setup_watchpoint(), this does not
> > solve other unexpected behaviour we may get due recursing into functions
> > that may not be tolerant to such recursion:
> >         __srcu_read_lock -> kcsan -> ... -> __srcu_read_lock
> >
> > Therefore, switch to using prandom_u32_state(), which is uninstrumented,
> > and does not have a tracepoint.
> >
> > Link: https://lkml.kernel.org/r/20200821063043.1949509-1-elver@google.com
> > Link: https://lkml.kernel.org/r/20200820172046.GA177701@elver.google.com
> > Signed-off-by: Marco Elver <elver@...gle.com>
> > ---
> > Applies to latest -rcu/dev only.
> >
> > Let's wait a bit to see what happens with
> >   https://lkml.kernel.org/r/20200821063043.1949509-1-elver@google.com,
> > just in case there's a better solution that might make this patch redundant.
> 
> Paul, feel free to pick this up.
> 
> I wanted to wait until after plumbers to see what happens, but maybe
> it's better to give the heads-up now, so this is in time for the next
> pull-request. It seems that prandom_u32() will keep its tracepoint,
> which means we definitely need this to make KCSAN compatible with
> tracing again.

Queued and pushed, thank you!

							Thanx, Paul