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

[Marco Elver]

On Fri, 21 Aug 2020 at 14:31, Marco Elver <elver@xxxxxxxxxx> 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@xxxxxxxxxx
> Link: https://lkml.kernel.org/r/20200820172046.GA177701@xxxxxxxxxxxxxxxx
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx,
> 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.

Thanks,
-- Marco