Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

Marco Elver
2024-05-13T14:14:15Z

On Fri, 10 May 2024 at 16:11, Paul E. McKenney <paulmck@kernel.org> wrote:
[...]
> > > Does this mean that KCSAN/etc treats the files in kernel/rcu/
> > > differently than the "Rest of Kernel"? Or what?
> > >
> > > And how is it enforced?
> >
> > I can only find the strnstr(buf, "rcu") checks in skip_report(),
> > but they only cover the KCSAN_REPORT_VALUE_CHANGE_ONLY case...
>
> Huh, new one on me! When I run KCSAN, I set CONFIG_KCSAN_STRICT=y,
> which implies CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n, which should
> prevent skip_report() from even being invoked.

The strnstr hack goes back to the first version of KCSAN released in
v5.8 [1]. It was added in response to Paul wanting the "stricter"
treatment for RCU even while KCSAN was still in development, and back
then syzbot was the first test system using KCSAN. Shortly after Paul
added a KCSAN config for rcutorture with a laundry list of options to
make KCSAN "strict" (before we eventually added CONFIG_KCSAN_STRICT
which greatly simplified that).

While the strnstr(.., "rcu") rules are redundant when using the
stricter rules (at least CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n is
set), we're keeping the "rcu" special case around because there are
test robots and fuzzers that use the default KCSAN config (unlike
rcutorture). And because we know that RCU likes the stricter rules,
the "value change only" filter is ignored in all KCSAN configs for
rcu-related functions.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/kcsan/report.c?id=v5.8

Back then syzbot occasionally reported RCU data races, but these days
rcutorture probably finds all of them before syzbot ever gets its
hands on new code.

Thanks,
-- Marco