Re: [PATCH 0/7] kcsan: Introduce CONFIG_KCSAN_PERMISSIVE

[Mark Rutland]

Hi Marco,

On Mon, Jun 07, 2021 at 02:56:46PM +0200, Marco Elver wrote:
> While investigating a number of data races, we've encountered data-racy
> accesses on flags variables to be very common. The typical pattern is a
> reader masking all but one bit, and the writer setting/clearing only 1
> bit (current->flags being a frequently encountered case; mm/sl[au]b.c
> disables KCSAN for this reason currently).

As a heads up, I just sent out the series I promised for
thread_info::flags, at:

  https://lore.kernel.org/lkml/20210609122001.18277-1-mark.rutland@xxxxxxx/T/#t

... which I think is complementary to this (IIUC it should help with the
multi-bit cases you mention below), and may help to make the checks more
stringent in future.

FWIW, for this series:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Thanks,
Mark.

> Since these types of "trivial" data races are common (assuming they're
> intentional and hard to miscompile!), having the option to filter them
> (like we currently do for other types of data races) will avoid forcing
> everyone to mark them, and deliberately left to preference at this time.
> 
> The primary motivation is to move closer towards more easily filtering
> interesting data races (like [1], [2], [3]) on CI systems (e.g. syzbot),
> without the churn to mark all such "trivial" data races.
> [1] https://lkml.kernel.org/r/20210527092547.2656514-1-elver@xxxxxxxxxx
> [2] https://lkml.kernel.org/r/20210527104711.2671610-1-elver@xxxxxxxxxx
> [3] https://lkml.kernel.org/r/20210209112701.3341724-1-elver@xxxxxxxxxx
> 
> Notably, the need for further built-in filtering has become clearer as
> we notice some other CI systems (without active moderation) trying to
> employ KCSAN, but usually have to turn it down quickly because their
> reports are quickly met with negative feedback:
> https://lkml.kernel.org/r/YHSPfiJ/h/f3ky5n@xxxxxxxxxxxxxxxx
> 
> The rules are implemented and guarded by a new option
> CONFIG_KCSAN_PERMISSIVE. With it, we will ignore data races with only
> 1-bit value changes. Please see more details in in patch 7/7.
> 
> The rest of the patches are cleanups and improving configuration.
> 
> I ran some experiments to see what data races we're left with. With
> CONFIG_KCSAN_PERMISSIVE=y paired with syzbot's current KCSAN config
> (minimal kernel, most permissive KCSAN options), we're "just" about ~100
> reports away to a pretty silent KCSAN kernel:
> 
>   https://github.com/google/ktsan/tree/kcsan-permissive-with-dataraces
>   [ !!Disclaimer!! None of the commits are usable patches nor guaranteed
>     to be correct -- they merely resolve a data race so it wouldn't be
>     shown again and then moved on. Expect that simply marking is not
>     enough for some! ]
> 
> Most of the data races look interesting enough, and only few already had
> a comment nearby explaining what's happening.
> 
> All data races on current->flags, and most other flags are absent
> (unlike before). Those that were reported all had value changes with >1
> bit. A limitation is that few data races are still reported where the
> reader is only interested in 1 bit but the writer changed more than 1
> bit. A complete approach would require compiler changes in addition to
> the changes in this series -- but since that would further reduce the
> data races reported, the simpler and conservative approach is to stick
> to the value-change based rules for now.
> 
> Marco Elver (7):
>   kcsan: Improve some Kconfig comments
>   kcsan: Remove CONFIG_KCSAN_DEBUG
>   kcsan: Introduce CONFIG_KCSAN_STRICT
>   kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint()
>   kcsan: Rework atomic.h into permissive.h
>   kcsan: Print if strict or non-strict during init
>   kcsan: permissive: Ignore data-racy 1-bit value changes
> 
>  Documentation/dev-tools/kcsan.rst | 12 ++++
>  kernel/kcsan/atomic.h             | 23 --------
>  kernel/kcsan/core.c               | 77 ++++++++++++++++---------
>  kernel/kcsan/kcsan_test.c         | 32 +++++++++++
>  kernel/kcsan/permissive.h         | 94 +++++++++++++++++++++++++++++++
>  lib/Kconfig.kcsan                 | 39 +++++++++----
>  6 files changed, 215 insertions(+), 62 deletions(-)
>  delete mode 100644 kernel/kcsan/atomic.h
>  create mode 100644 kernel/kcsan/permissive.h
> 
> -- 
> 2.32.0.rc1.229.g3e70b5a671-goog
>