Date:   Tue, 4 Feb 2020 16:28:47 +0100
From:   Marco Elver <elver@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kcsan: Add option to assume plain writes up to word
 size are atomic

On Tue, 4 Feb 2020 at 15:04, Marco Elver <elver@...gle.com> wrote:
>
> This adds option KCSAN_ASSUME_PLAIN_WRITES_ATOMIC. If enabled, plain
> writes up to word size are also assumed to be atomic, and also not
> subject to other unsafe compiler optimizations resulting in data races.

I just realized we should probably also check for alignedness. Would
this be fair to add as an additional constraint? It would be my
preference.

Thanks,
-- Marco

> This option has been enabled by default to reflect current kernel-wide
> preferences.
>
> Signed-off-by: Marco Elver <elver@...gle.com>
> ---
>  kernel/kcsan/core.c | 20 +++++++++++++++-----
>  lib/Kconfig.kcsan   | 26 +++++++++++++++++++-------
>  2 files changed, 34 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> index 64b30f7716a12..3bd1bf8d6bfeb 100644
> --- a/kernel/kcsan/core.c
> +++ b/kernel/kcsan/core.c
> @@ -169,10 +169,19 @@ static __always_inline struct kcsan_ctx *get_ctx(void)
>         return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
>  }
>
> -static __always_inline bool is_atomic(const volatile void *ptr)
> +static __always_inline bool
> +is_atomic(const volatile void *ptr, size_t size, int type)
>  {
> -       struct kcsan_ctx *ctx = get_ctx();
> +       struct kcsan_ctx *ctx;
> +
> +       if ((type & KCSAN_ACCESS_ATOMIC) != 0)
> +               return true;
>
> +       if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
> +           (type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long))
> +               return true; /* Assume all writes up to word size are atomic. */
> +
> +       ctx = get_ctx();
>         if (unlikely(ctx->atomic_next > 0)) {
>                 /*
>                  * Because we do not have separate contexts for nested
> @@ -193,7 +202,8 @@ static __always_inline bool is_atomic(const volatile void *ptr)
>         return kcsan_is_atomic(ptr);
>  }
>
> -static __always_inline bool should_watch(const volatile void *ptr, int type)
> +static __always_inline bool
> +should_watch(const volatile void *ptr, size_t size, int type)
>  {
>         /*
>          * Never set up watchpoints when memory operations are atomic.
> @@ -202,7 +212,7 @@ static __always_inline bool should_watch(const volatile void *ptr, int type)
>          * should not count towards skipped instructions, and (2) to actually
>          * decrement kcsan_atomic_next for consecutive instruction stream.
>          */
> -       if ((type & KCSAN_ACCESS_ATOMIC) != 0 || is_atomic(ptr))
> +       if (is_atomic(ptr, size, type))
>                 return false;
>
>         if (this_cpu_dec_return(kcsan_skip) >= 0)
> @@ -460,7 +470,7 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
>         if (unlikely(watchpoint != NULL))
>                 kcsan_found_watchpoint(ptr, size, type, watchpoint,
>                                        encoded_watchpoint);
> -       else if (unlikely(should_watch(ptr, type)))
> +       else if (unlikely(should_watch(ptr, size, type)))
>                 kcsan_setup_watchpoint(ptr, size, type);
>  }
>
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index 3552990abcfe5..08972376f0454 100644
> --- a/lib/Kconfig.kcsan
> +++ b/lib/Kconfig.kcsan
> @@ -91,13 +91,13 @@ config KCSAN_REPORT_ONCE_IN_MS
>           limiting reporting to avoid flooding the console with reports.
>           Setting this to 0 disables rate limiting.
>
> -# Note that, while some of the below options could be turned into boot
> -# parameters, to optimize for the common use-case, we avoid this because: (a)
> -# it would impact performance (and we want to avoid static branch for all
> -# {READ,WRITE}_ONCE, atomic_*, bitops, etc.), and (b) complicate the design
> -# without real benefit. The main purpose of the below options is for use in
> -# fuzzer configs to control reported data races, and they are not expected
> -# to be switched frequently by a user.
> +# The main purpose of the below options is to control reported data races (e.g.
> +# in fuzzer configs), and are not expected to be switched frequently by other
> +# users. We could turn some of them into boot parameters, but given they should
> +# not be switched normally, let's keep them here to simplify configuration.
> +#
> +# The defaults below are chosen to be very conservative, and may miss certain
> +# bugs.
>
>  config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
>         bool "Report races of unknown origin"
> @@ -116,6 +116,18 @@ config KCSAN_REPORT_VALUE_CHANGE_ONLY
>           the data value of the memory location was observed to remain
>           unchanged, do not report the data race.
>
> +config KCSAN_ASSUME_PLAIN_WRITES_ATOMIC
> +       bool "Assume that plain writes up to word size are atomic"
> +       default y
> +       help
> +         Assume that plain writes up to word size are atomic by default, and
> +         also not subject to other unsafe compiler optimizations resulting in
> +         data races. This will cause KCSAN to not report data races due to
> +         conflicts where the only plain accesses are writes up to word size:
> +         conflicts between marked reads and plain writes up to word size will
> +         not be reported as data races; notice that data races between two
> +         conflicting plain writes will also not be reported.
> +
>  config KCSAN_IGNORE_ATOMICS
>         bool "Do not instrument marked atomic accesses"
>         help
> --
> 2.25.0.341.g760bfbb309-goog
>

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