Re: [PATCH v2] kcsan: Add option to allow watcher interruptions

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Feb 21, 2020 at 11:02:09PM +0100, Marco Elver wrote:
> Add option to allow interrupts while a watchpoint is set up. This can be
> enabled either via CONFIG_KCSAN_INTERRUPT_WATCHER or via the boot
> parameter 'kcsan.interrupt_watcher=1'.
> 
> Note that, currently not all safe per-CPU access primitives and patterns
> are accounted for, which could result in false positives. For example,
> asm-generic/percpu.h uses plain operations, which by default are
> instrumented. On interrupts and subsequent accesses to the same
> variable, KCSAN would currently report a data race with this option.
> 
> Therefore, this option should currently remain disabled by default, but
> may be enabled for specific test scenarios.
> 
> To avoid new warnings, changes all uses of smp_processor_id() to use the
> raw version (as already done in kcsan_found_watchpoint()). The exact SMP
> processor id is for informational purposes in the report, and
> correctness is not affected.
> 
> Signed-off-by: Marco Elver <elver@google.com>

Applied in place of V1, thank you!

							Thanx, Paul

> ---
> v2:
> * Change smp_processor_id() to raw_smp_processor_id() as already used in
>   kcsan_found_watchpoint() to avoid warnings.
> ---
>  kernel/kcsan/core.c | 34 ++++++++++------------------------
>  lib/Kconfig.kcsan   | 11 +++++++++++
>  2 files changed, 21 insertions(+), 24 deletions(-)
> 
> diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> index 589b1e7f0f253..e7387fec66795 100644
> --- a/kernel/kcsan/core.c
> +++ b/kernel/kcsan/core.c
> @@ -21,6 +21,7 @@ static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE);
>  static unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
>  static unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
>  static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
> +static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER);
>  
>  #ifdef MODULE_PARAM_PREFIX
>  #undef MODULE_PARAM_PREFIX
> @@ -30,6 +31,7 @@ module_param_named(early_enable, kcsan_early_enable, bool, 0);
>  module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
>  module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
>  module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
> +module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444);
>  
>  bool kcsan_enabled;
>  
> @@ -354,7 +356,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
>  	unsigned long access_mask;
>  	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
>  	unsigned long ua_flags = user_access_save();
> -	unsigned long irq_flags;
> +	unsigned long irq_flags = 0;
>  
>  	/*
>  	 * Always reset kcsan_skip counter in slow-path to avoid underflow; see
> @@ -370,26 +372,9 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
>  		goto out;
>  	}
>  
> -	/*
> -	 * Disable interrupts & preemptions to avoid another thread on the same
> -	 * CPU accessing memory locations for the set up watchpoint; this is to
> -	 * avoid reporting races to e.g. CPU-local data.
> -	 *
> -	 * An alternative would be adding the source CPU to the watchpoint
> -	 * encoding, and checking that watchpoint-CPU != this-CPU. There are
> -	 * several problems with this:
> -	 *   1. we should avoid stealing more bits from the watchpoint encoding
> -	 *      as it would affect accuracy, as well as increase performance
> -	 *      overhead in the fast-path;
> -	 *   2. if we are preempted, but there *is* a genuine data race, we
> -	 *      would *not* report it -- since this is the common case (vs.
> -	 *      CPU-local data accesses), it makes more sense (from a data race
> -	 *      detection point of view) to simply disable preemptions to ensure
> -	 *      as many tasks as possible run on other CPUs.
> -	 *
> -	 * Use raw versions, to avoid lockdep recursion via IRQ flags tracing.
> -	 */
> -	raw_local_irq_save(irq_flags);
> +	if (!kcsan_interrupt_watcher)
> +		/* Use raw to avoid lockdep recursion via IRQ flags tracing. */
> +		raw_local_irq_save(irq_flags);
>  
>  	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
>  	if (watchpoint == NULL) {
> @@ -507,7 +492,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
>  		if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
>  			kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES);
>  
> -		kcsan_report(ptr, size, type, value_change, smp_processor_id(),
> +		kcsan_report(ptr, size, type, value_change, raw_smp_processor_id(),
>  			     KCSAN_REPORT_RACE_SIGNAL);
>  	} else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
>  		/* Inferring a race, since the value should not have changed. */
> @@ -518,13 +503,14 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
>  
>  		if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
>  			kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE,
> -				     smp_processor_id(),
> +				     raw_smp_processor_id(),
>  				     KCSAN_REPORT_RACE_UNKNOWN_ORIGIN);
>  	}
>  
>  	kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
>  out_unlock:
> -	raw_local_irq_restore(irq_flags);
> +	if (!kcsan_interrupt_watcher)
> +		raw_local_irq_restore(irq_flags);
>  out:
>  	user_access_restore(ua_flags);
>  }
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index f0b791143c6ab..081ed2e1bf7b1 100644
> --- a/lib/Kconfig.kcsan
> +++ b/lib/Kconfig.kcsan
> @@ -88,6 +88,17 @@ config KCSAN_SKIP_WATCH_RANDOMIZE
>  	  KCSAN_WATCH_SKIP. If false, the chosen value is always
>  	  KCSAN_WATCH_SKIP.
>  
> +config KCSAN_INTERRUPT_WATCHER
> +	bool "Interruptible watchers"
> +	help
> +	  If enabled, a task that set up a watchpoint may be interrupted while
> +	  delayed. This option will allow KCSAN to detect races between
> +	  interrupted tasks and other threads of execution on the same CPU.
> +
> +	  Currently disabled by default, because not all safe per-CPU access
> +	  primitives and patterns may be accounted for, and therefore could
> +	  result in false positives.
> +
>  config KCSAN_REPORT_ONCE_IN_MS
>  	int "Duration in milliseconds, in which any given race is only reported once"
>  	default 3000
> -- 
> 2.25.0.265.gbab2e86ba0-goog
>