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Date:   Fri, 14 Feb 2020 15:40:04 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Marco Elver <elver@...gle.com>
Cc:     andreyknvl@...gle.com, glider@...gle.com, dvyukov@...gle.com,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mingo@...hat.com, x86@...nel.org,
        Qian Cai <cai@....pw>
Subject: Re: [PATCH v2] kcsan, trace: Make KCSAN compatible with tracing

On Fri, Feb 14, 2020 at 10:10:35PM +0100, Marco Elver wrote:
> Previously the system would lock up if ftrace was enabled together with
> KCSAN. This is due to recursion on reporting if the tracer code is
> instrumented with KCSAN.
> 
> To avoid this for all types of tracing, disable KCSAN instrumentation
> for all of kernel/trace.
> 
> Furthermore, since KCSAN relies on udelay() to introduce delay, we have
> to disable ftrace for udelay() (currently done for x86) in case KCSAN is
> used together with lockdep and ftrace. The reason is that it may corrupt
> lockdep IRQ flags tracing state due to a peculiar case of recursion
> (details in Makefile comment).
> 
> Signed-off-by: Marco Elver <elver@...gle.com>
> Reported-by: Qian Cai <cai@....pw>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>

Queued for review and further testing, thank you!

Qian, does this also fix things for you?

							Thanx, Paul

> ---
> v2:
> *  Fix KCSAN+lockdep+ftrace compatibility.
> ---
>  arch/x86/lib/Makefile | 5 +++++
>  kernel/kcsan/Makefile | 2 ++
>  kernel/trace/Makefile | 3 +++
>  3 files changed, 10 insertions(+)
> 
> diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
> index 432a077056775..6110bce7237bd 100644
> --- a/arch/x86/lib/Makefile
> +++ b/arch/x86/lib/Makefile
> @@ -8,6 +8,11 @@ KCOV_INSTRUMENT_delay.o	:= n
>  
>  # KCSAN uses udelay for introducing watchpoint delay; avoid recursion.
>  KCSAN_SANITIZE_delay.o := n
> +ifdef CONFIG_KCSAN
> +# In case KCSAN+lockdep+ftrace are enabled, disable ftrace for delay.o to avoid
> +# lockdep -> [other libs] -> KCSAN -> udelay -> ftrace -> lockdep recursion.
> +CFLAGS_REMOVE_delay.o = $(CC_FLAGS_FTRACE)
> +endif
>  
>  # Early boot use of cmdline; don't instrument it
>  ifdef CONFIG_AMD_MEM_ENCRYPT
> diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> index df6b7799e4927..d4999b38d1be5 100644
> --- a/kernel/kcsan/Makefile
> +++ b/kernel/kcsan/Makefile
> @@ -4,6 +4,8 @@ KCOV_INSTRUMENT := n
>  UBSAN_SANITIZE := n
>  
>  CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_debugfs.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_report.o = $(CC_FLAGS_FTRACE)
>  
>  CFLAGS_core.o := $(call cc-option,-fno-conserve-stack,) \
>  	$(call cc-option,-fno-stack-protector,)
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index f9dcd19165fa2..6b601d88bf71e 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -6,6 +6,9 @@ ifdef CONFIG_FUNCTION_TRACER
>  ORIG_CFLAGS := $(KBUILD_CFLAGS)
>  KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
>  
> +# Avoid recursion due to instrumentation.
> +KCSAN_SANITIZE := n
> +
>  ifdef CONFIG_FTRACE_SELFTEST
>  # selftest needs instrumentation
>  CFLAGS_trace_selftest_dynamic.o = $(CC_FLAGS_FTRACE)
> -- 
> 2.25.0.265.gbab2e86ba0-goog
> 

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