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Date:   Mon, 17 Feb 2020 12:06:14 -0500
From:   Qian Cai <cai@....pw>
To:     paulmck@...nel.org, 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
Subject: Re: [PATCH v2] kcsan, trace: Make KCSAN compatible with tracing

On Fri, 2020-02-14 at 15:40 -0800, Paul E. McKenney wrote:
> 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?

It works fine. Feel free to use,

Tested-by: Qian Cai <cai@....pw>

> 
> 							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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