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From:   paulmck@kernel.org
To:     linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        kernel-team@fb.com, mingo@kernel.org
Cc:     elver@google.com, andreyknvl@google.com, glider@google.com,
        dvyukov@google.com, cai@lca.pw, boqun.feng@gmail.com,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: [PATCH kcsan 26/32] kcsan, trace: Make KCSAN compatible with tracing
Date:   Mon,  9 Mar 2020 12:04:14 -0700
Message-Id: <20200309190420.6100-26-paulmck@kernel.org>
In-Reply-To: <20200309190359.GA5822@paulmck-ThinkPad-P72>
References: <20200309190359.GA5822@paulmck-ThinkPad-P72>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Marco Elver <elver@google.com>

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@google.com>
Reported-by: Qian Cai <cai@lca.pw>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Qian Cai <cai@lca.pw>
---
 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 432a077..6110bce7 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 df6b779..d4999b3 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 0e63db6..9072486 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)
-- 
2.9.5