[tip: locking/kcsan] kcsan: Restrict supported compilers

[tip-bot2 for Marco Elver]

The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID:     0d473b1d6e5c240f8ffed02715c718024802d0fa
Gitweb:        https://git.kernel.org/tip/0d473b1d6e5c240f8ffed02715c718024802d0fa
Author:        Marco Elver <elver@xxxxxxxxxx>
AuthorDate:    Thu, 21 May 2020 16:20:42 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 22 May 2020 14:46:02 +02:00

kcsan: Restrict supported compilers

The first version of Clang that supports -tsan-distinguish-volatile will
be able to support KCSAN. The first Clang release to do so, will be
Clang 11. This is due to satisfying all the following requirements:

1. Never emit calls to __tsan_func_{entry,exit}.

2. __no_kcsan functions should not call anything, not even
   kcsan_{enable,disable}_current(), when using __{READ,WRITE}_ONCE => Requires
   leaving them plain!

3. Support atomic_{read,set}*() with KCSAN, which rely on
   arch_atomic_{read,set}*() using __{READ,WRITE}_ONCE() => Because of
   #2, rely on Clang 11's -tsan-distinguish-volatile support. We will
   double-instrument atomic_{read,set}*(), but that's reasonable given
   it's still lower cost than the data_race() variant due to avoiding 2
   extra calls (kcsan_{en,dis}able_current() calls).

4. __always_inline functions inlined into __no_kcsan functions are never
   instrumented.

5. __always_inline functions inlined into instrumented functions are
   instrumented.

6. __no_kcsan_or_inline functions may be inlined into __no_kcsan functions =>
   Implies leaving 'noinline' off of __no_kcsan_or_inline.

7. Because of #6, __no_kcsan and __no_kcsan_or_inline functions should never be
   spuriously inlined into instrumented functions, causing the accesses of the
   __no_kcsan function to be instrumented.

Older versions of Clang do not satisfy #3. The latest GCC currently
doesn't support at least #1, #3, and #7.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/CANpmjNMTsY_8241bS7=XAfqvZHFLrVEkv_uM4aDUWE_kh3Rvbw@xxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20200521142047.169334-7-elver@xxxxxxxxxx
---
 lib/Kconfig.kcsan |  9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
index b5d88ac..5ee88e5 100644
--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -3,6 +3,12 @@
 config HAVE_ARCH_KCSAN
 	bool
 
+config HAVE_KCSAN_COMPILER
+	def_bool CC_IS_CLANG && $(cc-option,-fsanitize=thread -mllvm -tsan-distinguish-volatile=1)
+	help
+	  For the list of compilers that support KCSAN, please see
+	  <file:Documentation/dev-tools/kcsan.rst>.
+
 config KCSAN_KCOV_BROKEN
 	def_bool KCOV && CC_HAS_SANCOV_TRACE_PC
 	depends on CC_IS_CLANG
@@ -15,7 +21,8 @@ config KCSAN_KCOV_BROKEN
 
 menuconfig KCSAN
 	bool "KCSAN: dynamic data race detector"
-	depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN
+	depends on HAVE_ARCH_KCSAN && HAVE_KCSAN_COMPILER
+	depends on DEBUG_KERNEL && !KASAN
 	depends on !KCSAN_KCOV_BROKEN
 	select STACKTRACE
 	help