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From: Marco Elver <elver@google.com>
To: elver@google.com, "Paul E. McKenney" <paulmck@kernel.org>
Cc: Alexander Potapenko <glider@google.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	 Borislav Petkov <bp@alien8.de>,
	Dmitry Vyukov <dvyukov@google.com>,
	Ingo Molnar <mingo@kernel.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	 Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>,
	 kasan-dev@googlegroups.com, linux-arch@vger.kernel.org,
	 linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org,
	 linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	llvm@lists.linux.dev,  x86@kernel.org
Subject: [PATCH v3 25/25] kcsan: Support WEAK_MEMORY with Clang where no objtool support exists
Date: Tue, 30 Nov 2021 12:44:33 +0100	[thread overview]
Message-ID: <20211130114433.2580590-26-elver@google.com> (raw)
In-Reply-To: <20211130114433.2580590-1-elver@google.com>

Clang and GCC behave a little differently when it comes to the
__no_sanitize_thread attribute, which has valid reasons, and depending
on context either one could be right.

Traditionally, user space ThreadSanitizer [1] still expects instrumented
builtin atomics (to avoid false positives) and __tsan_func_{entry,exit}
(to generate meaningful stack traces), even if the function has the
attribute no_sanitize("thread").

[1] https://clang.llvm.org/docs/ThreadSanitizer.html#attribute-no-sanitize-thread

GCC doesn't follow the same policy (for better or worse), and removes
all kinds of instrumentation if no_sanitize is added. Arguably, since
this may be a problem for user space ThreadSanitizer, we expect this may
change in future.

Since KCSAN != ThreadSanitizer, the likelihood of false positives even
without barrier instrumentation everywhere, is much lower by design.

At least for Clang, however, to fully remove all sanitizer
instrumentation, we must add the disable_sanitizer_instrumentation
attribute, which is available since Clang 14.0.

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* New patch.
---
 include/linux/compiler_types.h | 13 ++++++++++++-
 lib/Kconfig.kcsan              |  2 +-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1d32f4c03c9e..3c1795fdb568 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -198,9 +198,20 @@ struct ftrace_likely_data {
 # define __no_kasan_or_inline __always_inline
 #endif
 
-#define __no_kcsan __no_sanitize_thread
 #ifdef __SANITIZE_THREAD__
+/*
+ * Clang still emits instrumentation for __tsan_func_{entry,exit}() and builtin
+ * atomics even with __no_sanitize_thread (to avoid false positives in userspace
+ * ThreadSanitizer). The kernel's requirements are stricter and we really do not
+ * want any instrumentation with __no_kcsan.
+ *
+ * Therefore we add __disable_sanitizer_instrumentation where available to
+ * disable all instrumentation. See Kconfig.kcsan where this is mandatory.
+ */
+# define __no_kcsan __no_sanitize_thread __disable_sanitizer_instrumentation
 # define __no_sanitize_or_inline __no_kcsan notrace __maybe_unused
+#else
+# define __no_kcsan
 #endif
 
 #ifndef __no_sanitize_or_inline
diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
index e4394ea8068b..63b70b8c5551 100644
--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -198,7 +198,7 @@ config KCSAN_WEAK_MEMORY
 	# We can either let objtool nop __tsan_func_{entry,exit}() and builtin
 	# atomics instrumentation in .noinstr.text, or use a compiler that can
 	# implement __no_kcsan to really remove all instrumentation.
-	depends on STACK_VALIDATION || CC_IS_GCC
+	depends on STACK_VALIDATION || CC_IS_GCC || CLANG_VERSION >= 140000
 	help
 	  Enable support for modeling a subset of weak memory, which allows
 	  detecting a subset of data races due to missing memory barriers.
-- 
2.34.0.rc2.393.gf8c9666880-goog


      parent reply	other threads:[~2021-11-30 11:46 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-30 11:44 [PATCH v3 00/25] kcsan: Support detecting a subset of missing memory barriers Marco Elver
2021-11-30 11:44 ` [PATCH v3 01/25] kcsan: Refactor reading of instrumented memory Marco Elver
2021-11-30 11:44 ` [PATCH v3 02/25] kcsan: Remove redundant zero-initialization of globals Marco Elver
2021-11-30 11:44 ` [PATCH v3 03/25] kcsan: Avoid checking scoped accesses from nested contexts Marco Elver
2021-11-30 11:44 ` [PATCH v3 04/25] kcsan: Add core support for a subset of weak memory modeling Marco Elver
2021-12-03  8:56   ` Marco Elver
2021-12-03 16:50     ` Paul E. McKenney
2021-12-03 21:08       ` Paul E. McKenney
2021-12-03 23:42         ` Marco Elver
2021-12-03 23:42         ` Paul E. McKenney
2021-12-03 23:45           ` Marco Elver
2021-12-04  1:14             ` Paul E. McKenney
2021-11-30 11:44 ` [PATCH v3 05/25] kcsan: Add core memory barrier instrumentation functions Marco Elver
2021-11-30 11:44 ` [PATCH v3 06/25] kcsan, kbuild: Add option for barrier instrumentation only Marco Elver
2021-11-30 11:44 ` [PATCH v3 07/25] kcsan: Call scoped accesses reordered in reports Marco Elver
2021-11-30 11:44 ` [PATCH v3 08/25] kcsan: Show location access was reordered to Marco Elver
2021-12-06  5:03   ` Boqun Feng
2021-12-06  7:16     ` Marco Elver
2021-12-06 14:31       ` Boqun Feng
2021-12-06 16:04         ` Marco Elver
2021-12-06 17:16           ` Boqun Feng
2021-12-06 17:38             ` Paul E. McKenney
2021-11-30 11:44 ` [PATCH v3 09/25] kcsan: Document modeling of weak memory Marco Elver
2021-11-30 11:44 ` [PATCH v3 10/25] kcsan: test: Match reordered or normal accesses Marco Elver
2021-11-30 11:44 ` [PATCH v3 11/25] kcsan: test: Add test cases for memory barrier instrumentation Marco Elver
2021-11-30 11:44 ` [PATCH v3 12/25] kcsan: Ignore GCC 11+ warnings about TSan runtime support Marco Elver
2021-11-30 11:44 ` [PATCH v3 13/25] kcsan: selftest: Add test case to check memory barrier instrumentation Marco Elver
2021-11-30 11:44 ` [PATCH v3 14/25] locking/barriers, kcsan: Add instrumentation for barriers Marco Elver
2021-11-30 11:44 ` [PATCH v3 15/25] locking/barriers, kcsan: Support generic instrumentation Marco Elver
2021-11-30 11:44 ` [PATCH v3 16/25] locking/atomics, kcsan: Add instrumentation for barriers Marco Elver
2021-11-30 11:44 ` [PATCH v3 17/25] asm-generic/bitops, " Marco Elver
2021-11-30 11:44 ` [PATCH v3 18/25] x86/barriers, kcsan: Use generic instrumentation for non-smp barriers Marco Elver
2021-11-30 11:44 ` [PATCH v3 19/25] x86/qspinlock, kcsan: Instrument barrier of pv_queued_spin_unlock() Marco Elver
2021-11-30 11:44 ` [PATCH v3 20/25] mm, kcsan: Enable barrier instrumentation Marco Elver
2021-11-30 11:44 ` [PATCH v3 21/25] sched, kcsan: Enable memory " Marco Elver
2021-11-30 11:44 ` [PATCH v3 22/25] objtool, kcsan: Add memory barrier instrumentation to whitelist Marco Elver
2021-11-30 11:44 ` [PATCH v3 23/25] objtool, kcsan: Remove memory barrier instrumentation from noinstr Marco Elver
2021-11-30 11:44 ` [PATCH v3 24/25] compiler_attributes.h: Add __disable_sanitizer_instrumentation Marco Elver
2021-11-30 11:44 ` Marco Elver [this message]

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