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Date:   Thu, 27 May 2021 21:44:48 +0200
Message-Id: <20210527194448.3470080-1-elver@google.com>
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Subject: [PATCH v3] kcov: add __no_sanitize_coverage to fix noinstr for all architectures
From:   Marco Elver <elver@google.com>
To:     elver@google.com, Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, nathan@kernel.org,
        ndesaulniers@google.com, ojeda@kernel.org, keescook@chromium.org,
        peterz@infradead.org, will@kernel.org, nivedita@alum.mit.edu,
        luc.vanoostenryck@gmail.com, masahiroy@kernel.org,
        samitolvanen@google.com, arnd@arndb.de,
        clang-built-linux@googlegroups.com,
        Dmitry Vyukov <dvyukov@google.com>,
        Mark Rutland <mark.rutland@arm.com>, kasan-dev@googlegroups.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Until now no compiler supported an attribute to disable coverage
instrumentation as used by KCOV.

To work around this limitation on x86, noinstr functions have their
coverage instrumentation turned into nops by objtool. However, this
solution doesn't scale automatically to other architectures, such as
arm64, which are migrating to use the generic entry code.

Clang [1] and GCC [2] have added support for the attribute recently.
[1] https://github.com/llvm/llvm-project/commit/280333021e9550d80f5c1152a34e33e81df1e178
[2] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cec4d4a6782c9bd8d071839c50a239c49caca689
The changes will appear in Clang 13 and GCC 12.

Add __no_sanitize_coverage for both compilers, and add it to noinstr.

Note: In the Clang case, __has_feature(coverage_sanitizer) is only true
if the feature is enabled, and therefore we do not require an additional
defined(CONFIG_KCOV) (like in the GCC case where __has_attribute(..) is
always true) to avoid adding redundant attributes to functions if KCOV
is off. That being said, compilers that support the attribute will not
generate errors/warnings if the attribute is redundantly used; however,
where possible let's avoid it as it reduces preprocessed code size and
associated compile-time overheads.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
---
v3:
* Add comment explaining __has_feature() in Clang.
* Add Miguel's Reviewed-by.

v2:
* Implement __has_feature(coverage_sanitizer) in Clang
  (https://reviews.llvm.org/D103159) and use instead of version check.
* Add Peter's Ack.
---
 include/linux/compiler-clang.h | 17 +++++++++++++++++
 include/linux/compiler-gcc.h   |  6 ++++++
 include/linux/compiler_types.h |  2 +-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index adbe76b203e2..49b0ac8b6fd3 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -13,6 +13,12 @@
 /* all clang versions usable with the kernel support KASAN ABI version 5 */
 #define KASAN_ABI_VERSION 5
 
+/*
+ * Note: Checking __has_feature(*_sanitizer) is only true if the feature is
+ * enabled. Therefore it is not required to additionally check defined(CONFIG_*)
+ * to avoid adding redundant attributes in other configurations.
+ */
+
 #if __has_feature(address_sanitizer) || __has_feature(hwaddress_sanitizer)
 /* Emulate GCC's __SANITIZE_ADDRESS__ flag */
 #define __SANITIZE_ADDRESS__
@@ -45,6 +51,17 @@
 #define __no_sanitize_undefined
 #endif
 
+/*
+ * Support for __has_feature(coverage_sanitizer) was added in Clang 13 together
+ * with no_sanitize("coverage"). Prior versions of Clang support coverage
+ * instrumentation, but cannot be queried for support by the preprocessor.
+ */
+#if __has_feature(coverage_sanitizer)
+#define __no_sanitize_coverage __attribute__((no_sanitize("coverage")))
+#else
+#define __no_sanitize_coverage
+#endif
+
 /*
  * Not all versions of clang implement the type-generic versions
  * of the builtin overflow checkers. Fortunately, clang implements
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 5d97ef738a57..cb9217fc60af 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -122,6 +122,12 @@
 #define __no_sanitize_undefined
 #endif
 
+#if defined(CONFIG_KCOV) && __has_attribute(__no_sanitize_coverage__)
+#define __no_sanitize_coverage __attribute__((no_sanitize_coverage))
+#else
+#define __no_sanitize_coverage
+#endif
+
 #if GCC_VERSION >= 50100
 #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
 #endif
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index d29bda7f6ebd..cc2bee7f0977 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -210,7 +210,7 @@ struct ftrace_likely_data {
 /* Section for code which can't be instrumented at all */
 #define noinstr								\
 	noinline notrace __attribute((__section__(".noinstr.text")))	\
-	__no_kcsan __no_sanitize_address
+	__no_kcsan __no_sanitize_address __no_sanitize_coverage
 
 #endif /* __KERNEL__ */
 
-- 
2.32.0.rc0.204.g9fa02ecfa5-goog