linux-kernel - [PATCH] compiler_types: mark __compiletime_assert failure as _

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Message-ID: <20211014132331.GA4811@kernel.org>
Date:   Thu, 14 Oct 2021 15:23:31 +0200
From:   Miguel Ojeda <ojeda@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: [PATCH] compiler_types: mark __compiletime_assert failure as
 __noreturn

`__compiletime_assert` declares a fake `extern` function
which appears (to the compiler) to be called when the test fails.

Therefore, compilers may emit possibly-uninitialized warnings
in some cases, even if it will be an error anyway (for compilers
supporting the `error` attribute, e.g. GCC and Clang >= 14)
or a link failure (for those that do not, e.g. Clang < 14).

Annotating the fake function as `__noreturn` gives them
the information they need to avoid the warning,
e.g. see https://godbolt.org/z/x1v69jjYY.

Link: https://lore.kernel.org/llvm/202110100514.3h9CI4s0-lkp@intel.com/
Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
 include/linux/compiler_types.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index b6ff83a714ca..ca1a66b8cd2f 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -298,7 +298,13 @@ struct ftrace_likely_data {
 #ifdef __OPTIMIZE__
 # define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
-		extern void prefix ## suffix(void) __compiletime_error(msg); \
+		/*							\
+		 * __noreturn is needed to give the compiler enough	\
+		 * information to avoid certain possibly-uninitialized	\
+		 * warnings (regardless of the build failing).		\
+		 */							\
+		__noreturn extern void prefix ## suffix(void)		\
+			__compiletime_error(msg);			\
 		if (!(condition))					\
 			prefix ## suffix();				\
 	} while (0)
-- 
2.33.1