linux-kernel - [PATCH 2/2] include/linux/typecheck.h: Zero initialize dummy variables

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Message-Id: <20250501-default-const-init-clang-v1-2-3d2c6c185dbb@kernel.org>
Date: Thu, 01 May 2025 16:00:22 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Masahiro Yamada <masahiroy@...nel.org>
Cc: Nicolas Schier <nicolas.schier@...ux.dev>, 
 Andrew Morton <akpm@...ux-foundation.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Linus Torvalds <torvalds@...ux-foundation.org>, 
 linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, patches@...ts.linux.dev, stable@...r.kernel.org, 
 Linux Kernel Functional Testing <lkft@...aro.org>, 
 Marcus Seyfarth <m.seyfarth@...il.com>, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH 2/2] include/linux/typecheck.h: Zero initialize dummy
 variables

A new on by default warning in clang [1] aims to flags instances where
const variables without static or thread local storage are not
initialized because it can lead to an indeterminate value. The __dummy
variables in the typecheck() macro are the only places within the kernel
where this warning currently occurs.

  drivers/gpu/drm/i915/gt/intel_ring.h:62:2: error: default initialization of an object of type 'typeof (ring->size)' (aka 'const unsigned int') leaves the object uninitialized and is incompatible with C++ [-Werror,-Wdefault-const-init-var-unsafe]
     62 |         typecheck(typeof(ring->size), next);
        |         ^
  include/linux/typecheck.h:10:9: note: expanded from macro 'typecheck'
     10 | ({      type __dummy; \
        |              ^

  include/net/ip.h:478:14: error: default initialization of an object of type 'typeof (rt->dst.expires)' (aka 'const unsigned long') leaves the object uninitialized and is incompatible with C++ [-Werror,-Wdefault-const-init-var-unsafe]
    478 |                 if (mtu && time_before(jiffies, rt->dst.expires))
        |                            ^
  include/linux/jiffies.h:138:26: note: expanded from macro 'time_before'
    138 | #define time_before(a,b)        time_after(b,a)
        |                                 ^
  include/linux/jiffies.h:128:3: note: expanded from macro 'time_after'
    128 |         (typecheck(unsigned long, a) && \
        |          ^
  include/linux/typecheck.h:11:12: note: expanded from macro 'typecheck'
     11 |         typeof(x) __dummy2; \
        |                   ^

Zero initialize the variables to silence the warning while not impacting
the final code generation because the comparison only matters at compile
time, as suggested on the PR of [1] by the clang maintainer.

Cc: stable@...r.kernel.org
Link: https://github.com/llvm/llvm-project/commit/576161cb6069e2c7656a8ef530727a0f4aefff30 [1]
Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
Closes: https://lore.kernel.org/CA+G9fYuNjKcxFKS_MKPRuga32XbndkLGcY-PVuoSwzv6VWbY=w@mail.gmail.com/
Reported-by: Marcus Seyfarth <m.seyfarth@...il.com>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2088
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 include/linux/typecheck.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h
index 46b15e2aaefb4e7a4d21c8797ec4d1578998981c..5b473c9905ae7fce58b7226b57b668f9ddaccaca 100644
--- a/include/linux/typecheck.h
+++ b/include/linux/typecheck.h
@@ -7,8 +7,8 @@
  * Always evaluates to 1 so you may use it easily in comparisons.
  */
 #define typecheck(type,x) \
-({	type __dummy; \
-	typeof(x) __dummy2; \
+({	type __dummy = {}; \
+	typeof(x) __dummy2 = {}; \
 	(void)(&__dummy == &__dummy2); \
 	1; \
 })

-- 
2.49.0