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@vger.kernel.org Link: https://github.com/llvm/llvm-project/commit/576161cb6069e2c7656a8ef530727a0f... [1] Reported-by: Linux Kernel Functional Testing lkft@linaro.org Closes: https://lore.kernel.org/CA+G9fYuNjKcxFKS_MKPRuga32XbndkLGcY-PVuoSwzv6VWbY=w@... Reported-by: Marcus Seyfarth m.seyfarth@gmail.com Closes: https://github.com/ClangBuiltLinux/linux/issues/2088 Signed-off-by: Nathan Chancellor nathan@kernel.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; \ })