patch: add patch for uninit dummy vars in typecheck macro

Author: Locietta

patches/0003-clang-zero-init-dummy-vars.patch

@@ -0,0 +1,105 @@
+From: Nathan Chancellor <[email protected]>
+Date: Thu, 01 May 2025 16:00:22 -0700
+Subject: [PATCH 2/2] include/linux/typecheck.h: Zero initialize dummy
+ variables
+Precedence: bulk
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+To: Andrew Morton <[email protected]>,
+ Masahiro Yamada <[email protected]>
+Cc: Nicolas Schier <[email protected]>,
+ Andrew Morton <[email protected]>,
+ Nick Desaulniers <[email protected]>,
+ Bill Wendling <[email protected]>, Justin Stitt <[email protected]>,
+ Linus Torvalds <[email protected]>,
+ [email protected], [email protected],
+ [email protected], [email protected], [email protected],
+ Linux Kernel Functional Testing <[email protected]>,
+ Marcus Seyfarth <[email protected]>,
+ Nathan Chancellor <[email protected]>
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+
+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') leave=
+s 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 ty=
+pe 'typeof (rt->dst.expires)' (aka 'const unsigned long') leaves the object=
+ uninitialized and is incompatible with C++ [-Werror,-Wdefault-const-init-v=
+ar-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: [email protected]
+Link: https://github.com/llvm/llvm-project/commit/576161cb6069e2c7656a8ef53=
+0727a0f4aefff30 [1]
+Reported-by: Linux Kernel Functional Testing <[email protected]>
+Closes: https://lore.kernel.org/CA+G9fYuNjKcxFKS_MKPRuga32XbndkLGcY-PVuoSwz=
+[email protected]/
+Reported-by: Marcus Seyfarth <[email protected]>
+Closes: https://github.com/ClangBuiltLinux/linux/issues/2088
+Signed-off-by: Nathan Chancellor <[email protected]>
+---
+ 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..5b473c9905ae7fce58b7226b57b=
+668f9ddaccaca 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 =3D {}; \
++	typeof(x) __dummy2 =3D {}; \
+ 	(void)(&__dummy =3D=3D &__dummy2); \
+ 	1; \
+ })
+
+--
+2.49.0