[PATCH v3] gcov: Disable GCOV_PROFILE_ALL on 32-bit UML with Clang 20/21

[Kees Cook]

Clang 20 and 21 miscompute __builtin_object_size() when -fprofile-arcs
is active on 32-bit UML targets, which passes incorrect object size
calculations for local variables through always_inline copy_to_user()
and check_copy_size(), causing spurious compile-time errors:

  include/linux/ucopysize.h:52:4: error: call to '__bad_copy_from' declared with 'error' attribute: copy source size is too small

The regression was introduced in LLVM commit 02b8ee281947 ("[llvm]
Improve llvm.objectsize computation by computing GEP, alloca and malloc
parameters bound"), which shipped in Clang 20. It was fixed in LLVM
by commit 45b697e610fd ("[MemoryBuiltins] Consider index type size
when aggregating gep offsets"), which was backported to the LLVM 22.x
release branch.

The bug requires 32-bit UML + GCOV_PROFILE_ALL (which uses -fprofile-arcs),
though the exact trigger depends on optimizer decisions influenced by other
enabled configs.

Prevent the bad combination by disabling UML's ARCH_HAS_GCOV_PROFILE_ALL
on 32-bit when using Clang 20.x or 21.x.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202604030531.O6FveVgn-lkp@xxxxxxxxx/
Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6[1m]
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
 v3: disable via ARCH_HAS_GCOV_PROFILE_ALL instead (Nathan)
 v2: https://lore.kernel.org/lkml/20260408162607.it.347-kees@xxxxxxxxxx/
 v1: https://lore.kernel.org/lkml/20260408005958.work.271-kees@xxxxxxxxxx/
Cc: Richard Weinberger <richard@xxxxxx>
Cc: Anton Ivanov <anton.ivanov@xxxxxxxxxxxxxxxxxx>
Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Cc: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>
Cc: Nick Desaulniers <nick.desaulniers+lkml@xxxxxxxxx>
Cc: Bill Wendling <morbo@xxxxxxxxxx>
Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
Cc: <llvm@xxxxxxxxxxxxxxx>
Cc: <linux-um@xxxxxxxxxxxxxxxxxxx>
---
 arch/um/Kconfig | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 098cda44db22..d9541d13d9eb 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -11,7 +11,9 @@ config UML
 	select ARCH_HAS_CACHE_LINE_SIZE
 	select ARCH_HAS_CPU_FINALIZE_INIT
 	select ARCH_HAS_FORTIFY_SOURCE
-	select ARCH_HAS_GCOV_PROFILE_ALL
+	# Clang 20 & 21 miscompute __builtin_object_size() under -fprofile-arcs
+	# on 32-bit, causing spurious compile-time errors in check_copy_size().
+	select ARCH_HAS_GCOV_PROFILE_ALL if !(!64BIT && CLANG_VERSION >= 200000 && CLANG_VERSION < 220100)
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_STRNCPY_FROM_USER
 	select ARCH_HAS_STRNLEN_USER
-- 
2.34.1