gcov: Disable GCOV_PROFILE_ALL on 32-bit UML with Clang 20/21

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 broken combination by disabling GCOV_PROFILE_ALL on 32-bit UML when using Clang 20.x or 21.x.

---
Cc: Peter Oberparleiter <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Justin Stitt <[email protected]>
Cc: <[email protected]>

---
 kernel/gcov/Kconfig | 3 +++
 1 file changed, 3 insertions(+)

index 04f4ebdc3cf5..11c377019d01 100644
@@ -42,6 +42,9 @@ config GCOV_PROFILE_ALL
 	depends on !COMPILE_TEST
 	depends on GCOV_KERNEL
 	depends on ARCH_HAS_GCOV_PROFILE_ALL
+	# Clang 20 & 21 miscompute __builtin_object_size() under -fprofile-arcs
+	# on 32-bit UML, causing spurious compile-time errors in check_copy_size().
+	depends on !(UML && !64BIT && CC_IS_CLANG && CLANG_VERSION >= 200000 && CLANG_VERSION <= 220100)
 	default n
 	help
 	This options activates profiling for the entire kernel.

2.34.1