Re: [PATCH v2] gcov: Disable GCOV_PROFILE_ALL on 32-bit UML with Clang 20/21

[Nathan Chancellor]

On Wed, Apr 08, 2026 at 09:26:12AM -0700, Kees Cook wrote:
> 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.
> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202604030531.O6FveVgn-lkp@xxxxxxxxx/
> Assisted-by: Claude:claude-opus-4-6[1m]
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
>  v2: fixed typo in version comparison: needed < not <= (Sashiko)
>  v1: https://lore.kernel.org/lkml/20260408005958.work.271-kees@xxxxxxxxxx/
> Cc: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Nick Desaulniers <nick.desaulniers+lkml@xxxxxxxxx>
> Cc: Bill Wendling <morbo@xxxxxxxxxx>
> Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
> Cc: <llvm@xxxxxxxxxxxxxxx>
> ---
>  kernel/gcov/Kconfig | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
> index 04f4ebdc3cf5..56abff785654 100644
> --- a/kernel/gcov/Kconfig
> +++ b/kernel/gcov/Kconfi
> @@ -42,6 +42,9 @@ config GCOV_PROFILE_ALL
>  	depends on !COMPILE_TEST
>  	depends on GCOV_KERNEL
>  	depends on ARCH_HAS_GCOV_PROFILE_ALL

Would it be better to avoid selecting this symbol in arch/um/Kconfig
when we have this condition?

> +	# 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)

Given that you have an inclusive range above 0, you could drop the CC_IS_CLANG.

With the suggestion above it would just become:

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
--

Cheers,
Nathan