From 6522fe5c1b007c376fc5f2de1016c99a18b0af8e Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Wed, 8 Apr 2026 22:20:42 -0700 Subject: [PATCH] um: 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 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 Closes: https://lore.kernel.org/oe-kbuild-all/202604030531.O6FveVgn-lkp@intel.com/ Suggested-by: Nathan Chancellor Assisted-by: Claude:claude-opus-4-6[1m] Signed-off-by: Kees Cook Link: https://patch.msgid.link/20260409052038.make.995-kees@kernel.org Signed-off-by: Johannes Berg --- 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