linux-hardening - [PATCH] hardening: Require clang 20.1.0 for __counted

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Message-Id: <20250807-fix-counted_by-clang-19-v1-1-902c86c1d515@kernel.org>
Date: Thu, 07 Aug 2025 14:36:28 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>, 
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Cc: linux-hardening@...r.kernel.org, llvm@...ts.linux.dev, 
 patches@...ts.linux.dev, Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] hardening: Require clang 20.1.0 for __counted_by

After an innocuous change in -next that modified a structure that
contains __counted_by, clang-19 start crashing when building certain
files in drivers/gpu/drm/xe. When assertions are enabled, the more
descriptive failure is:

  clang: clang/lib/AST/RecordLayoutBuilder.cpp:3335: const ASTRecordLayout &clang::ASTContext::getASTRecordLayout(const RecordDecl *) const: Assertion `D && "Cannot get layout of forward declarations!"' failed.

According to a reverse bisect, a tangential change to the LLVM IR
generation phase of clang during the LLVM 20 development cycle [1]
resolves this problem. Bump the version of clang that enables
CONFIG_CC_HAS_COUNTED_BY to 20.1.0 to ensure that this issue cannot be
hit.

Link: https://github.com/llvm/llvm-project/commit/160fb1121cdf703c3ef5e61fb26c5659eb581489 [1]
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
Should this be marked for stable? If this makes 6.17 final, I think we
are fine for mainline and newer but I wonder if a patch that adds
__counted_by to a structure would ever be backported and expose this
problem there.
---
 init/Kconfig | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f085455..a2675ee1ef8c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -117,10 +117,11 @@ config CC_HAS_NO_PROFILE_FN_ATTR

 config CC_HAS_COUNTED_BY
 	bool
-	# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
-	# https://github.com/llvm/llvm-project/pull/110497
-	# https://github.com/llvm/llvm-project/pull/112636
-	default y if CC_IS_CLANG && CLANG_VERSION >= 190103
+	# clang needs to be at least 20.1.0 to avoid potential crashes
+	# when building structures that contain __counted_by
+	# https://github.com/ClangBuiltLinux/linux/issues/2114
+	# https://github.com/llvm/llvm-project/commit/160fb1121cdf703c3ef5e61fb26c5659eb581489
+	default y if CC_IS_CLANG && CLANG_VERSION >= 200100
 	# supported since gcc 15.1.0
 	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
 	default y if CC_IS_GCC && GCC_VERSION >= 150100

---
base-commit: f627b51aaa041cba715b59026cf2d9cb1476c7ed
change-id: 20250807-fix-counted_by-clang-19-470d53f93aa0

Best regards,
--  
Nathan Chancellor <nathan@...nel.org>