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

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Message-ID: <20250811210906.GA924329@ax162>
Date: Mon, 11 Aug 2025 14:09:06 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
	patches@...ts.linux.dev
Subject: Re: [PATCH] hardening: Require clang 20.1.0 for __counted_by

On Mon, Aug 11, 2025 at 04:21:32PM +0200, Alexander Lobakin wrote:
> From: Nathan Chancellor <nathan@...nel.org>
> Date: Thu, 07 Aug 2025 14:36:28 -0700
> 
> > 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.
> 
> Any chance for this to go to the next 19.x (if it's planned at all)?
> I always use the latest HEAD from llvm-project, but 19 is still widely
> used across distros etc =\

Unfortunately not, LLVM does not maintain more than one branch at a
time, so LLVM 19 has been unsupported since LLVM 20.1.0-rc1 was
released back in February :/

Some distros may be willing to patch that in but there is not really a
good way for us to check for that here, so I am not sure it helps much.

Kees and Bill may have thoughts around working around this in the one
spot we know it happens but in my opinion, I would rather do the big
hammer. The deployment of __counted_by in the kernel is only growing so
it is totally possible for us to fix this one instance then run into
another case in a few months, which is more painful for clang than GCC
since it has been supported for a bigger number of releases.

Cheers,
Nathan