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

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Message-ID: <CAFhGd8pkSP9JTVnBzn7-Cm6JMtj-qBAgLMBP1JhLtt4unkruZQ@mail.gmail.com>
Date: Thu, 7 Aug 2025 15:12:06 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Bill Wendling <morbo@...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

Hi,

On Thu, Aug 7, 2025 at 2:36 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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:

Since this is just now breaking builds, it is OK to bump up the
minimum compiler version just in time.

For future travelers, the "innocuous change" popped up in -next as
Commit 535c445eb94c ("drm/xe: Add dependency scheduler for GT TLB
invalidations to bind queues"). I am not sure why the additional
struct member caused this crash but it seems like Bill's clang patch
solved it -- so I won't look into it any further.

>
>   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]

Reviewed-by: Justin Stitt <justinstitt@...gle.com>

> 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>
>