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Date: Thu, 25 Jan 2024 15:10:24 -0800
From: Kees Cook <keescook@chromium.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: akpm@linux-foundation.org, masahiroy@kernel.org, nicolas@fjasle.eu,
	linux-kbuild@vger.kernel.org, llvm@lists.linux.dev,
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Subject: Re: [PATCH 00/11] Bump the minimum supported version of LLVM to
 13.0.1
Message-ID: <202401251509.17795F434@keescook>
References: <20240125-bump-min-llvm-ver-to-13-0-1-v1-0-f5ff9bda41c5@kernel.org>
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In-Reply-To: <20240125-bump-min-llvm-ver-to-13-0-1-v1-0-f5ff9bda41c5@kernel.org>

On Thu, Jan 25, 2024 at 03:55:06PM -0700, Nathan Chancellor wrote:
> Hi all,
> 
> This series bumps the minimum supported version of LLVM for building the
> kernel to 13.0.1. The first patch does the bump and all subsequent
> patches clean up all the various workarounds and checks for earlier
> versions.
> 
> Quoting the first patch's commit message for those that were only on CC
> for the clean ups:
> 
>   When __builtin_mul_overflow() has arguments that differ in terms of
>   signedness and width, LLVM may generate a libcall to __muloti4 because
>   it performs the checks in terms of 65-bit multiplication. This issue
>   becomes harder to hit (but still possible) after LLVM 12.0.0, which
>   includes a special case for matching widths but different signs.
> 
>   To gain access to this special case, which the kernel can take advantage
>   of when calls to __muloti4 appear, bump the minimum supported version of
>   LLVM for building the kernel to 13.0.1. 13.0.1 was chosen because there
>   is minimal impact to distribution support while allowing a few more
>   workarounds to be dropped in the kernel source than if 12.0.0 were
>   chosen. Looking at container images of up to date distribution versions:
> 
>     archlinux:latest              clang version 16.0.6
>     debian:oldoldstable-slim      clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)
>     debian:oldstable-slim         Debian clang version 11.0.1-2
>     debian:stable-slim            Debian clang version 14.0.6
>     debian:testing-slim           Debian clang version 16.0.6 (19)
>     debian:unstable-slim          Debian clang version 16.0.6 (19)
>     fedora:38                     clang version 16.0.6 (Fedora 16.0.6-3.fc38)
>     fedora:latest                 clang version 17.0.6 (Fedora 17.0.6-1.fc39)
>     fedora:rawhide                clang version 17.0.6 (Fedora 17.0.6-1.fc40)
>     opensuse/leap:latest          clang version 15.0.7
>     opensuse/tumbleweed:latest    clang version 17.0.6
>     ubuntu:focal                  clang version 10.0.0-4ubuntu1
>     ubuntu:latest                 Ubuntu clang version 14.0.0-1ubuntu1.1
>     ubuntu:rolling                Ubuntu clang version 16.0.6 (15)
>     ubuntu:devel                  Ubuntu clang version 17.0.6 (3)
> 
>   The only distribution that gets left behind is Debian Bullseye, as the
>   default version is 11.0.1; other distributions either have a newer
>   version than 13.0.1 or one older than the current minimum of 11.0.0.
>   Debian has easy access to more recent LLVM versions through
>   apt.llvm.org, so this is not as much of a concern. There are also the
>   kernel.org LLVM toolchains, which should work with distributions with
>   glibc 2.28 and newer.
> 
>   Another benefit of slimming up the number of supported versions of LLVM
>   for building the kernel is reducing the build capacity needed to support
>   a matrix that builds with each supported version, which allows a matrix
>   to reallocate the freed up build capacity towards something else, such
>   as more configuration combinations.
> 
> This passes my build matrix with all supported versions.
> 
> This is based on Andrew's mm-nonmm-unstable to avoid trivial conflicts
> with my series to update the LLVM links across the repository [1] but I
> can easily rebase it to linux-kbuild if Masahiro would rather these
> patches go through there (and defer the conflict resolution to the merge
> window).
> 
> [1]: https://lore.kernel.org/20240109-update-llvm-links-v1-0-eb09b59db071@kernel.org/
> 
> ---
> Nathan Chancellor (11):
>       kbuild: Raise the minimum supported version of LLVM to 13.0.1
>       Makefile: Drop warn-stack-size plugin opt
>       x86: Drop stack-alignment plugin opt
>       ARM: Remove Thumb2 __builtin_thread_pointer workaround for Clang
>       arm64: Kconfig: Clean up tautological LLVM version checks
>       powerpc: Kconfig: Remove tautology in CONFIG_COMPAT
>       riscv: Remove MCOUNT_NAME workaround
>       riscv: Kconfig: Remove version dependency from CONFIG_CLANG_SUPPORTS_DYNAMIC_FTRACE
>       fortify: Drop Clang version check for 12.0.1 or newer
>       lib/Kconfig.debug: Update Clang version check in CONFIG_KCOV
>       compiler-clang.h: Update __diag_clang() macros for minimum version bump
> 
>  Documentation/process/changes.rst |  2 +-
>  Makefile                          |  8 --------
>  arch/arm/include/asm/current.h    |  8 +-------
>  arch/arm64/Kconfig                |  5 +----
>  arch/powerpc/Kconfig              |  1 -
>  arch/riscv/Kconfig                |  2 --
>  arch/riscv/include/asm/ftrace.h   | 14 ++------------
>  arch/riscv/kernel/mcount.S        | 10 +++++-----
>  arch/x86/Makefile                 |  6 ------
>  include/linux/compiler-clang.h    |  8 ++------
>  lib/Kconfig.debug                 |  2 +-
>  scripts/min-tool-version.sh       |  2 +-
>  scripts/recordmcount.pl           |  2 +-
>  security/Kconfig                  |  2 --
>  14 files changed, 15 insertions(+), 57 deletions(-)
> ---
> base-commit: 979741ebd48f75ed6d101c7290e3325340d361ff
> change-id: 20240124-bump-min-llvm-ver-to-13-0-1-39f84dd36b19
> 
> Best regards,
> -- 
> Nathan Chancellor <nathan@kernel.org>
> 

Yes, please. :) This looks reasonable -- I appreciate the review of
default Clang versions across distros!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook