linux-kernel - Re: Update15.x/kcfi to LLVM 15.0.3 (ZSTD + DWARFv5)

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Date:   Thu, 20 Oct 2022 09:56:50 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     "Sedat Dilek (DHL Supply Chain)" <sedat.dilek@....com>,
        Fangrui Song <maskray@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: Update15.x/kcfi to LLVM 15.0.3 (ZSTD + DWARFv5)

On Wed, Oct 19, 2022 at 10:44 PM Sedat Dilek (DHL Supply Chain)
<sedat.dilek@....com> wrote:
>
> -----Ursprüngliche Nachricht-----
> Von: Sami Tolvanen <samitolvanen@...gle.com>
> Gesendet: Dienstag, 18. Oktober 2022 18:00
> An: Sedat Dilek (DHL Supply Chain) <sedat.dilek@....com>
> Betreff: Re: Update15.x/kcfi to LLVM 15.0.3
>
> On Tue, Oct 18, 2022 at 4:54 AM Sedat Dilek (DHL Supply Chain) <sedat.dilek@....com> wrote:
> >
> [ CC Nick + Fangrui ]
>
> You happen to know or use the ZSTD cmake-option (see [1] and [2]) - came in with changes post-15.0.2?
>
> AFAICS Fangrui Song had some patches to use ZSTD debug-compression with DWARFv5.
>
> Unsure, if that combination is now possible in v15.0.3 or is a feature of LLVM-16.
>
> [3] says:
>
> [ lib/Kconfig.debug ]
>
> config DEBUG_INFO_COMPRESSED
>         bool "Compressed debugging information"
>         depends on $(cc-option,-gz=zlib)
>         depends on $(ld-option,--compress-debug-sections=zlib)
>         help
>           Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
>           5.0+, binutils 2.26+, and zlib.
>
>           Users of dpkg-deb via scripts/package/builddeb may find an increase in
>           size of their debug .deb packages with this config set, due to the
>           debug info being compressed with zlib, then the object files being
>           recompressed with a different compression scheme. But this is still
>           preferable to setting $KDEB_COMPRESS to "none" which would be even
>           larger.
>
> I am interested in the equivalent changes to cc-option/ld-option with ZSTD (usable with DWARFv5).

Good idea.  It looks like there's still a blocker to using -gz=zstd with clang:
$ clang -gz=zstd -x c - < /dev/null
clang-16: warning: cannot compress debug sections (zstd not enabled)
[-Wdebug-compression-unavailable]

https://github.com/facebook/zstd/issues/3271

Attaching a WIP patch.

>
> Thanks.
>
> Best regards,
> -Sedat-
>
> [1] https://github.com/llvm/llvm-project/commit/6fba7854a2f0b6b3899bb156c1a0c4ae35c96e24
> [2] https://github.com/llvm/llvm-project/commit/b4840279846e1eea44c3dca575395a90c9d77ca0
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/Kconfig.debug#n315
> [4] https://metadata.ftp-master.debian.org/changelogs//main/l/llvm-toolchain-15/llvm-toolchain-15_15.0.3-1~exp1_changelog
>
>
>


-- 
Thanks,
~Nick Desaulniers

View attachment "0001-WIP-Makefile.debug-support-for-gz-zstd.patch" of type "text/x-patch" (2836 bytes)