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From:   Eric Curtin <ecurtin@redhat.com>
Date:   Sun, 8 Jan 2023 15:18:01 +0000
Message-ID: <CAOgh=Fzc3_bCLCKE+6KVzyMbBUOcQ_s9ef6Rw33amD5+yu-_WA@mail.gmail.com>
Subject: Re: [PATCH v2] scripts: Exclude Rust CUs with pahole
To:     Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Neal Gompa <neal@gompa.dev>, bpf@vger.kernel.org,
        rust-for-linux@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>, Yonghong Song <yhs@fb.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
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On Sun, 8 Jan 2023 at 02:15, Martin Rodriguez Reboredo
<yakoyoku@gmail.com> wrote:
>
> Version 1.24 of pahole has the capability to exclude compilation units
> (CUs) of specific languages. Rust, as of writing, is not currently
> supported by pahole and if it's used with a build that has BTF debugging
> enabled it results in malformed kernel and module binaries (see
> Rust-for-Linux/linux#735). So it's better for pahole to exclude Rust
> CUs until support for it arrives.
>
> Reviewed-by: Eric Curtin <ecurtin@redhat.com>
> Tested-by: Eric Curtin <ecurtin@redhat.com>
> Reviewed-by: Neal Gompa <neal@gompa.dev>
> Tested-by: Neal Gompa <neal@gompa.dev>
> Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> ---
> V1 -> V2: Removed dependency on auto.conf
>
>  init/Kconfig              | 2 +-
>  lib/Kconfig.debug         | 9 +++++++++
>  scripts/Makefile.modfinal | 4 ++++
>  scripts/link-vmlinux.sh   | 4 ++++
>  4 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 694f7c160c9c..360aef8d7292 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1913,7 +1913,7 @@ config RUST
>         depends on !MODVERSIONS
>         depends on !GCC_PLUGINS
>         depends on !RANDSTRUCT
> -       depends on !DEBUG_INFO_BTF
> +       depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
>         select CONSTRUCTORS
>         help
>           Enables Rust support in the kernel.
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index ea4c903c9868..d473d491e709 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -364,6 +364,15 @@ config PAHOLE_HAS_BTF_TAG
>           btf_decl_tag) or not. Currently only clang compiler implements
>           these attributes, so make the config depend on CC_IS_CLANG.
>
> +config PAHOLE_HAS_LANG_EXCLUDE
> +       def_bool PAHOLE_VERSION >= 124
> +       help
> +         Support for the --lang_exclude flag which makes pahole exclude
> +         compilation units from the supplied language. Used in Kbuild to
> +         omit Rust CUs which are not supported in version 1.24 of pahole,
> +         otherwise it would emit malformed kernel and module binaries when
> +         using DEBUG_INFO_BTF_MODULES.
> +
>  config DEBUG_INFO_BTF_MODULES
>         def_bool y
>         depends on DEBUG_INFO_BTF && MODULES && PAHOLE_HAS_SPLIT_BTF
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index 25bedd83644b..a880f2d6918f 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -30,6 +30,10 @@ quiet_cmd_cc_o_c = CC [M]  $@
>
>  ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
>
> +ifdef CONFIG_RUST
> +PAHOLE_FLAGS += --lang_exclude=rust
> +endif
> +
>  quiet_cmd_ld_ko_o = LD [M]  $@
>        cmd_ld_ko_o +=                                                   \
>         $(LD) -r $(KBUILD_LDFLAGS)                                      \
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 918470d768e9..69eb0bea89bf 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -122,6 +122,10 @@ gen_btf()
>                 return 1
>         fi
>
> +       if is_enabled CONFIG_RUST; then
> +               PAHOLE_FLAGS="${PAHOLE_FLAGS} --lang_exclude=rust"
> +       fi

If it was me, I would do things more like v1 of the patch (instead
just checking pahole version), because this is the only flag set in
scripts/Makefile.modfinal, which is a little confusing and
inconsistent. It's ok to set --lang_exclude=rust in all cases, as long
as pahole_ver is recent enough.

+if [ "${pahole_ver}" -ge "124" ]; then
+       # see PAHOLE_HAS_LANG_EXCLUDE
+       extra_paholeopt="${extra_paholeopt} --lang_exclude=rust"
+fi

But I'm not too opinionated either on this so...

Reviewed-by: Eric Curtin <ecurtin@redhat.com>

can be reapplied. I'm gonna test this again to see if it works in a
Fedora Asahi rpm build.


> +
>         vmlinux_link ${1}
>
>         info "BTF" ${2}
> --
> 2.39.0
>