linux-kernel - Re: [PATCH v2] Makefile: use -z pack-relative-relocs

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Message-ID: <CAKwvOd=QXGbrxct20cBia92=QonWtfWdC21WK4w2bRBprPXh=w@mail.gmail.com>
Date:   Fri, 7 Apr 2023 11:35:24 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Collingbourne <pcc@...gle.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2] Makefile: use -z pack-relative-relocs

On Fri, Mar 24, 2023 at 2:22 PM Fangrui Song <maskray@...gle.com> wrote:
>
> Commit 27f2a4db76e8 ("Makefile: fix GDB warning with CONFIG_RELR")
> added --use-android-relr-tags to fix a GDB warning
>
> BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'
>
> The GDB warning has been fixed in version 11.2.
>
> The DT_ANDROID_RELR tag was deprecated since DT_RELR was standardized.
> Thus, --use-android-relr-tags should be removed. While making the
> change, try -z pack-relative-relocs, which is supported since LLD 15.
> Keep supporting --pack-dyn-relocs=relr as well for older LLD versions.
>
> As of today, GNU ld supports the latter option for x86 and powerpc64
> ports and has no intention to support --pack-dyn-relocs=relr. In the
> absence of the glibc symbol version GLIBC_ABI_DT_RELR,
> --pack-dyn-relocs=relr and -z pack-relative-relocs are identical in
> ld.lld.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1057
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a619b58721f0a03fd91c27670d3e4c2fb0d88f1e
> Signed-off-by: Fangrui Song <maskray@...gle.com>

Thanks v2 looks better. IIUC, this will first try to test+use
`--pack-dyn-relocs=relr` in preference to `-z pack-relative-relocs`.
Do we want to reorder the preference, for both the test and actual
flag used?

> ---
>  Makefile                      | 3 ++-
>  scripts/tools-support-relr.sh | 8 ++++++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
> ---
> Changes from v1:
> * Keep supporting --pack-dyn-relocs=relr for older ld.lld versions
>
> diff --git a/Makefile b/Makefile
> index a2c310df2145..e23a85476d5d 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1113,7 +1113,8 @@ LDFLAGS_vmlinux   += -X
>  endif
>
>  ifeq ($(CONFIG_RELR),y)
> -LDFLAGS_vmlinux        += --pack-dyn-relocs=relr --use-android-relr-tags
> +# ld.lld before 15 did not support -z pack-relative-relocs.
> +LDFLAGS_vmlinux        += $(call ld-option,--pack-dyn-relocs=relr,-z pack-relative-relocs)
>  endif
>
>  # We never want expected sections to be placed heuristically by the
> diff --git a/scripts/tools-support-relr.sh b/scripts/tools-support-relr.sh
> index cb55878bd5b8..4c121946e517 100755
> --- a/scripts/tools-support-relr.sh
> +++ b/scripts/tools-support-relr.sh
> @@ -7,8 +7,12 @@ trap "rm -f $tmp_file.o $tmp_file $tmp_file.bin" EXIT
>  cat << "END" | $CC -c -x c - -o $tmp_file.o >/dev/null 2>&1
>  void *p = &p;
>  END
> -$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr \
> -  --use-android-relr-tags -o $tmp_file
> +
> +# ld.lld before 15 did not support -z pack-relative-relocs.
> +if ! $LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr -o $tmp_file 2>/dev/null; then
> +       $LD $tmp_file.o -shared -Bsymbolic -z pack-relative-relocs -o $tmp_file 2>&1 |
> +               grep -q pack-relative-relocs && exit 1
> +fi
>
>  # Despite printing an error message, GNU nm still exits with exit code 0 if it
>  # sees a relr section. So we need to check that nothing is printed to stderr.
> --
> 2.40.0.348.gf938b09366-goog
>


-- 
Thanks,
~Nick Desaulniers