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

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Date:   Thu, 23 Mar 2023 16:17:25 -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] Makefile: use -z pack-relative-relocs

Hi Fangrui,
Thanks for the patches!

On Thu, Mar 23, 2023 at 1:44 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, replace --pack-dyn-relocs=relr with -z pack-relative-relocs,
> which is supported since LLD 15.

Looks like
commit 4a8de2832a2a ("[ELF] Add -z pack-relative-relocs")

>
> 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.

Won't this regress support for CONFIG_TOOLS_SUPPORT_RELR for LLD
11-14? (which we still support, see Documentation/process/changes.rst;
I guess we don't list LLD explicitly, but it's effectively under the
llvm umbrella there).

IIUC, looks like lld 7.0+ should support SHT_RELR based on looking at
commit 11479daf2f06 ("lld: add experimental support for SHT_RELR sections.")

So we might want to invoke the linker twice, once with the new flag,
and again with the old if we fail the first time?

>
> 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>
> ---
>  Makefile                      | 2 +-
>  scripts/tools-support-relr.sh | 3 +--
>  2 files changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index a2c310df2145..30c2edc35813 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1113,7 +1113,7 @@ LDFLAGS_vmlinux   += -X
>  endif
>
>  ifeq ($(CONFIG_RELR),y)
> -LDFLAGS_vmlinux        += --pack-dyn-relocs=relr --use-android-relr-tags
> +LDFLAGS_vmlinux        += -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..58c2e8b02ce8 100755
> --- a/scripts/tools-support-relr.sh
> +++ b/scripts/tools-support-relr.sh
> @@ -7,8 +7,7 @@ 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 $tmp_file.o -shared -Bsymbolic -z pack-relative-relocs -o $tmp_file
>
>  # 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