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Date:   Fri, 24 Mar 2023 00:12:28 +0000
From:   Fangrui Song <maskray@google.com>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        Peter Collingbourne <pcc@google.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH] Makefile: use -z pack-relative-relocs
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On 2023-03-23, Nick Desaulniers wrote:
>Hi Fangrui,
>Thanks for the patches!
>
>On Thu, Mar 23, 2023 at 1:44 PM Fangrui Song <maskray@google.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?

Yes, I am aware of the increased linker and debugger requirement and was
thinking the CONFIG_RELR=y case with ld.lld<15 did not matter.

Since you raised this, it seems that we should ld.lld<15. How about the following diff?


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..355a9b1eb150 100755
--- a/scripts/tools-support-relr.sh
+++ b/scripts/tools-support-relr.sh
@@ -7,8 +7,9 @@ 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 $tmp_file
+# ld.lld before 15 did not support -z pack-relative-relocs.
+$LD $tmp_file.o -shared -Bsymbolic -z pack-relative-relocs -o $tmp_file || \
+	$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr -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


>>
>> 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@google.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