Re: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF

From: Fangrui Song <maskray@google.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	clang-built-linux@googlegroups.com,
	Stanislav Fomichev <sdf@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexei Starovoitov <ast@kernel.org>
Subject: Re: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF
Date: Mon, 16 Mar 2020 22:21:20 -0700	[thread overview]
Message-ID: <20200317052120.diawg3a75kxl5hkn@google.com> (raw)
In-Reply-To: <CAEf4BzYyimAo2_513kW6hrDWwmzSDhNjTYksjy01ugKKTPt+qA@mail.gmail.com>

On 2020-03-16, Andrii Nakryiko wrote:
>On Mon, Mar 16, 2020 at 8:37 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2020-03-16, Andrii Nakryiko wrote:
>> >On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song <maskray@google.com> wrote:
>> >>
>> >> Simplify gen_btf logic to make it work with llvm-objcopy and
>> >> llvm-objdump.  We just need to retain one section .BTF. To do so, we can
>> >> use a simple objcopy --only-section=.BTF instead of jumping all the
>> >> hoops via an architecture-less binary file.
>> >>
>> >> We use a dd comment to change the e_type field in the ELF header from
>> >> ET_EXEC to ET_REL so that .btf.vmlinux.bin.o will be accepted by lld.
>> >>
>> >> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
>> >> Cc: Stanislav Fomichev <sdf@google.com>
>> >> Cc: Nick Desaulniers <ndesaulniers@google.com>
>> >> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
>> >> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>> >> Link: https://github.com/ClangBuiltLinux/linux/issues/871
>> >> Signed-off-by: Fangrui Song <maskray@google.com>
>> >> ---
>> >>  scripts/link-vmlinux.sh | 13 ++-----------
>> >>  1 file changed, 2 insertions(+), 11 deletions(-)
>> >>
>> >> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> >> index dd484e92752e..84be8d7c361d 100755
>> >> --- a/scripts/link-vmlinux.sh
>> >> +++ b/scripts/link-vmlinux.sh
>> >> @@ -120,18 +120,9 @@ gen_btf()
>> >>
>> >>         info "BTF" ${2}
>> >>         vmlinux_link ${1}
>> >> -       LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
>> >
>> >Is it really tested? Seems like you just dropped .BTF generation step
>> >completely...
>>
>> Sorry, dropped the whole line:/
>> I don't know how to test .BTF . I can only check readelf -S...
>>
>> Attached the new patch.
>>
>>
>>  From 02afb9417d4f0f8d2175c94fc3797a94a95cc248 Mon Sep 17 00:00:00 2001
>> From: Fangrui Song <maskray@google.com>
>> Date: Mon, 16 Mar 2020 18:02:31 -0700
>> Subject: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for
>>   vmlinux BTF
>>
>> Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump.
>> We use a dd comment to change the e_type field in the ELF header from
>> ET_EXEC to ET_REL so that .btf.vmlinux.bin.o can be accepted by lld.
>>
>> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
>> Cc: Stanislav Fomichev <sdf@google.com>
>> Cc: Nick Desaulniers <ndesaulniers@google.com>
>> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/871
>> Signed-off-by: Fangrui Song <maskray@google.com>
>> ---
>>   scripts/link-vmlinux.sh | 14 +++-----------
>>   1 file changed, 3 insertions(+), 11 deletions(-)
>>
>> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> index dd484e92752e..b23313944c89 100755
>> --- a/scripts/link-vmlinux.sh
>> +++ b/scripts/link-vmlinux.sh
>> @@ -120,18 +120,10 @@ gen_btf()
>>
>>         info "BTF" ${2}
>>         vmlinux_link ${1}
>> -       LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
>> +       ${PAHOLE} -J ${1}
>
>I'm not sure why you are touching this line at all. LLVM_OBJCOPY part
>is necessary, pahole assumes llvm-objcopy by default, but that can
>(and should for objcopy) be overridden with LLVM_OBJCOPY.

Why is LLVM_OBJCOPY assumed? What if llvm-objcopy is not available?
This is confusing that one tool assumes llvm-objcopy while the block
below immediately uses GNU objcopy (without this patch).

e83b9f55448afce3fe1abcd1d10db9584f8042a6 "kbuild: add ability to
generate BTF type info for vmlinux" does not say why LLVM_OBJCOPY is
set.

>>
>> -       # dump .BTF section into raw binary file to link with final vmlinux
>> -       bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \
>> -               cut -d, -f1 | cut -d' ' -f2)
>> -       bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
>> -               awk '{print $4}')
>> -       ${OBJCOPY} --change-section-address .BTF=0 \
>> -               --set-section-flags .BTF=alloc -O binary \
>> -               --only-section=.BTF ${1} .btf.vmlinux.bin
>> -       ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
>> -               --rename-section .data=.BTF .btf.vmlinux.bin ${2}
>> +       # Extract .BTF section, change e_type to ET_REL, to link with final vmlinux
>> +       ${OBJCOPY} --only-section=.BTF ${1} ${2} && printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16
>>   }
>>
>>   # Create ${2} .o file with all symbols from the ${1} object file
>> --
>> 2.25.1.481.gfbce0eb801-goog
>>