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Date:   Mon, 16 Mar 2020 22:43:59 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux@...glegroups.com,
        Stanislav Fomichev <sdf@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: [PATCH bpf v3] bpf: Support llvm-objcopy and llvm-objdump for
 vmlinux BTF

On 2020-03-16, Andrii Nakryiko wrote:
>On Mon, Mar 16, 2020 at 10:21 PM Fangrui Song <maskray@...gle.com> wrote:
>>
>>
>> On 2020-03-16, Andrii Nakryiko wrote:
>> >On Mon, Mar 16, 2020 at 8:37 PM Fangrui Song <maskray@...gle.com> wrote:
>> >>
>> >> On 2020-03-16, Andrii Nakryiko wrote:
>> >> >On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song <maskray@...gle.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@...gle.com>
>> >> >> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
>> >> >> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> >> >> Reported-by: Nathan Chancellor <natechancellor@...il.com>
>> >> >> Link: https://github.com/ClangBuiltLinux/linux/issues/871
>> >> >> Signed-off-by: Fangrui Song <maskray@...gle.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@...gle.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@...gle.com>
>> >> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
>> >> Reported-by: Nathan Chancellor <natechancellor@...il.com>
>> >> Link: https://github.com/ClangBuiltLinux/linux/issues/871
>> >> Signed-off-by: Fangrui Song <maskray@...gle.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?
>
>It's pahole assumption that we have to live with. pahole assumes
>llvm-objcopy internally, unless it is overriden with LLVM_OBJCOPY env
>var. So please revert this line otherwise you are breaking it for GCC
>objcopy case.

Acknowledged. Uploaded v3.

I added back 2>/dev/null which was removed by a previous change, to
suppress GNU objcopy warnings. The warnings could be annoying in V=1
output.

>> 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
>> >>

 From ca3597477542453e9f63185c27c162da081a4baf Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@...gle.com>
Date: Mon, 16 Mar 2020 22:38:23 -0700
Subject: [PATCH bpf v3] bpf: Support llvm-objcopy and llvm-objdump for
  vmlinux BTF

Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump.
Add 2>/dev/null to suppress GNU objcopy (but not llvm-objcopy) warnings
"empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"
Our use of --only-section drops many SHF_ALLOC sections which will essentially nullify
program headers. When used as linker input, program headers are simply
ignored.

We use a dd command 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.
Accepting ET_EXEC as an input file is an extremely rare GNU ld feature
that lld does not intend to support, because this is very error-prone.

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Cc: Stanislav Fomichev <sdf@...gle.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Reported-by: Nathan Chancellor <natechancellor@...il.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Fangrui Song <maskray@...gle.com>
---
  scripts/link-vmlinux.sh | 12 ++----------
  1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..c3e808a89d4a 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -122,16 +122,8 @@ gen_btf()
  	vmlinux_link ${1}
  	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
  
-	# 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} ${2} 2> /dev/null && printf '\1' | dd of=${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

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