diff mbox series

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

Message ID 20200317033701.w7jwos7mvfnde2t2@google.com
State Changes Requested
Delegated to: BPF Maintainers
Headers show
Series [bpf,v2] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF | expand

Commit Message

Fangrui Song March 17, 2020, 3:37 a.m. UTC 
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(-)

Comments

Andrii Nakryiko March 17, 2020, 5:09 a.m. UTC | #1 
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.

>
> -       # 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
>
Fangrui Song March 17, 2020, 5:21 a.m. UTC | #2 
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
>>
Andrii Nakryiko March 17, 2020, 5:31 a.m. UTC | #3 
On Mon, Mar 16, 2020 at 10:21 PM Fangrui Song <maskray@google.com> wrote:
>
>
> 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?

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.

> 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
> >>
diff mbox series

Patch

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}
  
-	# 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