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

[Fangrui Song <maskray@google.com>]

On 2020-03-16, Stanislav Fomichev wrote:
>Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
>BTF") switched from --dump-section to
>--only-section/--change-section-address for BTF export assuming
>those ("legacy") options should cover all objcopy versions.
>
>Turns out llvm-objcopy doesn't implement --change-section-address [1],
>but it does support --dump-section. Let's partially roll back and
>try to use --dump-section first and fall back to
>--only-section/--change-section-address for the older binutils.
>
>1. https://bugs.llvm.org/show_bug.cgi?id=45217
>
>Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
>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: Stanislav Fomichev <sdf@google.com>
>---
> scripts/link-vmlinux.sh | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
>diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>index dd484e92752e..8ddf57cbc439 100755
>--- a/scripts/link-vmlinux.sh
>+++ b/scripts/link-vmlinux.sh
>@@ -127,6 +127,16 @@ gen_btf()
> 		cut -d, -f1 | cut -d' ' -f2)
> 	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> 		awk '{print $4}')
>+
>+	# Compatibility issues:
>+	# - pre-2.25 binutils objcopy doesn't support --dump-section
>+	# - llvm-objcopy doesn't support --change-section-address, but
>+	#   does support --dump-section
>+	#
>+	# Try to use --dump-section which should cover both recent
>+	# binutils and llvm-objcopy and fall back to --only-section
>+	# for pre-2.25 binutils.
>+	${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> 	${OBJCOPY} --change-section-address .BTF=0 \
> 		--set-section-flags .BTF=alloc -O binary \
> 		--only-section=.BTF ${1} .btf.vmlinux.bin
>-- 
>2.25.1.481.gfbce0eb801-goog

So let me take advantage of this email to ask some questions about
commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").

Does .BTF have the SHF_ALLOC flag?
Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?

Regarding

>Turns out llvm-objcopy doesn't implement --change-section-address [1],

This option will be difficult to implement in llvm-objcopy if we intend
it to have a GNU objcopy compatible behavior.
Without --only-section, it is not very clear how
--change-section-{address,vma,lma} will affect program headers.
There will be a debate even if we decide to implement them in llvm-objcopy.

Some PT_LOAD rewriting examples:

   objcopy --change-section-address .plt=0 a b
   objcopy --change-section-address .text=0 a b

There is another bug related to -B
(https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):

+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
-O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
+ objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
objcopy: architecture x86_64 unknown
+ echo 'Failed to generate BTF for vmlinux'

It should be i386:x86_64.