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

[Stanislav Fomichev <sdf@fomichev.me>]

On 03/16, Fangrui Song wrote:
> 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?
No, that's why we manually do '--set-section-flags .BTF=alloc' to
make --only-section work.

> 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?
I think that '--set-section-flags .BTF=alloc' causes objcopy to put
some non-zero (valid) sh_addr, that's why we need to reset it to 0.

(it's not clear if it's a feature or a bug and man isn't helpful)

> 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.
Here is what I get:

+ bin_arch=i386:x86-64
+ bin_format=elf64-x86-64
+ 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 i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.

Can you try to see where your x86_64 is coming from?