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

Andrii Nakryiko
2020-03-17T03:14:52Z

On Mon, Mar 16, 2020 at 6:09 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2020-03-16, Stanislav Fomichev wrote:
> >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?
>
> llvm-objdump -f does not print bfdarch (ARCH= in binutils-gdb/ld/emulparams/*.sh).
>
> % objdump -f .btf.vmlinux.bin.o
>
> .btf.vmlinux.bin.o: file format elf64-x86-64
> architecture: i386:x86-64, flags 0x00000010:
> HAS_SYMS
> start address 0x0000000000000000
>
> % llvm-objdump -f .btf.vmlinux.bin.o
>
> .btf.vmlinux.bin.o: file format elf64-x86-64
>
> architecture: x86_64
> start address: 0x0000000000000000
>
> % objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
> # happy
> % objcopy -I binary -O elf64-x86-64 -B x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
> objcopy: architecture x86-64 unknown
>
>
> As a non-x86 example, elf64-powerpcle / powerpc:common64:
>
> % powerpc64le-linux-gnu-objdump -f meow.btf.vmlinux.bin.o
>
> meow.btf.vmlinux.bin.o: file format elf64-powerpcle
> architecture: powerpc:common64, flags 0x00000010:
>
>
> Unfortunately, GNU objcopy<2.34 (before I complained about the redundant -B https://sourceware.org/bugzilla/show_bug.cgi?id=24968)
> could not infer -B from -O elf* .
> % objcopy -I binary -O elf64-x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o #<2.34
> % file .btf.vmlinux.bin.o
> .btf.vmlinux.bin.o: ELF 64-bit LSB relocatable, no machine, version 1 (SYSV), not stripped
> objcopy: architecture x86-64 unknown
>
> GNU ld and lld will error for e_machine==0.
>
>
>
> I will be a bit nervous to make llvm-objdump behave more BFD like.
> Adding i386:x86-64, powerpc:common64, etc does not look particularly clean.
> Fortunately, looking at the code, it seems that we only want to retain .BTF
> The following scheme may be simpler:
>
> objcopy --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin.o && printf '\1' | dd of=.btf.vmlinux.bin.o conv=notrunc bs=1 seek=16
>

This part looks pretty crazy. Would it be simpler to detect whether
$(OBJCOPY) is llvm-objcopy and handle that in a bit less hacky way?

> The command after && is to change e_type from ET_EXEC to ET_REL. GNU ld has an extremely rare feature that allows ET_EXEC to be linked,
> but lld is more rigid and will reject such an input file.
> https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/