Re: [PATCH bpf-next v6] bpf: Support llvm-objcopy for vmlinux BTF

On Wed, Mar 18, 2020 at 3:27 PM Fangrui Song [off-list ref] wrote:

Simplify gen_btf logic to make it work with llvm-objcopy. The existing
'file format' and 'architecture' parsing logic is brittle and does not
work with llvm-objcopy/llvm-objdump.
'file format' output of llvm-objdump>=11 will match GNU objdump, but
'architecture' (bfdarch) may not.

.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag
because it is part of vmlinux image used for introspection. C code can
reference the section via linker script defined __start_BTF and
__stop_BTF. This fixes a small problem that previous .BTF had the
SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).

Additionally, `objcopy -I binary` synthesized symbols
_binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not
used elsewhere) are replaced with more commonplace __start_BTF and
__stop_BTF.

Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns
"empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"

We use a dd command to change the e_type field in the ELF header from
ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o.  Accepting
ET_EXEC as an input file is an extremely rare GNU ld feature that lld
does not intend to support, because this is error-prone.

The output section description .BTF in include/asm-generic/vmlinux.lds.h
avoids potential subtle orphan section placement issues and suppresses
--orphan-handling=warn warnings.

v6:
- drop llvm-objdump from the title. We don't run objdump now
- delete unused local variables: bin_arch, bin_format and bin_file
- mention in the comment that lld does not allow an ET_EXEC input
- rename BTF back to .BTF . The section name is assumed by bpftool
- add output section description to include/asm-generic/vmlinux.lds.h
- mention cb0cc635c7a9 ("powerpc: Include .BTF section")

v5:
- rebase on top of bpf-next/master
- rename .BTF to BTF

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section")
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Fangrui Song <redacted>
Reported-by: Nathan Chancellor <redacted>
Reviewed-by: Stanislav Fomichev <redacted>
Tested-by: Stanislav Fomichev <redacted>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <redacted>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Kees Cook <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N. Rao <redacted>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: clang-built-linux@googlegroups.com
---

Thanks for detailed commit message and comments in the script, that's
very helpful. Looks good to me, I've tested with my local setup and
everything works across bpftool, selftests and my private BTF tool,
which doesn't use libbpf.

Acked-by: Andrii Nakryiko <redacted>
Tested-by: Andrii Nakryiko <redacted>

 arch/powerpc/kernel/vmlinux.lds.S |  6 ------
 include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++
 kernel/bpf/btf.c                  |  9 ++++-----
 kernel/bpf/sysfs_btf.c            | 11 +++++------
 scripts/link-vmlinux.sh           | 24 ++++++++++--------------
 5 files changed, 34 insertions(+), 31 deletions(-)

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