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

On 3/19/20 11:27 AM, Michael Ellerman wrote:

Fangrui Song [off-list ref] writes:

quoted

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

Applied, thanks everyone!