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

From: Daniel Borkmann <daniel@iogearbox.net>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Fangrui Song <maskray@google.com>,
	bpf@vger.kernel.org, netdev@vger.kernel.org
Cc: Nathan Chancellor <natechancellor@gmail.com>,
	Stanislav Fomichev <sdf@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Kees Cook <keescook@chromium.org>,
	"Naveen N . Rao" <naveen.n.rao@linux.vnet.ibm.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH bpf-next v6] bpf: Support llvm-objcopy for vmlinux BTF
Date: Thu, 19 Mar 2020 13:04:43 +0100	[thread overview]
Message-ID: <b5d153ad-23fe-a367-100b-ea9f19ae6958@iogearbox.net> (raw)
In-Reply-To: <87tv2kd4hn.fsf@mpe.ellerman.id.au>

On 3/19/20 11:27 AM, Michael Ellerman wrote:
> Fangrui Song <maskray@google.com> writes:
>> 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 <maskray@google.com>
>> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>> Reviewed-by: Stanislav Fomichev <sdf@google.com>
>> Tested-by: Stanislav Fomichev <sdf@google.com>
>> Cc: Alexei Starovoitov <ast@kernel.org>
>> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Cc: David S. Miller <davem@davemloft.net>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> Cc: Nick Desaulniers <ndesaulniers@google.com>
>> Cc: clang-built-linux@googlegroups.com

Applied, thanks everyone!