netdev - Re: [PATCH bpf-next] kbuild: add ability to generate BTF type info for vmlinux

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Date:   Wed, 3 Apr 2019 11:21:06 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     andrii.nakryiko@...il.com, bpf@...r.kernel.org,
        netdev@...r.kernel.org, kernel-team@...com,
        Andrii Nakryiko <andriin@...com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Alexei Starovoitov <ast@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] kbuild: add ability to generate BTF type info
 for vmlinux

On 04/03/2019 11:12 AM, Jiri Olsa wrote:
> On Wed, Apr 03, 2019 at 11:04:28AM +0200, Daniel Borkmann wrote:
>> On 04/03/2019 10:46 AM, Jiri Olsa wrote:
>>> On Tue, Apr 02, 2019 at 09:49:50AM -0700, andrii.nakryiko@...il.com wrote:
>>>> From: Andrii Nakryiko <andriin@...com>
>>>>
>>>> This patch adds new config option to trigger generation of BTF type
>>>> information from DWARF debuginfo for vmlinux and kernel modules through
>>>> pahole, which in turn relies on libbpf for btf_dedup() algorithm.
>>>>
>>>> The intent is to record compact type information of all types used
>>>> inside kernel, including all the structs/unions/typedefs/etc. This
>>>> enables BPF's compile-once-run-everywhere ([0]) approach, in which
>>>> tracing programs that are inspecting kernel's internal data (e.g.,
>>>> struct task_struct) can be compiled on a system running some kernel
>>>> version, but would be possible to run on other kernel versions (and
>>>> configurations) without recompilation, even if the layout of structs
>>>> changed and/or some of the fields were added, removed, or renamed.
>>>>
>>>> This is only possible if BPF loader can get kernel type info to adjust
>>>> all the offsets correctly. This patch is a first time in this direction,
>>>> making sure that BTF type info is part of Linux kernel image in
>>>> non-loadable ELF section.
>>>>
>>>> BTF deduplication ([1]) algorithm typically provides 100x savings
>>>> compared to DWARF data, so resulting .BTF section is not big as is
>>>> typically about 2MB in size.
>>>
>>> hi,
>>> I'm using the latest pahole from git tree:
>>>   https://github.com/acmel/dwarves.git
>>>
>>> and getting pahole crash:
>>>
>>>   LD      vmlinux
>>>   BTF     vmlinux
>>> die__process_inline_expansion: DW_TAG_label (0xa) @ <0x3eef8> not handled!
>>> scripts/link-vmlinux.sh: line 96: 31222 Segmentation fault      (core dumped) LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
>>> make[1]: *** [/home/jolsa/linux/Makefile:1025: vmlinux] Error 139
>>> make: *** [Makefile:170: sub-make] Error 2
>>>
>>> is there some other source/dependency I'm missing?
>>
>> Yesterday night, I've tested with [0] but seems to have the same HEAD as
>> the github repo you pointed out. Seems the above is coming from pahole's
>> __die__process_tag() bailing out with default for DW_TAG_label?
>>
>> On my side worked fine:
> 
> hum, I also had to change the version of pahole in the patch to allow
> version v1.12, because both latest pahole sources are on version 1.12,
> did u have to do that? looks like there's v1.13 somewhere ;-)

Yeah, I did for local testing. Tag still needs to be bumped by Arnaldo.