linux-kernel - Re: [Question] attributes encoding in BTF

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Message-ID: <CAADnVQ+T_s6dAwJ5JKqNqA8tq1P+MdfPvPw0EoY5yOf8PnGT4g@mail.gmail.com>
Date: Thu, 5 Jun 2025 09:09:53 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Alexis Lothoré <alexis.lothore@...tlin.com>
Cc: Ihor Solodrai <ihor.solodrai@...ux.dev>, Andrii Nakryiko <andrii.nakryiko@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Puranjay Mohan <puranjay@...nel.org>, Xu Kuohai <xukuohai@...weicloud.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Alexandre Torgue <alexandre.torgue@...s.st.com>, 
	Florent Revest <revest@...omium.org>, Bastien Curutchet <bastien.curutchet@...tlin.com>, 
	ebpf@...uxfoundation.org, Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, linux-stm32@...md-mailman.stormreply.com, 
	dwarves@...r.kernel.org
Subject: Re: [Question] attributes encoding in BTF

On Thu, Jun 5, 2025 at 12:35 AM Alexis Lothoré
<alexis.lothore@...tlin.com> wrote:
>
> Hi Ihor,
>
> On Wed Jun 4, 2025 at 7:31 PM CEST, Ihor Solodrai wrote:
> > On 6/4/25 2:02 AM, Alexis LothorÃ© wrote:
>
> [...]
>
> >> Could I be missing something obvious ? Or did I misunderstand the actual
> >> attribute encoding feature ?
> >
> > Hi Alexis.
> >
> > The changes recently landed in pahole and libbpf re attributes had a
> > very narrow goal: passing through particular attributes for some BPF
> > kfuncs from the kernel source to vmlinux.h
> >
> > BTF now has a way of encoding any attribute (as opposed to only bpf
> > type/decl tags) by setting type/decl tag kind flag [1]. So it is
> > possible to represent attributes like packed and aligned in BTF.
> >
> > However, the BTF tags need to be generated by something, in case of
> > vmlinux by pahole. Pahole generates BTF by parsing DWARF. And, as far as
> > I understand, attributes are not (can not be?) represented in DWARF in a
> > generic way, it really depends on specifics of the attribute.
> >
> > In order to support packed/aligned, pahole needs to know how to figure
> > them out from DWARF input and add the tags to BTF. And this does not
> > happen right now, which is why you don't see anything in bpftool output.
> >
> > [1]
> > https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/
>
> Thanks for the details ! I have missed this possibility, as I have been
> assuming that DWARF info was exposing the needed info. I'll take a look at
> it, but if those attributes can not be represented by DWARF, I'll have to
> find another way of getting those packing/alignment modifications on data
> type (eg: re-use/share btf__align_of from libbpf, as suggested by Andrii,
> but it may not able to cover all cases).

Not sure all the trouble is worth it.
I feel it's a corner case. Something we don't need to fix.