linux-kernel - Re: [PATCH bpf-next v2 4/4] resolve_btfids: change in-place update with raw binary output

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Message-ID: <728c615d-85d7-4a2f-a68e-dc63baf2c4aa@linux.dev>
Date: Wed, 3 Dec 2025 20:42:31 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Alan Maguire <alan.maguire@...cle.com>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, 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>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 Nicolas Schier <nicolas.schier@...ux.dev>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Donglin Peng <dolinux.peng@...il.com>, bpf@...r.kernel.org,
 dwarves@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kbuild@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 4/4] resolve_btfids: change in-place update
 with raw binary output

On 12/3/25 10:48 AM, Alan Maguire wrote:
> On 01/12/2025 22:16, Andrii Nakryiko wrote:
>> On Thu, Nov 27, 2025 at 10:53 AM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>>
>> [...]
>>
>>> +if ! is_enabled CONFIG_DEBUG_INFO_BTF; then
>>> +       exit 0
>>> +fi
>>> +
>>> +gen_btf_data()
>>> +{
>>> +       info BTF "${ELF_FILE}"
>>> +       btf1="${ELF_FILE}.btf.1"
>>> +       ${PAHOLE} -J ${PAHOLE_FLAGS}                    \
>>> +               ${BTF_BASE:+--btf_base ${BTF_BASE}}     \
>>> +               --btf_encode_detached=${btf1}           \
>>
>> please double-check what pahole version has --btf_encode_detached, we
>> might need to change minimal supported pahole version because of this
>>
> 
> yeah, this landed in v1.22 [1]

Thank you for checking!

> 
> One thing worth thinking about; are there aspects of the gen_btf.sh
> script that could be moved to Makefile.btf to avoid having to compute them
> repeatedly for each module? For example computing resolve_btfids 
> flags based on CONFIG_WERROR could be done there I think. You could
> also determine whether the script is needed at all in Makefile.btf; i.e.
> 
> gen-btf-y				=
> gen-btf-$(CONFIG_DEBUG_INFO_BTF)	= scripts/gen-btf.sh
> 
> export GEN_BTF := $(gen-btf-y)
> 
> That would allow you to get rid of the is_enabled() I think.

Good point. I'll try moving most relevant flags to Makefile.btf

> 
> I'm building this now, but I was wondering if the linking/objcopy changes pose
> any risk to kernel address computations in kallsyms or anything like that? IIRC
> Stephen ran into some issues with global variable addresses as a consequence of
> linking BTF sections [2], but not sure if there are additional concerns here.

This series doesn't change the fact that .BTF is *linked* into final vmlinux,
so a problem described in [2] stands.

That said, AFAIU the suggested change in the linker script will still work.

> 
> [1] https://github.com/acmel/dwarves/releases/tag/v1.22
> [2] https://lore.kernel.org/bpf/20250207012045.2129841-2-stephen.s.brennan@oracle.com/