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: <707080716569c7de7c3cb5869b67d62b55a96b68.camel@gmail.com>
Date: Thu, 04 Dec 2025 10:06:20 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>, Alexei Starovoitov
 <ast@...nel.org>,  Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,  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>, Alan Maguire <alan.maguire@...cle.com>,
 Donglin Peng	 <dolinux.peng@...il.com>
Cc: 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 Thu, 2025-12-04 at 09:29 -0800, Ihor Solodrai wrote:

[...]

> Ok, it seems you're conflating two separate issues.
> 
> There is a requirement to *link* .BTF section into vmlinux, because it
> must have a SHF_ALLOC flag, which makes objcopying the section data
> insufficient: linker has to do some magic under the hood.
> 
> The patch doesn't change this behavior, and this was (and is) covered
> in the script comments.
> 
> A separate issue is what resolve_btfids does: updates ELF in-place
> (before the patch) or outputs detached section data (after patch).
> 
> The paragraph in the commit message attempted to explain the decision
> to output raw section data. And apparently I did a bad job of
> that. I'll rewrite this part it in the next revision.
> 
> And I feel I should clarify that I didn't claim that libelf is buggy.
> I meant that using it is complicated, which makes resolve_btfids buggy.

So, pahole does the following:
- elf_begin(fildes: fd, cmd: ELF_C_RDWR, ref: NULL);
- selects a section to modify and modifies it
- elf_flagdata(data: btf_data, cmd: ELF_C_SET, flags: ELF_F_DIRTY);
- elf_update(elf, cmd: ELF_C_WRITE)
- elf_end(elf)

What exactly is complicated about that?

[...]