linux-kernel - Re: [PATCH] libbpf: fix BTF dedup to support recursive typedef definitions

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Message-ID: <CA+aJb_27fAdAXNwkYCxTKaWCCOdiWmJc7a_qhpmykXqxMjJYMA@mail.gmail.com>
Date: Fri, 7 Nov 2025 21:49:09 +0100
From: polo <paulhoussel2@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	martin.lau@...ux.dev, song@...nel.org, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Paul Houssel <paul.houssel@...nge.com>, 
	Martin Horth <martin.horth@...ecom-sudparis.eu>, 
	Ouail Derghal <ouail.derghal@...-atlantique.fr>, 
	Guilhem Jazeron <guilhem.jazeron@...ia.fr>, Ludovic Paillat <ludovic.paillat@...ia.fr>, 
	Robin Theveniaut <robin.theveniaut@...t.fr>, "Tristan d'Audibert" <tristan.daudibert@...il.com>
Subject: Re: [PATCH] libbpf: fix BTF dedup to support recursive typedef definitions

Hello Eduard,

On Fri, 7 Nov 2025 at 20:45, Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Fri, 2025-11-07 at 16:34 +0100, paulhoussel2@...il.com wrote:
> > From: Paul Houssel <paul.houssel@...nge.com>
> >
> > Handle recursive typedefs in BTF deduplication
> >
> > Pahole fails to encode BTF for some Go projects (e.g. Kubernetes and
> > Podman) due to recursive type definitions that create reference loops
> > not representable in C. These recursive typedefs trigger a failure in
> > the BTF deduplication algorithm.
> >
> > This patch extends btf_dedup_ref_type() to properly handle potential
> > recursion for BTF_KIND_TYPEDEF, similar to how recursion is already
> > handled for BTF_KIND_STRUCT. This allows pahole to successfully
> > generate BTF for Go binaries using recursive types without impacting
> > existing C-based workflows.
> >
> > Co-developed-by: Martin Horth <martin.horth@...ecom-sudparis.eu>
> > Signed-off-by: Martin Horth <martin.horth@...ecom-sudparis.eu>
> > Co-developed-by: Ouail Derghal <ouail.derghal@...-atlantique.fr>
> > Signed-off-by: Ouail Derghal <ouail.derghal@...-atlantique.fr>
> > Co-developed-by: Guilhem Jazeron <guilhem.jazeron@...ia.fr>
> > Signed-off-by: Guilhem Jazeron <guilhem.jazeron@...ia.fr>
> > Co-developed-by: Ludovic Paillat <ludovic.paillat@...ia.fr>
> > Signed-off-by: Ludovic Paillat <ludovic.paillat@...ia.fr>
> > Co-developed-by: Robin Theveniaut <robin.theveniaut@...t.fr>
> > Signed-off-by: Robin Theveniaut <robin.theveniaut@...t.fr>
> > Suggested-by: Tristan d'Audibert <tristan.daudibert@...il.com>
> > Signed-off-by: Paul Houssel <paul.houssel@...nge.com>
> >
> > ---
> > The issue was originally observed when attempting to encode BTF for
> > Kubernetes binaries (kubectl, kubeadm):
> >
> > $ git clone --depth 1 https://github.com/kubernetes/kubernetes
> > $ cd ./kubernetes
> > $ make kubeadm DBG=1
> > $ pahole --btf_encode_detached=kubeadm.btf _output/bin/kubeadm
> > btf_encoder__encode: btf__dedup failed!
> > Failed to encode BTF
>
> Hi Paul,
>
> Could you please provide some details on why would you like to use BTF
> for golang programs?

We would like to use BTF for Golang programs in order to trace
compiled Go user-space applications using eBPF uprobe programs.
Tetragon [1] implements the use of the BTF file to resolve paths to
attributes in hook parameters, and therefore if we can obtain the BTF
for Go programs, we will be able to start reading any attributes.
Recently, this feature has been extended to support uprobes [2].

[1] https://tetragon.io/docs/concepts/tracing-policy/hooks/#attribute-resolution
[2] https://github.com/cilium/tetragon/pull/4286#pullrequestreview-3427725698

> Also, is this the only scenario when golang
> generated DWARF has loops not possible in C code?

This is the only scenario we’ve identified where Golang DWARF contains
loops, which are not possible in C. We’re not aware of any other
Go-specific characteristics that could cause additional DWARF loops.
We tested BTF generation on a set of Go projects that are quite large
and representative of the diversity of Go programs, and we only
observed loops for this specific typedef usage.

Paul Houssel