linux-kernel - Re: [PATCH] rust: Disallow BTF generation with Rust + LTO

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Message-ID: <Z3_7MM30-wwY5ihQ@x1>
Date: Thu, 9 Jan 2025 13:37:04 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Tamir Duberstein <tamird@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Neal Gompa <neal@...pa.dev>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Matthew Maurer <mmaurer@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org,
	Matthias Maennich <maennich@...gle.com>, bpf <bpf@...r.kernel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Eric Curtin <ecurtin@...hat.com>,
	Martin Reboredo <yakoyoku@...il.com>,
	Alessandro Decina <alessandro.d@...il.com>,
	Michal Rostecki <vadorovsky@...tonmail.com>,
	Dave Tucker <dave@...cker.co.uk>
Subject: Re: [PATCH] rust: Disallow BTF generation with Rust + LTO

On Thu, Jan 09, 2025 at 01:29:47PM -0300, Arnaldo Carvalho de Melo wrote:
> On Thu, Jan 09, 2025 at 10:49:49AM -0500, Tamir Duberstein wrote:
> > On Thu, Jan 9, 2025 at 10:47 AM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
 
> > > I was thinking about it after reading this thread yesterday, i.e. we
> > > could encode constructs from Rust that can be represented in BTF and
> > > skip the ones that can't, pruning types that depend on non BTF
> > > representable types, etc.
  
> > Yep, this is what bpf-linker does, along with some other things[0]. I
> > highly recommend reading the code I linked to avoid re-discovering
> > these things.
 
> Sure, thanks for pointing it out and suggest I read it while
> experimenting with having the same concept in pahole, I'll try a quick
> hack and then look at it to see how close I got to what you guys came up
> with :-)

BTW, its "funny" how the DWARF loader can get things from Rust, golang,
fortran and end up with things like:

Rust:

https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=d744d859768d6951cacd146604891c108b39f6a1
https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=c4eb1897d1f3841d291ee39dc969c4212750cf2c
https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=2e8cd6a435d96335c4794794147019369b6a7b6a

FORTRAN:

https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=f5847773d94d4875e04e47de9b677098f34c6510

Go:

https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=31bc0d7410572f6e03e3ed9da7c8c6f0d8df23c8

Now its a matter of making the BTF encoder be more permissive and just
skip things it can't express in BTF.

:-)

- Arnaldo