Date:   Mon, 13 Feb 2023 18:43:32 +0100
From:   Andrea Righi <andrea.righi@canonical.com>
To:     Eric Curtin <ecurtin@redhat.com>
Cc:     Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
        miguel.ojeda.sandonis@gmail.com, alex.gaynor@gmail.com,
        bjorn3_gh@protonmail.com, boqun.feng@gmail.com, dxu@dxuuu.xyz,
        gary@garyguo.net, linux-kernel@vger.kernel.org, ojeda@kernel.org,
        rust-for-linux@vger.kernel.org, wedsonaf@gmail.com
Subject: Re: [PATCH] rust: fix regexp in scripts/is_rust_module.sh
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On Mon, Feb 13, 2023 at 04:03:46PM +0000, Eric Curtin wrote:
> On Mon, 13 Feb 2023 at 15:48, Andrea Righi <andrea.righi@canonical.com> wrote:
> >
> > On Mon, Feb 13, 2023 at 12:13:39PM -0300, Martin Rodriguez Reboredo wrote:
> > > On Mon, Feb 13, 2023 at 1:19 PM Miguel Ojeda <ojeda@kernel.org> wrote:
> > > > On Fri, Feb 10, 2023 at 4:26 PM Andrea Righi <andrea.righi@canonical.com> wrote:
> > > > >
> > > > > nm can use "R" or "r" to show read-only data sections, but
> > > > > scripts/is_rust_module.sh can only recognize "r", so with some versions
> > > > > of binutils it can fail to detect if a module is a Rust module or not.
> > > >
> > > > Do you know which versions? If so, it would be nice to document it here.
> > > >
> > > > > Moreover, with this patch applied I can also relax the constraint of
> > > > > "RUST depends on !DEBUG_INFO_BTF" and build a kernel with Rust and BTF
> > > > > enabled at the same time (of course BTF generation is still skipped for
> > > > > Rust modules).
> > > >
> > > > Even if that build succeeds, can you load the modules? i.e. the
> > > > constraint was there due to
> > > > https://github.com/Rust-for-Linux/linux/issues/735.
> > >
> > > Issue was that the kernel couldn't even load C modules, Rust CUs must be
> > > skipped in vmlinux for it to work, and because of that the constraint
> > > !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE [1] cannot be relaxed until
> > > version 1.24+ of Pahole is the minimum requirement by the kernel. I'm OK
> > > with this change but keep in mind that it won't remedy that condition.
> > >
> > > Link: https://lore.kernel.org/bpf/20230111152050.559334-1-yakoyoku@gmail.com/ [1]
> >
> > Oh yes, I'm using pahole 1.24, that's probably why it's all working for
> > me. So we can't just drop !DEBUG_INFO_BTF, but we can definitely relax
> > the constraint to "!DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE".
> 
> Yup the patch is ok, but we can only build with RUST and
> DEBUG_INFO_BTF if we have pahole 1.24+ because that is the version
> smart enough to exclude the Rust CUs.
> 
> If you have a version of pahole < 1.24 and you want to include RUST
> you basically should not have DEBUG_INFO_BTF on at all because your
> build will fail (unless this change alters the list of CUs passed to
> pahole).
> 
> What is a little confusing is scripts/is_rust_module.sh and pahole
> "--lang_exclude=rust" feature both attempt to do the same thing,
> recognize and skip Rust CUs

Yep, "pahole --lang_exclude=rust" makes scripts/is_rust_module.sh
obsolete, so with this applied:
https://lore.kernel.org/bpf/20230111152050.559334-1-yakoyoku@gmail.com/

I can just drop scripts/is_rust_module.sh and everything is still
working.

So, if that patch is applied we can do another patch to get rid of
is_rust_module.sh completely (because to my knowledge this script is
only used to exclude the BTF generation for the Rust modules).

-Andrea