linux-kernel - Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

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Date:   Wed, 6 Dec 2023 14:59:11 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...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@...sung.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

On Thu, Nov 30, 2023 at 01:46:36PM +0100, Christian Brauner wrote:
> On Wed, Nov 29, 2023 at 05:48:15PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 29, 2023 at 05:28:27PM +0100, Christian Brauner wrote:
> > 
> > > > +pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
> > > > +				  struct pid_namespace *ns)
> > > > +{
> > > > +	return task_tgid_nr_ns(tsk, ns);
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(rust_helper_task_tgid_nr_ns);
> > > 
> > > I'm a bit puzzled by all these rust_helper_*() calls. Can you explain
> > > why they are needed? Because they are/can be static inlines and that
> > > somehow doesn't work?
> > 
> > Correct, because Rust can only talk to C ABI, it cannot use C headers.
> > Bindgen would need to translate the full C headers into valid Rust for
> > that to work.
> > 
> > I really think the Rust peoples should spend more effort on that,
> > because you are quite right, all this wrappery is tedious at best.

I suspect even if the manpower existed to go that route we'd end up
regretting it, because then the Rust compiler would need to be able to
handle _all_ the craziness a modern C compiler knows how to do -
preprocessor magic/devilry isn't even the worst of it, it gets even
worse when you start to consider things like bitfields and all the crazy
__attributes__(()) people have invented.

Swift went that route, but they have Apple funding them, and I doubt
even they would want anything to do with Linux kernel C.

IOW: yes, the extra friction from not being able to do full C -> Rust
translation is annoying now, but probably a good thing in the long run.

> The problem is that we end up with a long list of explicit exports that
> also are all really weirdly named like rust_helper_*(). I wouldn't even
> complain if it they were somehow auto-generated but as you say that
> might be out of scope.

I think we'd need help from the C side to auto generate them - what we
really want is for them to be inline, not static inline, but of course
that has never really worked for functions used across a single C file.
But maybe C compiler people are smarter these days?

Just a keyword to to tell the C compiler "take this static inline and
generate a compiled version in this .c file" would be all we need.

I could see it being handy for other things, too: as Linus has been
saying, we tend to inline too much code these days, and part of the
reason for that is we make a function inline because of the _one_
fastpath that needs it, but there's 3 more slowpaths that don't. 

And right now we don't have any sane way of having a function be
available with both inlined and outlined versions, besides the same kind
of manual wrappers the Rust people are doing here... so we should
probably just fix that.