linux-kernel - Re: [RFC v2 09/20] PCI/CMA: Expose in sysfs whether devices are authenticated

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Message-ID: <CAH5fLghbScOTBnLLRDMdhE4RBhaPfhaqPr=Xivh8VL09wd5XGQ@mail.gmail.com>
Date: Thu, 27 Feb 2025 13:11:01 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Alistair Francis <alistair@...stair23.me>, linux-cxl@...r.kernel.org, 
	linux-kernel@...r.kernel.org, lukas@...ner.de, linux-pci@...r.kernel.org, 
	bhelgaas@...gle.com, Jonathan.Cameron@...wei.com, 
	rust-for-linux@...r.kernel.org, akpm@...ux-foundation.org, 
	boqun.feng@...il.com, bjorn3_gh@...tonmail.com, wilfred.mallawa@....com, 
	ojeda@...nel.org, alistair23@...il.com, a.hindborg@...nel.org, 
	tmgross@...ch.edu, gary@...yguo.net, alex.gaynor@...il.com, 
	benno.lossin@...ton.me, Alistair Francis <alistair.francis@....com>
Subject: Re: [RFC v2 09/20] PCI/CMA: Expose in sysfs whether devices are authenticated

On Thu, Feb 27, 2025 at 1:01 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Thu, Feb 27, 2025 at 12:52:02PM +0100, Alice Ryhl wrote:
> > On Thu, Feb 27, 2025 at 12:17 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Thu, Feb 27, 2025 at 01:09:41PM +1000, Alistair Francis wrote:
> > > > +     return rust_authenticated_show(spdm_state, buf);
> > >
> > > Here you have C code calling into Rust code.  I'm not complaining about
> > > it, but I think it will be the first use of this, and I didn't think
> > > that the rust maintainers were willing to do that just yet.
> > >
> > > Has that policy changed?
> > >
> > > The issue here is that the C signature for this is not being
> > > auto-generated, you have to manually keep it in sync (as you did above),
> > > with the Rust side.  That's not going to scale over time at all, you
> > > MUST have a .h file somewhere for C to know how to call into this and
> > > for the compiler to check that all is sane on both sides.
> > >
> > > And you are passing a random void * into the Rust side, what could go
> > > wrong?  I think this needs more thought as this is fragile-as-f***.
> >
> > I don't think we have a policy against it? I'm pretty sure the QR code
> > thing does it.
>
> Sorry, you are right, it does, and of course it happens (otherwise how
> would bindings work), but for small functions like this, how is the C
> code kept in sync with the rust side?  Where is the .h file that C
> should include?

I don't think there is tooling for it today. We need the opposite of
bindgen, which does exist in a tool called cbindgen. Unfortunately,
cbindgen is written to only work in cargo-based build systems, so we
cannot use it.

One trick you could do is write the signature in a header file, and
then compare what bindgen generates to the real signature like this:

const _: () = {
    if true {
        bindings::my_function
    } else {
        my_function
    };
};

This would only compile if the two function pointers have the same signature.

Alice