linux-kernel - Re: [PATCH] rust: fix building firmware abstraction on 32bit arm

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Message-ID: <Z_0WRohxxMYqKxM5@google.com>
Date: Mon, 14 Apr 2025 14:05:58 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, 
	Christian Schrefl <chrisi.schrefl@...il.com>, Luis Chamberlain <mcgrof@...nel.org>, 
	Russ Weight <russ.weight@...ux.dev>, Danilo Krummrich <dakr@...nel.org>, 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>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] rust: fix building firmware abstraction on 32bit arm

On Sat, Apr 12, 2025 at 10:01:22AM +0000, Benno Lossin wrote:
> On Fri Apr 11, 2025 at 4:15 PM CEST, Miguel Ojeda wrote:
> > On Fri, Apr 11, 2025 at 2:46 PM Benno Lossin <benno.lossin@...ton.me> wrote:
> >>
> >> Ah I overlooked this, you should be using `kernel::ffi` (or
> >> `crate::ffi`) instead of `core`. (for `c_char` it doesn't matter, but we
> >> shouldn't be using `core::ffi`, since we have our own mappings).
> >
> > In 6.6, C `char` changed to unsigned, but `core::ffi::c_char` is
> > signed (in x86_64 at least).
> >
> > We should just never use `core::ffi` (except in `rust/ffi.rs`, of
> > course) -- I think we should just add the C types to the prelude
> > (which we discussed in the past) so that it is easy to avoid the
> > mistake (something like the patch attached as the end result, but
> > tested and across a kernel cycle or two) and mention it in the Coding
> > Guidelines. Thoughts?
> 
> Yeah sounds like a good idea.
> 
> > I tried to use Clippy's `disallowed-types` too:
> >
> >     disallowed-types = [
> >         { path = "core::ffi::c_void", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_char", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_schar", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_uchar", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_short", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_ushort", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_int", reason = "the `kernel::ffi` types
> > should be used instead" },
> >         { path = "core::ffi::c_uint", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_long", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_ulong", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_longlong", reason = "the `kernel::ffi`
> > types should be used instead" },
> >         { path = "core::ffi::c_ulonglong", reason = "the `kernel::ffi`
> > types should be used instead" },
> >     ]
> >
> > But it goes across aliases.
> 
> We could make the types in `ffi` be transparent newtypes. But not sure
> if that could interfere with kCFI or other stuff.

Transparent newtypes for all integers would be super inconvenient.

Alice