linux-kernel - Re: [PATCH v5 6/6] rust: use strict provenance APIs

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Message-ID: <CAJ-ks9k5XZUN_vuH648rr6-e+v0my_dR2zo+986rzx+A5ZLxng@mail.gmail.com>
Date: Tue, 18 Mar 2025 10:08:37 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, 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>, Danilo Krummrich <dakr@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Brendan Higgins <brendan.higgins@...ux.dev>, David Gow <davidgow@...gle.com>, 
	Rae Moar <rmoar@...gle.com>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Luis Chamberlain <mcgrof@...nel.org>, Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, 
	linux-pci@...r.kernel.org, linux-block@...r.kernel.org, 
	devicetree@...r.kernel.org
Subject: Re: [PATCH v5 6/6] rust: use strict provenance APIs

On Tue, Mar 18, 2025 at 8:29 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Mon, Mar 17, 2025 at 10:23:56AM -0400, Tamir Duberstein wrote:
> > Throughout the tree, use the strict provenance APIs stabilized in Rust
> > 1.84.0[1]. Retain backwards-compatibility by introducing forwarding
> > functions at the `kernel` crate root along with polyfills for rustc <
> > 1.84.0.
> >
> > Use `#[allow(clippy::incompatible_msrv)]` to avoid warnings on rustc <
> > 1.84.0 as our MSRV is 1.78.0.
> >
> > In the `kernel` crate, enable the strict provenance lints on rustc >=
> > 1.84.0; do this in `lib.rs` rather than `Makefile` to avoid introducing
> > compiler flags that are dependent on the rustc version in use.
> >
> > Link: https://blog.rust-lang.org/2025/01/09/Rust-1.84.0.html#strict-provenance-apis [1]
> > Suggested-by: Benno Lossin <benno.lossin@...ton.me>
> > Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@proton.me/
> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
>
> I'm not convinced that the pros of this change outweigh the cons. I
> think this is going to be too confusing for the C developers who look at
> this code.
>
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index 719b0a48ff55..96393bcf6bd7 100644
> > --- a/rust/kernel/uaccess.rs
> > +++ b/rust/kernel/uaccess.rs
> > @@ -226,7 +226,9 @@ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
> >          }
> >          // SAFETY: `out_ptr` points into a mutable slice of length `len`, so we may write
> >          // that many bytes to it.
> > -        let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr as *const c_void, len) };
> > +        let res = unsafe {
> > +            bindings::copy_from_user(out_ptr, crate::with_exposed_provenance(self.ptr), len)
> > +        };
> >          if res != 0 {
> >              return Err(EFAULT);
> >          }
> > @@ -264,7 +266,7 @@ pub fn read<T: FromBytes>(&mut self) -> Result<T> {
> >          let res = unsafe {
> >              bindings::_copy_from_user(
> >                  out.as_mut_ptr().cast::<c_void>(),
> > -                self.ptr as *const c_void,
> > +                crate::with_exposed_provenance(self.ptr),
> >                  len,
> >              )
> >          };
>
> That's especially true for cases like this. These are userspace pointers
> that are never dereferenced. It's not useful to care about provenance
> here.
>
> Alice

Let's just drop this last patch. It can be revisited later or not at
all. Perhaps in the future I need to be more willing to say no to
scope creep.