On Tue, Apr 15, 2025 at 2:18 PM Boqun Feng boqun.feng@gmail.com wrote:
On Tue, Apr 15, 2025 at 01:58:41PM -0400, Tamir Duberstein wrote:
Hi Boqun, thanks for having a look!
On Tue, Apr 15, 2025 at 1:37 PM Boqun Feng boqun.feng@gmail.com wrote:
On Wed, Apr 09, 2025 at 10:47:23AM -0400, Tamir Duberstein wrote:
In Rust 1.78.0, Clippy introduced the `ref_as_ptr` lint [1]:
Using `as` casts may result in silently changing mutability or type.
While this doesn't eliminate unchecked `as` conversions, it makes such conversions easier to scrutinize. It also has the slight benefit of removing a degree of freedom on which to bikeshed. Thus apply the changes and enable the lint -- no functional change intended.
Link: https://rust-lang.github.io/rust-clippy/master/index.html#ref_as_ptr [1] Suggested-by: Benno Lossin benno.lossin@proton.me Link: https://lore.kernel.org/all/D8PGG7NTWB6U.3SS3A5LN4XWMN@proton.me/ Signed-off-by: Tamir Duberstein tamird@gmail.com
Makefile | 1 + rust/bindings/lib.rs | 1 + rust/kernel/device_id.rs | 3 ++- rust/kernel/fs/file.rs | 3 ++- rust/kernel/str.rs | 6 ++++-- rust/kernel/uaccess.rs | 10 ++++------ rust/uapi/lib.rs | 1 + 7 files changed, 15 insertions(+), 10 deletions(-)
diff --git a/Makefile b/Makefile index eb5a942241a2..2a16e02f26db 100644 --- a/Makefile +++ b/Makefile @@ -485,6 +485,7 @@ export rust_common_flags := --edition=2021 \ -Wclippy::no_mangle_with_rust_abi \ -Wclippy::ptr_as_ptr \ -Wclippy::ptr_cast_constness \
-Wclippy::ref_as_ptr \ -Wclippy::undocumented_unsafe_blocks \ -Wclippy::unnecessary_safety_comment \ -Wclippy::unnecessary_safety_doc \diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs index b105a0d899cc..2b69016070c6 100644 --- a/rust/bindings/lib.rs +++ b/rust/bindings/lib.rs @@ -27,6 +27,7 @@ #[allow(dead_code)] #[allow(clippy::cast_lossless)] #[allow(clippy::ptr_as_ptr)] +#[allow(clippy::ref_as_ptr)] #[allow(clippy::undocumented_unsafe_blocks)] mod bindings_raw { // Manual definition for blocklisted types. diff --git a/rust/kernel/device_id.rs b/rust/kernel/device_id.rs index 4063f09d76d9..37cc03d1df4c 100644 --- a/rust/kernel/device_id.rs +++ b/rust/kernel/device_id.rs @@ -136,7 +136,8 @@ impl<T: RawDeviceId, U, const N: usize> IdTable<T, U> for IdArray<T, U, N> { fn as_ptr(&self) -> *const T::RawType { // This cannot be `self.ids.as_ptr()`, as the return pointer must have correct provenance // to access the sentinel.
(self as *const Self).cast()
let this: *const Self = self;Hmm.. so this lint usually just requires to use a let statement instead of as expression when casting a reference to a pointer? Not 100% convinced this results into better code TBH..
The rationale is in the lint description and quoted in the commit message: "Using `as` casts may result in silently changing mutability or type.".
Could you show me how you can silently change the mutability or type? A simple try like below doesn't compile:
let x = &42; let ptr = x as *mut i32; // <- error let another_ptr = x as *const i64; // <- error
I think the point is that the meaning of an `as` cast can change when the type of `x` changes, which can happen at a distance. The example shown in the clippy docs uses `as _`, which is where you get into real trouble.
also from the link document you shared, looks like the suggestion is to use core::ptr::from_{ref,mut}(), was this ever considered?
I considered it, but I thought it was ugly. We don't have a linter to enforce it, so I'd be surprised if people reached for it.
this.cast()}
fn id(&self, index: usize) -> &T::RawType {
diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs index 791f493ada10..559a4bfa123f 100644 --- a/rust/kernel/fs/file.rs +++ b/rust/kernel/fs/file.rs @@ -359,12 +359,13 @@ impl core::ops::Deref for File { type Target = LocalFile; #[inline] fn deref(&self) -> &LocalFile {
let this: *const Self = self; // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a // valid file for the desired duration. // // By the type invariants, there are no `fdget_pos` calls that did not take the // `f_pos_lock` mutex.
unsafe { LocalFile::from_raw_file((self as *const Self).cast()) }
unsafe { LocalFile::from_raw_file(this.cast()) }}
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs index 40034f77fc2f..75b4a18c67c4 100644 --- a/rust/kernel/str.rs +++ b/rust/kernel/str.rs @@ -28,8 +28,9 @@ pub const fn is_empty(&self) -> bool { /// Creates a [`BStr`] from a `[u8]`. #[inline] pub const fn from_bytes(bytes: &[u8]) -> &Self {
let bytes: *const [u8] = bytes; // SAFETY: `BStr` is transparent to `[u8]`.
unsafe { &*(bytes as *const [u8] as *const BStr) }
unsafe { &*(bytes as *const BStr) }unsafe { &*(bytes.cast::<BStr>()) }? I'm curious why this dodged the other lint (ptr_as_ptr).
The reason it has to be written this way is that BStr is !Sized, and `pointer::cast` has an implicit Sized bound.
Perhaps the lint is smart enough to avoid the suggestion in that case? Seems like yes: https://github.com/rust-lang/rust-clippy/blob/d3267e9230940757fde2fcb608605b....
Oh, I see, so fat pointers get their way from this check? Hmm... however fat pointers also suffer the same problem that ptr_as_ptr prevents, right? How should we avoid that?
Probably the proper solution is to remove the `Sized` bound from `pointer::cast`. Short of that, I'm not sure how -- but I don't think this deficiency should prevent us from the benefits of this change, even if they are a bit limited.