kernel/debugfs/
callback_adapters.rs

1// SPDX-License-Identifier: GPL-2.0
2// Copyright (C) 2025 Google LLC.
3
4//! Adapters which allow the user to supply a write or read implementation as a value rather
5//! than a trait implementation. If provided, it will override the trait implementation.
6
7use super::{Reader, Writer};
8use crate::fmt;
9use crate::prelude::*;
10use crate::uaccess::UserSliceReader;
11use core::marker::PhantomData;
12use core::ops::Deref;
13
14/// # Safety
15///
16/// To implement this trait, it must be safe to cast a `&Self` to a `&Inner`.
17/// It is intended for use in unstacking adapters out of `FileOps` backings.
18pub(crate) unsafe trait Adapter {
19    type Inner;
20}
21
22/// Adapter to implement `Reader` via a callback with the same representation as `T`.
23///
24/// * Layer it on top of `WriterAdapter` if you want to add a custom callback for `write`.
25/// * Layer it on top of `NoWriter` to pass through any support present on the underlying type.
26///
27/// # Invariants
28///
29/// If an instance for `WritableAdapter<_, W>` is constructed, `W` is inhabited.
30#[repr(transparent)]
31pub(crate) struct WritableAdapter<D, W> {
32    inner: D,
33    _writer: PhantomData<W>,
34}
35
36// SAFETY: Stripping off the adapter only removes constraints
37unsafe impl<D, W> Adapter for WritableAdapter<D, W> {
38    type Inner = D;
39}
40
41impl<D: Writer, W> Writer for WritableAdapter<D, W> {
42    fn write(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
43        self.inner.write(fmt)
44    }
45}
46
47impl<D: Deref, W> Reader for WritableAdapter<D, W>
48where
49    W: Fn(&D::Target, &mut UserSliceReader) -> Result + Send + Sync + 'static,
50{
51    fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result {
52        // SAFETY: WritableAdapter<_, W> can only be constructed if W is inhabited
53        let w: &W = unsafe { materialize_zst() };
54        w(self.inner.deref(), reader)
55    }
56}
57
58/// Adapter to implement `Writer` via a callback with the same representation as `T`.
59///
60/// # Invariants
61///
62/// If an instance for `FormatAdapter<_, F>` is constructed, `F` is inhabited.
63#[repr(transparent)]
64pub(crate) struct FormatAdapter<D, F> {
65    inner: D,
66    _formatter: PhantomData<F>,
67}
68
69impl<D, F> Deref for FormatAdapter<D, F> {
70    type Target = D;
71    fn deref(&self) -> &D {
72        &self.inner
73    }
74}
75
76impl<D, F> Writer for FormatAdapter<D, F>
77where
78    F: Fn(&D, &mut fmt::Formatter<'_>) -> fmt::Result + 'static,
79{
80    fn write(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
81        // SAFETY: FormatAdapter<_, F> can only be constructed if F is inhabited
82        let f: &F = unsafe { materialize_zst() };
83        f(&self.inner, fmt)
84    }
85}
86
87// SAFETY: Stripping off the adapter only removes constraints
88unsafe impl<D, F> Adapter for FormatAdapter<D, F> {
89    type Inner = D;
90}
91
92#[repr(transparent)]
93pub(crate) struct NoWriter<D> {
94    inner: D,
95}
96
97// SAFETY: Stripping off the adapter only removes constraints
98unsafe impl<D> Adapter for NoWriter<D> {
99    type Inner = D;
100}
101
102impl<D> Deref for NoWriter<D> {
103    type Target = D;
104    fn deref(&self) -> &D {
105        &self.inner
106    }
107}
108
109/// For types with a unique value, produce a static reference to it.
110///
111/// # Safety
112///
113/// The caller asserts that F is inhabited
114unsafe fn materialize_zst<F>() -> &'static F {
115    const { assert!(core::mem::size_of::<F>() == 0) };
116    let zst_dangle: core::ptr::NonNull<F> = core::ptr::NonNull::dangling();
117    // SAFETY: While the pointer is dangling, it is a dangling pointer to a ZST, based on the
118    // assertion above. The type is also inhabited, by the caller's assertion. This means
119    // we can materialize it.
120    unsafe { zst_dangle.as_ref() }
121}
kernel/debugfs/callback_adapters.rs

kernel/debugfs/
callback_adapters.rs
