1 // SPDX-License-Identifier: GPL-2.0 2 3 // Copyright (C) 2024 Google LLC. 4 5 //! Helpers for implementing list traits safely. 6 7 use crate::list::ListLinks; 8 9 /// Declares that this type has a `ListLinks<ID>` field at a fixed offset. 10 /// 11 /// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented 12 /// manually, then this trait is not needed. Use the [`impl_has_list_links!`] macro to implement 13 /// this trait. 14 /// 15 /// # Safety 16 /// 17 /// All values of this type must have a `ListLinks<ID>` field at the given offset. 18 /// 19 /// The behavior of `raw_get_list_links` must not be changed. 20 pub unsafe trait HasListLinks<const ID: u64 = 0> { 21 /// The offset of the `ListLinks` field. 22 const OFFSET: usize; 23 24 /// Returns a pointer to the [`ListLinks<T, ID>`] field. 25 /// 26 /// # Safety 27 /// 28 /// The provided pointer must point at a valid struct of type `Self`. 29 /// 30 /// [`ListLinks<T, ID>`]: ListLinks 31 // We don't really need this method, but it's necessary for the implementation of 32 // `impl_has_list_links!` to be correct. 33 #[inline] 34 unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> { 35 // SAFETY: The caller promises that the pointer is valid. The implementer promises that the 36 // `OFFSET` constant is correct. 37 unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> } 38 } 39 } 40 41 /// Implements the [`HasListLinks`] trait for the given type. 42 #[macro_export] 43 macro_rules! impl_has_list_links { 44 ($(impl$(<$($implarg:ident),*>)? 45 HasListLinks$(<$id:tt>)? 46 for $self:ident $(<$($selfarg:ty),*>)? 47 { self$(.$field:ident)* } 48 )*) => {$( 49 // SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the 50 // right type. 51 // 52 // The behavior of `raw_get_list_links` is not changed since the `addr_of_mut!` macro is 53 // equivalent to the pointer offset operation in the trait definition. 54 unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for 55 $self $(<$($selfarg),*>)? 56 { 57 const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize; 58 59 #[inline] 60 unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? { 61 // SAFETY: The caller promises that the pointer is not dangling. We know that this 62 // expression doesn't follow any pointers, as the `offset_of!` invocation above 63 // would otherwise not compile. 64 unsafe { ::core::ptr::addr_of_mut!((*ptr)$(.$field)*) } 65 } 66 } 67 )*}; 68 } 69 pub use impl_has_list_links; 70 71 /// Implements the [`ListItem`] trait for the given type. 72 /// 73 /// Requires that the type implements [`HasListLinks`]. Use the [`impl_has_list_links!`] macro to 74 /// implement that trait. 75 /// 76 /// [`ListItem`]: crate::list::ListItem 77 #[macro_export] 78 macro_rules! impl_list_item { 79 ( 80 $(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty { 81 using ListLinks; 82 })* 83 ) => {$( 84 // SAFETY: See GUARANTEES comment on each method. 85 unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t { 86 // GUARANTEES: 87 // * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert` 88 // is implemented in terms of `view_links`. 89 // * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when 90 // this value is not in a list. 91 unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> { 92 // SAFETY: The caller guarantees that `me` points at a valid value of type `Self`. 93 unsafe { 94 <Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut()) 95 } 96 } 97 98 // GUARANTEES: 99 // * `me` originates from the most recent call to `prepare_to_insert`, which just added 100 // `offset` to the pointer passed to `prepare_to_insert`. This method subtracts 101 // `offset` from `me` so it returns the pointer originally passed to 102 // `prepare_to_insert`. 103 // * The pointer remains valid until the next call to `post_remove` because the caller 104 // of the most recent call to `prepare_to_insert` promised to retain ownership of the 105 // `ListArc` containing `Self` until the next call to `post_remove`. The value cannot 106 // be destroyed while a `ListArc` reference exists. 107 unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self { 108 let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET; 109 // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it 110 // points at the field at offset `offset` in a value of type `Self`. Thus, 111 // subtracting `offset` from `me` is still in-bounds of the allocation. 112 unsafe { (me as *const u8).sub(offset) as *const Self } 113 } 114 115 // GUARANTEES: 116 // This implementation of `ListItem` will not give out exclusive access to the same 117 // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove` 118 // must alternate and exclusive access is given up when `post_remove` is called. 119 // 120 // Other invocations of `impl_list_item!` also cannot give out exclusive access to the 121 // same `ListLinks` because you can only implement `ListItem` once for each value of 122 // `ID`, and the `ListLinks` fields only work with the specified `ID`. 123 unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> { 124 // SAFETY: The caller promises that `me` points at a valid value. 125 unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) } 126 } 127 128 // GUARANTEES: 129 // * `me` originates from the most recent call to `prepare_to_insert`, which just added 130 // `offset` to the pointer passed to `prepare_to_insert`. This method subtracts 131 // `offset` from `me` so it returns the pointer originally passed to 132 // `prepare_to_insert`. 133 unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self { 134 let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET; 135 // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it 136 // points at the field at offset `offset` in a value of type `Self`. Thus, 137 // subtracting `offset` from `me` is still in-bounds of the allocation. 138 unsafe { (me as *const u8).sub(offset) as *const Self } 139 } 140 } 141 )*}; 142 } 143 pub use impl_list_item; 144