1 // SPDX-License-Identifier: GPL-2.0 2 3 //! Traits for transmuting types. 4 5 use core::mem::size_of; 6 7 /// Types for which any bit pattern is valid. 8 /// 9 /// Not all types are valid for all values. For example, a `bool` must be either zero or one, so 10 /// reading arbitrary bytes into something that contains a `bool` is not okay. 11 /// 12 /// It's okay for the type to have padding, as initializing those bytes has no effect. 13 /// 14 /// # Examples 15 /// 16 /// ``` 17 /// use kernel::transmute::FromBytes; 18 /// 19 /// # fn test() -> Option<()> { 20 /// let raw = [1, 2, 3, 4]; 21 /// 22 /// let result = u32::from_bytes(&raw)?; 23 /// 24 /// #[cfg(target_endian = "little")] 25 /// assert_eq!(*result, 0x4030201); 26 /// 27 /// #[cfg(target_endian = "big")] 28 /// assert_eq!(*result, 0x1020304); 29 /// 30 /// # Some(()) } 31 /// # test().ok_or(EINVAL)?; 32 /// # Ok::<(), Error>(()) 33 /// ``` 34 /// 35 /// # Safety 36 /// 37 /// All bit-patterns must be valid for this type. This type must not have interior mutability. 38 pub unsafe trait FromBytes { 39 /// Converts a slice of bytes to a reference to `Self`. 40 /// 41 /// Succeeds if the reference is properly aligned, and the size of `bytes` is equal to that of 42 /// `T` and different from zero. 43 /// 44 /// Otherwise, returns [`None`]. 45 fn from_bytes(bytes: &[u8]) -> Option<&Self> 46 where 47 Self: Sized, 48 { 49 let slice_ptr = bytes.as_ptr().cast::<Self>(); 50 let size = size_of::<Self>(); 51 52 if bytes.len() == size && slice_ptr.is_aligned() { 53 // SAFETY: Size and alignment were just checked. 54 unsafe { Some(&*slice_ptr) } 55 } else { 56 None 57 } 58 } 59 60 /// Converts the beginning of `bytes` to a reference to `Self`. 61 /// 62 /// This method is similar to [`Self::from_bytes`], with the difference that `bytes` does not 63 /// need to be the same size of `Self` - the appropriate portion is cut from the beginning of 64 /// `bytes`, and the remainder returned alongside `Self`. 65 fn from_bytes_prefix(bytes: &[u8]) -> Option<(&Self, &[u8])> 66 where 67 Self: Sized, 68 { 69 let (prefix, remainder) = bytes.split_at_checked(size_of::<Self>())?; 70 71 Self::from_bytes(prefix).map(|s| (s, remainder)) 72 } 73 74 /// Converts a mutable slice of bytes to a reference to `Self`. 75 /// 76 /// Succeeds if the reference is properly aligned, and the size of `bytes` is equal to that of 77 /// `T` and different from zero. 78 /// 79 /// Otherwise, returns [`None`]. 80 fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self> 81 where 82 Self: AsBytes + Sized, 83 { 84 let slice_ptr = bytes.as_mut_ptr().cast::<Self>(); 85 let size = size_of::<Self>(); 86 87 if bytes.len() == size && slice_ptr.is_aligned() { 88 // SAFETY: Size and alignment were just checked. 89 unsafe { Some(&mut *slice_ptr) } 90 } else { 91 None 92 } 93 } 94 95 /// Converts the beginning of `bytes` to a mutable reference to `Self`. 96 /// 97 /// This method is similar to [`Self::from_bytes_mut`], with the difference that `bytes` does 98 /// not need to be the same size of `Self` - the appropriate portion is cut from the beginning 99 /// of `bytes`, and the remainder returned alongside `Self`. 100 fn from_bytes_mut_prefix(bytes: &mut [u8]) -> Option<(&mut Self, &mut [u8])> 101 where 102 Self: AsBytes + Sized, 103 { 104 let (prefix, remainder) = bytes.split_at_mut_checked(size_of::<Self>())?; 105 106 Self::from_bytes_mut(prefix).map(|s| (s, remainder)) 107 } 108 109 /// Creates an owned instance of `Self` by copying `bytes`. 110 /// 111 /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this method can be used on 112 /// non-aligned data at the cost of a copy. 113 fn from_bytes_copy(bytes: &[u8]) -> Option<Self> 114 where 115 Self: Sized, 116 { 117 if bytes.len() == size_of::<Self>() { 118 // SAFETY: we just verified that `bytes` has the same size as `Self`, and per the 119 // invariants of `FromBytes`, any byte sequence of the correct length is a valid value 120 // for `Self`. 121 Some(unsafe { core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) }) 122 } else { 123 None 124 } 125 } 126 127 /// Creates an owned instance of `Self` from the beginning of `bytes`. 128 /// 129 /// This method is similar to [`Self::from_bytes_copy`], with the difference that `bytes` does 130 /// not need to be the same size of `Self` - the appropriate portion is cut from the beginning 131 /// of `bytes`, and the remainder returned alongside `Self`. 132 fn from_bytes_copy_prefix(bytes: &[u8]) -> Option<(Self, &[u8])> 133 where 134 Self: Sized, 135 { 136 let (prefix, remainder) = bytes.split_at_checked(size_of::<Self>())?; 137 138 Self::from_bytes_copy(prefix).map(|s| (s, remainder)) 139 } 140 } 141 142 macro_rules! impl_frombytes { 143 ($($({$($generics:tt)*})? $t:ty, )*) => { 144 // SAFETY: Safety comments written in the macro invocation. 145 $(unsafe impl$($($generics)*)? FromBytes for $t {})* 146 }; 147 } 148 149 impl_frombytes! { 150 // SAFETY: Inhabited ZSTs only have one possible bit pattern, and these two have no invariant. 151 (), 152 {<T>} core::marker::PhantomData<T>, 153 154 // SAFETY: All bit patterns are acceptable values of the types below. 155 u8, u16, u32, u64, usize, 156 i8, i16, i32, i64, isize, 157 158 // SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit 159 // patterns are also acceptable for arrays of that type. 160 {<T: FromBytes>} [T], 161 {<T: FromBytes, const N: usize>} [T; N], 162 } 163 164 /// Types that can be viewed as an immutable slice of initialized bytes. 165 /// 166 /// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This 167 /// means that it should not have any padding, as padding bytes are uninitialized. Reading 168 /// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive 169 /// information on the stack to userspace. 170 /// 171 /// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered 172 /// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so 173 /// this is a correctness requirement, but not a safety requirement. 174 /// 175 /// # Safety 176 /// 177 /// Values of this type may not contain any uninitialized bytes. This type must not have interior 178 /// mutability. 179 pub unsafe trait AsBytes { 180 /// Returns `self` as a slice of bytes. 181 fn as_bytes(&self) -> &[u8] { 182 // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized. 183 let data = core::ptr::from_ref(self).cast::<u8>(); 184 let len = core::mem::size_of_val(self); 185 186 // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes. 187 unsafe { core::slice::from_raw_parts(data, len) } 188 } 189 190 /// Returns `self` as a mutable slice of bytes. 191 fn as_bytes_mut(&mut self) -> &mut [u8] 192 where 193 Self: FromBytes, 194 { 195 // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self` 196 // bi-directionally transmutable to `[u8; size_of_val(self)]`. 197 let data = core::ptr::from_mut(self).cast::<u8>(); 198 let len = core::mem::size_of_val(self); 199 200 // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()` 201 // bytes. 202 unsafe { core::slice::from_raw_parts_mut(data, len) } 203 } 204 } 205 206 macro_rules! impl_asbytes { 207 ($($({$($generics:tt)*})? $t:ty, )*) => { 208 // SAFETY: Safety comments written in the macro invocation. 209 $(unsafe impl$($($generics)*)? AsBytes for $t {})* 210 }; 211 } 212 213 impl_asbytes! { 214 // SAFETY: Inhabited ZSTs only have one possible bit pattern, and these two have no invariant. 215 (), 216 {<T>} core::marker::PhantomData<T>, 217 218 // SAFETY: Instances of the following types have no uninitialized portions. 219 u8, u16, u32, u64, usize, 220 i8, i16, i32, i64, isize, 221 bool, 222 char, 223 str, 224 225 // SAFETY: If individual values in an array have no uninitialized portions, then the array 226 // itself does not have any uninitialized portions either. 227 {<T: AsBytes>} [T], 228 {<T: AsBytes, const N: usize>} [T; N], 229 } 230