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 #[allow(clippy::incompatible_msrv)] 53 if bytes.len() == size && slice_ptr.is_aligned() { 54 // SAFETY: Size and alignment were just checked. 55 unsafe { Some(&*slice_ptr) } 56 } else { 57 None 58 } 59 } 60 61 /// Converts the beginning of `bytes` to a reference to `Self`. 62 /// 63 /// This method is similar to [`Self::from_bytes`], with the difference that `bytes` does not 64 /// need to be the same size of `Self` - the appropriate portion is cut from the beginning of 65 /// `bytes`, and the remainder returned alongside `Self`. 66 fn from_bytes_prefix(bytes: &[u8]) -> Option<(&Self, &[u8])> 67 where 68 Self: Sized, 69 { 70 if bytes.len() < size_of::<Self>() { 71 None 72 } else { 73 // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at` cannot 74 // panic. 75 // TODO: replace with `split_at_checked` once the MSRV is >= 1.80. 76 let (prefix, remainder) = bytes.split_at(size_of::<Self>()); 77 78 Self::from_bytes(prefix).map(|s| (s, remainder)) 79 } 80 } 81 82 /// Converts a mutable slice of bytes to a reference to `Self`. 83 /// 84 /// Succeeds if the reference is properly aligned, and the size of `bytes` is equal to that of 85 /// `T` and different from zero. 86 /// 87 /// Otherwise, returns [`None`]. 88 fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self> 89 where 90 Self: AsBytes + Sized, 91 { 92 let slice_ptr = bytes.as_mut_ptr().cast::<Self>(); 93 let size = size_of::<Self>(); 94 95 #[allow(clippy::incompatible_msrv)] 96 if bytes.len() == size && slice_ptr.is_aligned() { 97 // SAFETY: Size and alignment were just checked. 98 unsafe { Some(&mut *slice_ptr) } 99 } else { 100 None 101 } 102 } 103 104 /// Converts the beginning of `bytes` to a mutable reference to `Self`. 105 /// 106 /// This method is similar to [`Self::from_bytes_mut`], with the difference that `bytes` does 107 /// not need to be the same size of `Self` - the appropriate portion is cut from the beginning 108 /// of `bytes`, and the remainder returned alongside `Self`. 109 fn from_bytes_mut_prefix(bytes: &mut [u8]) -> Option<(&mut Self, &mut [u8])> 110 where 111 Self: AsBytes + Sized, 112 { 113 if bytes.len() < size_of::<Self>() { 114 None 115 } else { 116 // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at_mut` cannot 117 // panic. 118 // TODO: replace with `split_at_mut_checked` once the MSRV is >= 1.80. 119 let (prefix, remainder) = bytes.split_at_mut(size_of::<Self>()); 120 121 Self::from_bytes_mut(prefix).map(|s| (s, remainder)) 122 } 123 } 124 125 /// Creates an owned instance of `Self` by copying `bytes`. 126 /// 127 /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this method can be used on 128 /// non-aligned data at the cost of a copy. 129 fn from_bytes_copy(bytes: &[u8]) -> Option<Self> 130 where 131 Self: Sized, 132 { 133 if bytes.len() == size_of::<Self>() { 134 // SAFETY: we just verified that `bytes` has the same size as `Self`, and per the 135 // invariants of `FromBytes`, any byte sequence of the correct length is a valid value 136 // for `Self`. 137 Some(unsafe { core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) }) 138 } else { 139 None 140 } 141 } 142 143 /// Creates an owned instance of `Self` from the beginning of `bytes`. 144 /// 145 /// This method is similar to [`Self::from_bytes_copy`], with the difference that `bytes` does 146 /// not need to be the same size of `Self` - the appropriate portion is cut from the beginning 147 /// of `bytes`, and the remainder returned alongside `Self`. 148 fn from_bytes_copy_prefix(bytes: &[u8]) -> Option<(Self, &[u8])> 149 where 150 Self: Sized, 151 { 152 if bytes.len() < size_of::<Self>() { 153 None 154 } else { 155 // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at` cannot 156 // panic. 157 // TODO: replace with `split_at_checked` once the MSRV is >= 1.80. 158 let (prefix, remainder) = bytes.split_at(size_of::<Self>()); 159 160 Self::from_bytes_copy(prefix).map(|s| (s, remainder)) 161 } 162 } 163 } 164 165 macro_rules! impl_frombytes { 166 ($($({$($generics:tt)*})? $t:ty, )*) => { 167 // SAFETY: Safety comments written in the macro invocation. 168 $(unsafe impl$($($generics)*)? FromBytes for $t {})* 169 }; 170 } 171 172 impl_frombytes! { 173 // SAFETY: All bit patterns are acceptable values of the types below. 174 u8, u16, u32, u64, usize, 175 i8, i16, i32, i64, isize, 176 177 // SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit 178 // patterns are also acceptable for arrays of that type. 179 {<T: FromBytes>} [T], 180 {<T: FromBytes, const N: usize>} [T; N], 181 } 182 183 /// Types that can be viewed as an immutable slice of initialized bytes. 184 /// 185 /// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This 186 /// means that it should not have any padding, as padding bytes are uninitialized. Reading 187 /// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive 188 /// information on the stack to userspace. 189 /// 190 /// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered 191 /// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so 192 /// this is a correctness requirement, but not a safety requirement. 193 /// 194 /// # Safety 195 /// 196 /// Values of this type may not contain any uninitialized bytes. This type must not have interior 197 /// mutability. 198 pub unsafe trait AsBytes { 199 /// Returns `self` as a slice of bytes. 200 fn as_bytes(&self) -> &[u8] { 201 // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized. 202 let data = core::ptr::from_ref(self).cast::<u8>(); 203 let len = core::mem::size_of_val(self); 204 205 // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes. 206 unsafe { core::slice::from_raw_parts(data, len) } 207 } 208 209 /// Returns `self` as a mutable slice of bytes. 210 fn as_bytes_mut(&mut self) -> &mut [u8] 211 where 212 Self: FromBytes, 213 { 214 // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self` 215 // bi-directionally transmutable to `[u8; size_of_val(self)]`. 216 let data = core::ptr::from_mut(self).cast::<u8>(); 217 let len = core::mem::size_of_val(self); 218 219 // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()` 220 // bytes. 221 unsafe { core::slice::from_raw_parts_mut(data, len) } 222 } 223 } 224 225 macro_rules! impl_asbytes { 226 ($($({$($generics:tt)*})? $t:ty, )*) => { 227 // SAFETY: Safety comments written in the macro invocation. 228 $(unsafe impl$($($generics)*)? AsBytes for $t {})* 229 }; 230 } 231 232 impl_asbytes! { 233 // SAFETY: Instances of the following types have no uninitialized portions. 234 u8, u16, u32, u64, usize, 235 i8, i16, i32, i64, isize, 236 bool, 237 char, 238 str, 239 240 // SAFETY: If individual values in an array have no uninitialized portions, then the array 241 // itself does not have any uninitialized portions either. 242 {<T: AsBytes>} [T], 243 {<T: AsBytes, const N: usize>} [T; N], 244 } 245