1// Copyright 2024 The Fuchsia Authors
2//
3// Licensed under the 2-Clause BSD License <LICENSE-BSD or
4// https://opensource.org/license/bsd-2-clause>, Apache License, Version 2.0
5// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
6// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
7// This file may not be copied, modified, or distributed except according to
8// those terms.
9
10use core::{mem, num::NonZeroUsize};
11
12use crate::util;
13
14/// The target pointer width, counted in bits.
15const POINTER_WIDTH_BITS: usize = mem::size_of::<usize>() * 8;
16
17/// The layout of a type which might be dynamically-sized.
18///
19/// `DstLayout` describes the layout of sized types, slice types, and "slice
20/// DSTs" - ie, those that are known by the type system to have a trailing slice
21/// (as distinguished from `dyn Trait` types - such types *might* have a
22/// trailing slice type, but the type system isn't aware of it).
23///
24/// Note that `DstLayout` does not have any internal invariants, so no guarantee
25/// is made that a `DstLayout` conforms to any of Rust's requirements regarding
26/// the layout of real Rust types or instances of types.
27#[doc(hidden)]
28#[allow(missing_debug_implementations, missing_copy_implementations)]
29#[cfg_attr(any(kani, test), derive(Copy, Clone, Debug, PartialEq, Eq))]
30pub struct DstLayout {
31 pub(crate) align: NonZeroUsize,
32 pub(crate) size_info: SizeInfo,
33}
34
35#[cfg_attr(any(kani, test), derive(Debug, PartialEq, Eq))]
36#[derive(Copy, Clone)]
37pub(crate) enum SizeInfo<E = usize> {
38 Sized { size: usize },
39 SliceDst(TrailingSliceLayout<E>),
40}
41
42#[cfg_attr(any(kani, test), derive(Debug, PartialEq, Eq))]
43#[derive(Copy, Clone)]
44pub(crate) struct TrailingSliceLayout<E = usize> {
45 // The offset of the first byte of the trailing slice field. Note that this
46 // is NOT the same as the minimum size of the type. For example, consider
47 // the following type:
48 //
49 // struct Foo {
50 // a: u16,
51 // b: u8,
52 // c: [u8],
53 // }
54 //
55 // In `Foo`, `c` is at byte offset 3. When `c.len() == 0`, `c` is followed
56 // by a padding byte.
57 pub(crate) offset: usize,
58 // The size of the element type of the trailing slice field.
59 pub(crate) elem_size: E,
60}
61
62impl SizeInfo {
63 /// Attempts to create a `SizeInfo` from `Self` in which `elem_size` is a
64 /// `NonZeroUsize`. If `elem_size` is 0, returns `None`.
65 #[allow(unused)]
66 const fn try_to_nonzero_elem_size(&self) -> Option<SizeInfo<NonZeroUsize>> {
67 Some(match *self {
68 SizeInfo::Sized { size: usize } => SizeInfo::Sized { size },
69 SizeInfo::SliceDst(TrailingSliceLayout { offset: usize, elem_size: usize }) => {
70 if let Some(elem_size) = NonZeroUsize::new(elem_size) {
71 SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
72 } else {
73 return None;
74 }
75 }
76 })
77 }
78}
79
80#[doc(hidden)]
81#[derive(Copy, Clone)]
82#[cfg_attr(test, derive(Debug))]
83#[allow(missing_debug_implementations)]
84pub enum CastType {
85 Prefix,
86 Suffix,
87}
88
89#[cfg_attr(test, derive(Debug))]
90pub(crate) enum MetadataCastError {
91 Alignment,
92 Size,
93}
94
95impl DstLayout {
96 /// The minimum possible alignment of a type.
97 const MIN_ALIGN: NonZeroUsize = match NonZeroUsize::new(1) {
98 Some(min_align) => min_align,
99 None => const_unreachable!(),
100 };
101
102 /// The maximum theoretic possible alignment of a type.
103 ///
104 /// For compatibility with future Rust versions, this is defined as the
105 /// maximum power-of-two that fits into a `usize`. See also
106 /// [`DstLayout::CURRENT_MAX_ALIGN`].
107 pub(crate) const THEORETICAL_MAX_ALIGN: NonZeroUsize =
108 match NonZeroUsize::new(1 << (POINTER_WIDTH_BITS - 1)) {
109 Some(max_align) => max_align,
110 None => const_unreachable!(),
111 };
112
113 /// The current, documented max alignment of a type \[1\].
114 ///
115 /// \[1\] Per <https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers>:
116 ///
117 /// The alignment value must be a power of two from 1 up to
118 /// 2<sup>29</sup>.
119 #[cfg(not(kani))]
120 pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 28) {
121 Some(max_align) => max_align,
122 None => const_unreachable!(),
123 };
124
125 /// Constructs a `DstLayout` for a zero-sized type with `repr_align`
126 /// alignment (or 1). If `repr_align` is provided, then it must be a power
127 /// of two.
128 ///
129 /// # Panics
130 ///
131 /// This function panics if the supplied `repr_align` is not a power of two.
132 ///
133 /// # Safety
134 ///
135 /// Unsafe code may assume that the contract of this function is satisfied.
136 #[doc(hidden)]
137 #[must_use]
138 #[inline]
139 pub const fn new_zst(repr_align: Option<NonZeroUsize>) -> DstLayout {
140 let align = match repr_align {
141 Some(align) => align,
142 None => Self::MIN_ALIGN,
143 };
144
145 const_assert!(align.get().is_power_of_two());
146
147 DstLayout { align, size_info: SizeInfo::Sized { size: 0 } }
148 }
149
150 /// Constructs a `DstLayout` which describes `T`.
151 ///
152 /// # Safety
153 ///
154 /// Unsafe code may assume that `DstLayout` is the correct layout for `T`.
155 #[doc(hidden)]
156 #[must_use]
157 #[inline]
158 pub const fn for_type<T>() -> DstLayout {
159 // SAFETY: `align` is correct by construction. `T: Sized`, and so it is
160 // sound to initialize `size_info` to `SizeInfo::Sized { size }`; the
161 // `size` field is also correct by construction.
162 DstLayout {
163 align: match NonZeroUsize::new(mem::align_of::<T>()) {
164 Some(align) => align,
165 None => const_unreachable!(),
166 },
167 size_info: SizeInfo::Sized { size: mem::size_of::<T>() },
168 }
169 }
170
171 /// Constructs a `DstLayout` which describes `[T]`.
172 ///
173 /// # Safety
174 ///
175 /// Unsafe code may assume that `DstLayout` is the correct layout for `[T]`.
176 pub(crate) const fn for_slice<T>() -> DstLayout {
177 // SAFETY: The alignment of a slice is equal to the alignment of its
178 // element type, and so `align` is initialized correctly.
179 //
180 // Since this is just a slice type, there is no offset between the
181 // beginning of the type and the beginning of the slice, so it is
182 // correct to set `offset: 0`. The `elem_size` is correct by
183 // construction. Since `[T]` is a (degenerate case of a) slice DST, it
184 // is correct to initialize `size_info` to `SizeInfo::SliceDst`.
185 DstLayout {
186 align: match NonZeroUsize::new(mem::align_of::<T>()) {
187 Some(align) => align,
188 None => const_unreachable!(),
189 },
190 size_info: SizeInfo::SliceDst(TrailingSliceLayout {
191 offset: 0,
192 elem_size: mem::size_of::<T>(),
193 }),
194 }
195 }
196
197 /// Like `Layout::extend`, this creates a layout that describes a record
198 /// whose layout consists of `self` followed by `next` that includes the
199 /// necessary inter-field padding, but not any trailing padding.
200 ///
201 /// In order to match the layout of a `#[repr(C)]` struct, this method
202 /// should be invoked for each field in declaration order. To add trailing
203 /// padding, call `DstLayout::pad_to_align` after extending the layout for
204 /// all fields. If `self` corresponds to a type marked with
205 /// `repr(packed(N))`, then `repr_packed` should be set to `Some(N)`,
206 /// otherwise `None`.
207 ///
208 /// This method cannot be used to match the layout of a record with the
209 /// default representation, as that representation is mostly unspecified.
210 ///
211 /// # Safety
212 ///
213 /// If a (potentially hypothetical) valid `repr(C)` Rust type begins with
214 /// fields whose layout are `self`, and those fields are immediately
215 /// followed by a field whose layout is `field`, then unsafe code may rely
216 /// on `self.extend(field, repr_packed)` producing a layout that correctly
217 /// encompasses those two components.
218 ///
219 /// We make no guarantees to the behavior of this method if these fragments
220 /// cannot appear in a valid Rust type (e.g., the concatenation of the
221 /// layouts would lead to a size larger than `isize::MAX`).
222 #[doc(hidden)]
223 #[must_use]
224 #[inline]
225 pub const fn extend(self, field: DstLayout, repr_packed: Option<NonZeroUsize>) -> Self {
226 use util::{max, min, padding_needed_for};
227
228 // If `repr_packed` is `None`, there are no alignment constraints, and
229 // the value can be defaulted to `THEORETICAL_MAX_ALIGN`.
230 let max_align = match repr_packed {
231 Some(max_align) => max_align,
232 None => Self::THEORETICAL_MAX_ALIGN,
233 };
234
235 const_assert!(max_align.get().is_power_of_two());
236
237 // We use Kani to prove that this method is robust to future increases
238 // in Rust's maximum allowed alignment. However, if such a change ever
239 // actually occurs, we'd like to be notified via assertion failures.
240 #[cfg(not(kani))]
241 {
242 const_debug_assert!(self.align.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
243 const_debug_assert!(field.align.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
244 if let Some(repr_packed) = repr_packed {
245 const_debug_assert!(repr_packed.get() <= DstLayout::CURRENT_MAX_ALIGN.get());
246 }
247 }
248
249 // The field's alignment is clamped by `repr_packed` (i.e., the
250 // `repr(packed(N))` attribute, if any) [1].
251 //
252 // [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
253 //
254 // The alignments of each field, for the purpose of positioning
255 // fields, is the smaller of the specified alignment and the alignment
256 // of the field's type.
257 let field_align = min(field.align, max_align);
258
259 // The struct's alignment is the maximum of its previous alignment and
260 // `field_align`.
261 let align = max(self.align, field_align);
262
263 let size_info = match self.size_info {
264 // If the layout is already a DST, we panic; DSTs cannot be extended
265 // with additional fields.
266 SizeInfo::SliceDst(..) => const_panic!("Cannot extend a DST with additional fields."),
267
268 SizeInfo::Sized { size: preceding_size } => {
269 // Compute the minimum amount of inter-field padding needed to
270 // satisfy the field's alignment, and offset of the trailing
271 // field. [1]
272 //
273 // [1] Per https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers:
274 //
275 // Inter-field padding is guaranteed to be the minimum
276 // required in order to satisfy each field's (possibly
277 // altered) alignment.
278 let padding = padding_needed_for(preceding_size, field_align);
279
280 // This will not panic (and is proven to not panic, with Kani)
281 // if the layout components can correspond to a leading layout
282 // fragment of a valid Rust type, but may panic otherwise (e.g.,
283 // combining or aligning the components would create a size
284 // exceeding `isize::MAX`).
285 let offset = match preceding_size.checked_add(padding) {
286 Some(offset) => offset,
287 None => const_panic!("Adding padding to `self`'s size overflows `usize`."),
288 };
289
290 match field.size_info {
291 SizeInfo::Sized { size: field_size } => {
292 // If the trailing field is sized, the resulting layout
293 // will be sized. Its size will be the sum of the
294 // preceeding layout, the size of the new field, and the
295 // size of inter-field padding between the two.
296 //
297 // This will not panic (and is proven with Kani to not
298 // panic) if the layout components can correspond to a
299 // leading layout fragment of a valid Rust type, but may
300 // panic otherwise (e.g., combining or aligning the
301 // components would create a size exceeding
302 // `usize::MAX`).
303 let size = match offset.checked_add(field_size) {
304 Some(size) => size,
305 None => const_panic!("`field` cannot be appended without the total size overflowing `usize`"),
306 };
307 SizeInfo::Sized { size }
308 }
309 SizeInfo::SliceDst(TrailingSliceLayout {
310 offset: trailing_offset,
311 elem_size,
312 }) => {
313 // If the trailing field is dynamically sized, so too
314 // will the resulting layout. The offset of the trailing
315 // slice component is the sum of the offset of the
316 // trailing field and the trailing slice offset within
317 // that field.
318 //
319 // This will not panic (and is proven with Kani to not
320 // panic) if the layout components can correspond to a
321 // leading layout fragment of a valid Rust type, but may
322 // panic otherwise (e.g., combining or aligning the
323 // components would create a size exceeding
324 // `usize::MAX`).
325 let offset = match offset.checked_add(trailing_offset) {
326 Some(offset) => offset,
327 None => const_panic!("`field` cannot be appended without the total size overflowing `usize`"),
328 };
329 SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
330 }
331 }
332 }
333 };
334
335 DstLayout { align, size_info }
336 }
337
338 /// Like `Layout::pad_to_align`, this routine rounds the size of this layout
339 /// up to the nearest multiple of this type's alignment or `repr_packed`
340 /// (whichever is less). This method leaves DST layouts unchanged, since the
341 /// trailing padding of DSTs is computed at runtime.
342 ///
343 /// In order to match the layout of a `#[repr(C)]` struct, this method
344 /// should be invoked after the invocations of [`DstLayout::extend`]. If
345 /// `self` corresponds to a type marked with `repr(packed(N))`, then
346 /// `repr_packed` should be set to `Some(N)`, otherwise `None`.
347 ///
348 /// This method cannot be used to match the layout of a record with the
349 /// default representation, as that representation is mostly unspecified.
350 ///
351 /// # Safety
352 ///
353 /// If a (potentially hypothetical) valid `repr(C)` type begins with fields
354 /// whose layout are `self` followed only by zero or more bytes of trailing
355 /// padding (not included in `self`), then unsafe code may rely on
356 /// `self.pad_to_align(repr_packed)` producing a layout that correctly
357 /// encapsulates the layout of that type.
358 ///
359 /// We make no guarantees to the behavior of this method if `self` cannot
360 /// appear in a valid Rust type (e.g., because the addition of trailing
361 /// padding would lead to a size larger than `isize::MAX`).
362 #[doc(hidden)]
363 #[must_use]
364 #[inline]
365 pub const fn pad_to_align(self) -> Self {
366 use util::padding_needed_for;
367
368 let size_info = match self.size_info {
369 // For sized layouts, we add the minimum amount of trailing padding
370 // needed to satisfy alignment.
371 SizeInfo::Sized { size: unpadded_size } => {
372 let padding = padding_needed_for(unpadded_size, self.align);
373 let size = match unpadded_size.checked_add(padding) {
374 Some(size) => size,
375 None => const_panic!("Adding padding caused size to overflow `usize`."),
376 };
377 SizeInfo::Sized { size }
378 }
379 // For DST layouts, trailing padding depends on the length of the
380 // trailing DST and is computed at runtime. This does not alter the
381 // offset or element size of the layout, so we leave `size_info`
382 // unchanged.
383 size_info @ SizeInfo::SliceDst(_) => size_info,
384 };
385
386 DstLayout { align: self.align, size_info }
387 }
388
389 /// Validates that a cast is sound from a layout perspective.
390 ///
391 /// Validates that the size and alignment requirements of a type with the
392 /// layout described in `self` would not be violated by performing a
393 /// `cast_type` cast from a pointer with address `addr` which refers to a
394 /// memory region of size `bytes_len`.
395 ///
396 /// If the cast is valid, `validate_cast_and_convert_metadata` returns
397 /// `(elems, split_at)`. If `self` describes a dynamically-sized type, then
398 /// `elems` is the maximum number of trailing slice elements for which a
399 /// cast would be valid (for sized types, `elem` is meaningless and should
400 /// be ignored). `split_at` is the index at which to split the memory region
401 /// in order for the prefix (suffix) to contain the result of the cast, and
402 /// in order for the remaining suffix (prefix) to contain the leftover
403 /// bytes.
404 ///
405 /// There are three conditions under which a cast can fail:
406 /// - The smallest possible value for the type is larger than the provided
407 /// memory region
408 /// - A prefix cast is requested, and `addr` does not satisfy `self`'s
409 /// alignment requirement
410 /// - A suffix cast is requested, and `addr + bytes_len` does not satisfy
411 /// `self`'s alignment requirement (as a consequence, since all instances
412 /// of the type are a multiple of its alignment, no size for the type will
413 /// result in a starting address which is properly aligned)
414 ///
415 /// # Safety
416 ///
417 /// The caller may assume that this implementation is correct, and may rely
418 /// on that assumption for the soundness of their code. In particular, the
419 /// caller may assume that, if `validate_cast_and_convert_metadata` returns
420 /// `Some((elems, split_at))`, then:
421 /// - A pointer to the type (for dynamically sized types, this includes
422 /// `elems` as its pointer metadata) describes an object of size `size <=
423 /// bytes_len`
424 /// - If this is a prefix cast:
425 /// - `addr` satisfies `self`'s alignment
426 /// - `size == split_at`
427 /// - If this is a suffix cast:
428 /// - `split_at == bytes_len - size`
429 /// - `addr + split_at` satisfies `self`'s alignment
430 ///
431 /// Note that this method does *not* ensure that a pointer constructed from
432 /// its return values will be a valid pointer. In particular, this method
433 /// does not reason about `isize` overflow, which is a requirement of many
434 /// Rust pointer APIs, and may at some point be determined to be a validity
435 /// invariant of pointer types themselves. This should never be a problem so
436 /// long as the arguments to this method are derived from a known-valid
437 /// pointer (e.g., one derived from a safe Rust reference), but it is
438 /// nonetheless the caller's responsibility to justify that pointer
439 /// arithmetic will not overflow based on a safety argument *other than* the
440 /// mere fact that this method returned successfully.
441 ///
442 /// # Panics
443 ///
444 /// `validate_cast_and_convert_metadata` will panic if `self` describes a
445 /// DST whose trailing slice element is zero-sized.
446 ///
447 /// If `addr + bytes_len` overflows `usize`,
448 /// `validate_cast_and_convert_metadata` may panic, or it may return
449 /// incorrect results. No guarantees are made about when
450 /// `validate_cast_and_convert_metadata` will panic. The caller should not
451 /// rely on `validate_cast_and_convert_metadata` panicking in any particular
452 /// condition, even if `debug_assertions` are enabled.
453 #[allow(unused)]
454 #[inline(always)]
455 pub(crate) const fn validate_cast_and_convert_metadata(
456 &self,
457 addr: usize,
458 bytes_len: usize,
459 cast_type: CastType,
460 ) -> Result<(usize, usize), MetadataCastError> {
461 // `debug_assert!`, but with `#[allow(clippy::arithmetic_side_effects)]`.
462 macro_rules! __const_debug_assert {
463 ($e:expr $(, $msg:expr)?) => {
464 const_debug_assert!({
465 #[allow(clippy::arithmetic_side_effects)]
466 let e = $e;
467 e
468 } $(, $msg)?);
469 };
470 }
471
472 // Note that, in practice, `self` is always a compile-time constant. We
473 // do this check earlier than needed to ensure that we always panic as a
474 // result of bugs in the program (such as calling this function on an
475 // invalid type) instead of allowing this panic to be hidden if the cast
476 // would have failed anyway for runtime reasons (such as a too-small
477 // memory region).
478 //
479 // TODO(#67): Once our MSRV is 1.65, use let-else:
480 // https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#let-else-statements
481 let size_info = match self.size_info.try_to_nonzero_elem_size() {
482 Some(size_info) => size_info,
483 None => const_panic!("attempted to cast to slice type with zero-sized element"),
484 };
485
486 // Precondition
487 __const_debug_assert!(
488 addr.checked_add(bytes_len).is_some(),
489 "`addr` + `bytes_len` > usize::MAX"
490 );
491
492 // Alignment checks go in their own block to avoid introducing variables
493 // into the top-level scope.
494 {
495 // We check alignment for `addr` (for prefix casts) or `addr +
496 // bytes_len` (for suffix casts). For a prefix cast, the correctness
497 // of this check is trivial - `addr` is the address the object will
498 // live at.
499 //
500 // For a suffix cast, we know that all valid sizes for the type are
501 // a multiple of the alignment (and by safety precondition, we know
502 // `DstLayout` may only describe valid Rust types). Thus, a
503 // validly-sized instance which lives at a validly-aligned address
504 // must also end at a validly-aligned address. Thus, if the end
505 // address for a suffix cast (`addr + bytes_len`) is not aligned,
506 // then no valid start address will be aligned either.
507 let offset = match cast_type {
508 CastType::Prefix => 0,
509 CastType::Suffix => bytes_len,
510 };
511
512 // Addition is guaranteed not to overflow because `offset <=
513 // bytes_len`, and `addr + bytes_len <= usize::MAX` is a
514 // precondition of this method. Modulus is guaranteed not to divide
515 // by 0 because `align` is non-zero.
516 #[allow(clippy::arithmetic_side_effects)]
517 if (addr + offset) % self.align.get() != 0 {
518 return Err(MetadataCastError::Alignment);
519 }
520 }
521
522 let (elems, self_bytes) = match size_info {
523 SizeInfo::Sized { size } => {
524 if size > bytes_len {
525 return Err(MetadataCastError::Size);
526 }
527 (0, size)
528 }
529 SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
530 // Calculate the maximum number of bytes that could be consumed
531 // - any number of bytes larger than this will either not be a
532 // multiple of the alignment, or will be larger than
533 // `bytes_len`.
534 let max_total_bytes =
535 util::round_down_to_next_multiple_of_alignment(bytes_len, self.align);
536 // Calculate the maximum number of bytes that could be consumed
537 // by the trailing slice.
538 //
539 // TODO(#67): Once our MSRV is 1.65, use let-else:
540 // https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#let-else-statements
541 let max_slice_and_padding_bytes = match max_total_bytes.checked_sub(offset) {
542 Some(max) => max,
543 // `bytes_len` too small even for 0 trailing slice elements.
544 None => return Err(MetadataCastError::Size),
545 };
546
547 // Calculate the number of elements that fit in
548 // `max_slice_and_padding_bytes`; any remaining bytes will be
549 // considered padding.
550 //
551 // Guaranteed not to divide by zero: `elem_size` is non-zero.
552 #[allow(clippy::arithmetic_side_effects)]
553 let elems = max_slice_and_padding_bytes / elem_size.get();
554 // Guaranteed not to overflow on multiplication: `usize::MAX >=
555 // max_slice_and_padding_bytes >= (max_slice_and_padding_bytes /
556 // elem_size) * elem_size`.
557 //
558 // Guaranteed not to overflow on addition:
559 // - max_slice_and_padding_bytes == max_total_bytes - offset
560 // - elems * elem_size <= max_slice_and_padding_bytes == max_total_bytes - offset
561 // - elems * elem_size + offset <= max_total_bytes <= usize::MAX
562 #[allow(clippy::arithmetic_side_effects)]
563 let without_padding = offset + elems * elem_size.get();
564 // `self_bytes` is equal to the offset bytes plus the bytes
565 // consumed by the trailing slice plus any padding bytes
566 // required to satisfy the alignment. Note that we have computed
567 // the maximum number of trailing slice elements that could fit
568 // in `self_bytes`, so any padding is guaranteed to be less than
569 // the size of an extra element.
570 //
571 // Guaranteed not to overflow:
572 // - By previous comment: without_padding == elems * elem_size +
573 // offset <= max_total_bytes
574 // - By construction, `max_total_bytes` is a multiple of
575 // `self.align`.
576 // - At most, adding padding needed to round `without_padding`
577 // up to the next multiple of the alignment will bring
578 // `self_bytes` up to `max_total_bytes`.
579 #[allow(clippy::arithmetic_side_effects)]
580 let self_bytes =
581 without_padding + util::padding_needed_for(without_padding, self.align);
582 (elems, self_bytes)
583 }
584 };
585
586 __const_debug_assert!(self_bytes <= bytes_len);
587
588 let split_at = match cast_type {
589 CastType::Prefix => self_bytes,
590 // Guaranteed not to underflow:
591 // - In the `Sized` branch, only returns `size` if `size <=
592 // bytes_len`.
593 // - In the `SliceDst` branch, calculates `self_bytes <=
594 // max_toatl_bytes`, which is upper-bounded by `bytes_len`.
595 #[allow(clippy::arithmetic_side_effects)]
596 CastType::Suffix => bytes_len - self_bytes,
597 };
598
599 Ok((elems, split_at))
600 }
601}
602
603// TODO(#67): For some reason, on our MSRV toolchain, this `allow` isn't
604// enforced despite having `#![allow(unknown_lints)]` at the crate root, but
605// putting it here works. Once our MSRV is high enough that this bug has been
606// fixed, remove this `allow`.
607#[allow(unknown_lints)]
608#[cfg(test)]
609mod tests {
610 use super::*;
611
612 /// Tests of when a sized `DstLayout` is extended with a sized field.
613 #[allow(clippy::decimal_literal_representation)]
614 #[test]
615 fn test_dst_layout_extend_sized_with_sized() {
616 // This macro constructs a layout corresponding to a `u8` and extends it
617 // with a zero-sized trailing field of given alignment `n`. The macro
618 // tests that the resulting layout has both size and alignment `min(n,
619 // P)` for all valid values of `repr(packed(P))`.
620 macro_rules! test_align_is_size {
621 ($n:expr) => {
622 let base = DstLayout::for_type::<u8>();
623 let trailing_field = DstLayout::for_type::<elain::Align<$n>>();
624
625 let packs =
626 core::iter::once(None).chain((0..29).map(|p| NonZeroUsize::new(2usize.pow(p))));
627
628 for pack in packs {
629 let composite = base.extend(trailing_field, pack);
630 let max_align = pack.unwrap_or(DstLayout::CURRENT_MAX_ALIGN);
631 let align = $n.min(max_align.get());
632 assert_eq!(
633 composite,
634 DstLayout {
635 align: NonZeroUsize::new(align).unwrap(),
636 size_info: SizeInfo::Sized { size: align }
637 }
638 )
639 }
640 };
641 }
642
643 test_align_is_size!(1);
644 test_align_is_size!(2);
645 test_align_is_size!(4);
646 test_align_is_size!(8);
647 test_align_is_size!(16);
648 test_align_is_size!(32);
649 test_align_is_size!(64);
650 test_align_is_size!(128);
651 test_align_is_size!(256);
652 test_align_is_size!(512);
653 test_align_is_size!(1024);
654 test_align_is_size!(2048);
655 test_align_is_size!(4096);
656 test_align_is_size!(8192);
657 test_align_is_size!(16384);
658 test_align_is_size!(32768);
659 test_align_is_size!(65536);
660 test_align_is_size!(131072);
661 test_align_is_size!(262144);
662 test_align_is_size!(524288);
663 test_align_is_size!(1048576);
664 test_align_is_size!(2097152);
665 test_align_is_size!(4194304);
666 test_align_is_size!(8388608);
667 test_align_is_size!(16777216);
668 test_align_is_size!(33554432);
669 test_align_is_size!(67108864);
670 test_align_is_size!(33554432);
671 test_align_is_size!(134217728);
672 test_align_is_size!(268435456);
673 }
674
675 /// Tests of when a sized `DstLayout` is extended with a DST field.
676 #[test]
677 fn test_dst_layout_extend_sized_with_dst() {
678 // Test that for all combinations of real-world alignments and
679 // `repr_packed` values, that the extension of a sized `DstLayout`` with
680 // a DST field correctly computes the trailing offset in the composite
681 // layout.
682
683 let aligns = (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap());
684 let packs = core::iter::once(None).chain(aligns.clone().map(Some));
685
686 for align in aligns {
687 for pack in packs.clone() {
688 let base = DstLayout::for_type::<u8>();
689 let elem_size = 42;
690 let trailing_field_offset = 11;
691
692 let trailing_field = DstLayout {
693 align,
694 size_info: SizeInfo::SliceDst(TrailingSliceLayout { elem_size, offset: 11 }),
695 };
696
697 let composite = base.extend(trailing_field, pack);
698
699 let max_align = pack.unwrap_or(DstLayout::CURRENT_MAX_ALIGN).get();
700
701 let align = align.get().min(max_align);
702
703 assert_eq!(
704 composite,
705 DstLayout {
706 align: NonZeroUsize::new(align).unwrap(),
707 size_info: SizeInfo::SliceDst(TrailingSliceLayout {
708 elem_size,
709 offset: align + trailing_field_offset,
710 }),
711 }
712 )
713 }
714 }
715 }
716
717 /// Tests that calling `pad_to_align` on a sized `DstLayout` adds the
718 /// expected amount of trailing padding.
719 #[test]
720 fn test_dst_layout_pad_to_align_with_sized() {
721 // For all valid alignments `align`, construct a one-byte layout aligned
722 // to `align`, call `pad_to_align`, and assert that the size of the
723 // resulting layout is equal to `align`.
724 for align in (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap()) {
725 let layout = DstLayout { align, size_info: SizeInfo::Sized { size: 1 } };
726
727 assert_eq!(
728 layout.pad_to_align(),
729 DstLayout { align, size_info: SizeInfo::Sized { size: align.get() } }
730 );
731 }
732
733 // Test explicitly-provided combinations of unpadded and padded
734 // counterparts.
735
736 macro_rules! test {
737 (unpadded { size: $unpadded_size:expr, align: $unpadded_align:expr }
738 => padded { size: $padded_size:expr, align: $padded_align:expr }) => {
739 let unpadded = DstLayout {
740 align: NonZeroUsize::new($unpadded_align).unwrap(),
741 size_info: SizeInfo::Sized { size: $unpadded_size },
742 };
743 let padded = unpadded.pad_to_align();
744
745 assert_eq!(
746 padded,
747 DstLayout {
748 align: NonZeroUsize::new($padded_align).unwrap(),
749 size_info: SizeInfo::Sized { size: $padded_size },
750 }
751 );
752 };
753 }
754
755 test!(unpadded { size: 0, align: 4 } => padded { size: 0, align: 4 });
756 test!(unpadded { size: 1, align: 4 } => padded { size: 4, align: 4 });
757 test!(unpadded { size: 2, align: 4 } => padded { size: 4, align: 4 });
758 test!(unpadded { size: 3, align: 4 } => padded { size: 4, align: 4 });
759 test!(unpadded { size: 4, align: 4 } => padded { size: 4, align: 4 });
760 test!(unpadded { size: 5, align: 4 } => padded { size: 8, align: 4 });
761 test!(unpadded { size: 6, align: 4 } => padded { size: 8, align: 4 });
762 test!(unpadded { size: 7, align: 4 } => padded { size: 8, align: 4 });
763 test!(unpadded { size: 8, align: 4 } => padded { size: 8, align: 4 });
764
765 let current_max_align = DstLayout::CURRENT_MAX_ALIGN.get();
766
767 test!(unpadded { size: 1, align: current_max_align }
768 => padded { size: current_max_align, align: current_max_align });
769
770 test!(unpadded { size: current_max_align + 1, align: current_max_align }
771 => padded { size: current_max_align * 2, align: current_max_align });
772 }
773
774 /// Tests that calling `pad_to_align` on a DST `DstLayout` is a no-op.
775 #[test]
776 fn test_dst_layout_pad_to_align_with_dst() {
777 for align in (0..29).map(|p| NonZeroUsize::new(2usize.pow(p)).unwrap()) {
778 for offset in 0..10 {
779 for elem_size in 0..10 {
780 let layout = DstLayout {
781 align,
782 size_info: SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }),
783 };
784 assert_eq!(layout.pad_to_align(), layout);
785 }
786 }
787 }
788 }
789
790 // This test takes a long time when running under Miri, so we skip it in
791 // that case. This is acceptable because this is a logic test that doesn't
792 // attempt to expose UB.
793 #[test]
794 #[cfg_attr(miri, ignore)]
795 fn test_validate_cast_and_convert_metadata() {
796 #[allow(non_local_definitions)]
797 impl From<usize> for SizeInfo {
798 fn from(size: usize) -> SizeInfo {
799 SizeInfo::Sized { size }
800 }
801 }
802
803 #[allow(non_local_definitions)]
804 impl From<(usize, usize)> for SizeInfo {
805 fn from((offset, elem_size): (usize, usize)) -> SizeInfo {
806 SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size })
807 }
808 }
809
810 fn layout<S: Into<SizeInfo>>(s: S, align: usize) -> DstLayout {
811 DstLayout { size_info: s.into(), align: NonZeroUsize::new(align).unwrap() }
812 }
813
814 /// This macro accepts arguments in the form of:
815 ///
816 /// layout(_, _, _).validate(_, _, _), Ok(Some((_, _)))
817 /// | | | | | | | |
818 /// base_size ----+ | | | | | | |
819 /// align -----------+ | | | | | |
820 /// trailing_size ------+ | | | | |
821 /// addr ---------------------------+ | | | |
822 /// bytes_len -------------------------+ | | |
823 /// cast_type ----------------------------+ | |
824 /// elems ---------------------------------------------+ |
825 /// split_at ---------------------------------------------+
826 ///
827 /// `.validate` is shorthand for `.validate_cast_and_convert_metadata`
828 /// for brevity.
829 ///
830 /// Each argument can either be an iterator or a wildcard. Each
831 /// wildcarded variable is implicitly replaced by an iterator over a
832 /// representative sample of values for that variable. Each `test!`
833 /// invocation iterates over every combination of values provided by
834 /// each variable's iterator (ie, the cartesian product) and validates
835 /// that the results are expected.
836 ///
837 /// The final argument uses the same syntax, but it has a different
838 /// meaning:
839 /// - If it is `Ok(pat)`, then the pattern `pat` is supplied to
840 /// a matching assert to validate the computed result for each
841 /// combination of input values.
842 /// - If it is `Err(Some(msg) | None)`, then `test!` validates that the
843 /// call to `validate_cast_and_convert_metadata` panics with the given
844 /// panic message or, if the current Rust toolchain version is too
845 /// early to support panicking in `const fn`s, panics with *some*
846 /// message. In the latter case, the `const_panic!` macro is used,
847 /// which emits code which causes a non-panicking error at const eval
848 /// time, but which does panic when invoked at runtime. Thus, it is
849 /// merely difficult to predict the *value* of this panic. We deem
850 /// that testing against the real panic strings on stable and nightly
851 /// toolchains is enough to ensure correctness.
852 ///
853 /// Note that the meta-variables that match these variables have the
854 /// `tt` type, and some valid expressions are not valid `tt`s (such as
855 /// `a..b`). In this case, wrap the expression in parentheses, and it
856 /// will become valid `tt`.
857 macro_rules! test {
858 ($(:$sizes:expr =>)?
859 layout($size:tt, $align:tt)
860 .validate($addr:tt, $bytes_len:tt, $cast_type:tt), $expect:pat $(,)?
861 ) => {
862 itertools::iproduct!(
863 test!(@generate_size $size),
864 test!(@generate_align $align),
865 test!(@generate_usize $addr),
866 test!(@generate_usize $bytes_len),
867 test!(@generate_cast_type $cast_type)
868 ).for_each(|(size_info, align, addr, bytes_len, cast_type)| {
869 // Temporarily disable the panic hook installed by the test
870 // harness. If we don't do this, all panic messages will be
871 // kept in an internal log. On its own, this isn't a
872 // problem, but if a non-caught panic ever happens (ie, in
873 // code later in this test not in this macro), all of the
874 // previously-buffered messages will be dumped, hiding the
875 // real culprit.
876 let previous_hook = std::panic::take_hook();
877 // I don't understand why, but this seems to be required in
878 // addition to the previous line.
879 std::panic::set_hook(Box::new(|_| {}));
880 let actual = std::panic::catch_unwind(|| {
881 layout(size_info, align).validate_cast_and_convert_metadata(addr, bytes_len, cast_type)
882 }).map_err(|d| {
883 let msg = d.downcast::<&'static str>().ok().map(|s| *s.as_ref());
884 assert!(msg.is_some() || cfg!(not(zerocopy_panic_in_const_and_vec_try_reserve_1_57_0)), "non-string panic messages are not permitted when `--cfg zerocopy_panic_in_const_and_vec_try_reserve` is set");
885 msg
886 });
887 std::panic::set_hook(previous_hook);
888
889 assert!(
890 matches!(actual, $expect),
891 "layout({:?}, {}).validate_cast_and_convert_metadata({}, {}, {:?})" ,size_info, align, addr, bytes_len, cast_type
892 );
893 });
894 };
895 (@generate_usize _) => { 0..8 };
896 // Generate sizes for both Sized and !Sized types.
897 (@generate_size _) => {
898 test!(@generate_size (_)).chain(test!(@generate_size (_, _)))
899 };
900 // Generate sizes for both Sized and !Sized types by chaining
901 // specified iterators for each.
902 (@generate_size ($sized_sizes:tt | $unsized_sizes:tt)) => {
903 test!(@generate_size ($sized_sizes)).chain(test!(@generate_size $unsized_sizes))
904 };
905 // Generate sizes for Sized types.
906 (@generate_size (_)) => { test!(@generate_size (0..8)) };
907 (@generate_size ($sizes:expr)) => { $sizes.into_iter().map(Into::<SizeInfo>::into) };
908 // Generate sizes for !Sized types.
909 (@generate_size ($min_sizes:tt, $elem_sizes:tt)) => {
910 itertools::iproduct!(
911 test!(@generate_min_size $min_sizes),
912 test!(@generate_elem_size $elem_sizes)
913 ).map(Into::<SizeInfo>::into)
914 };
915 (@generate_fixed_size _) => { (0..8).into_iter().map(Into::<SizeInfo>::into) };
916 (@generate_min_size _) => { 0..8 };
917 (@generate_elem_size _) => { 1..8 };
918 (@generate_align _) => { [1, 2, 4, 8, 16] };
919 (@generate_opt_usize _) => { [None].into_iter().chain((0..8).map(Some).into_iter()) };
920 (@generate_cast_type _) => { [CastType::Prefix, CastType::Suffix] };
921 (@generate_cast_type $variant:ident) => { [CastType::$variant] };
922 // Some expressions need to be wrapped in parentheses in order to be
923 // valid `tt`s (required by the top match pattern). See the comment
924 // below for more details. This arm removes these parentheses to
925 // avoid generating an `unused_parens` warning.
926 (@$_:ident ($vals:expr)) => { $vals };
927 (@$_:ident $vals:expr) => { $vals };
928 }
929
930 const EVENS: [usize; 8] = [0, 2, 4, 6, 8, 10, 12, 14];
931 const ODDS: [usize; 8] = [1, 3, 5, 7, 9, 11, 13, 15];
932
933 // base_size is too big for the memory region.
934 test!(
935 layout(((1..8) | ((1..8), (1..8))), _).validate([0], [0], _),
936 Ok(Err(MetadataCastError::Size))
937 );
938 test!(
939 layout(((2..8) | ((2..8), (2..8))), _).validate([0], [1], Prefix),
940 Ok(Err(MetadataCastError::Size))
941 );
942 test!(
943 layout(((2..8) | ((2..8), (2..8))), _).validate([0x1000_0000 - 1], [1], Suffix),
944 Ok(Err(MetadataCastError::Size))
945 );
946
947 // addr is unaligned for prefix cast
948 test!(layout(_, [2]).validate(ODDS, _, Prefix), Ok(Err(MetadataCastError::Alignment)));
949 test!(layout(_, [2]).validate(ODDS, _, Prefix), Ok(Err(MetadataCastError::Alignment)));
950
951 // addr is aligned, but end of buffer is unaligned for suffix cast
952 test!(layout(_, [2]).validate(EVENS, ODDS, Suffix), Ok(Err(MetadataCastError::Alignment)));
953 test!(layout(_, [2]).validate(EVENS, ODDS, Suffix), Ok(Err(MetadataCastError::Alignment)));
954
955 // Unfortunately, these constants cannot easily be used in the
956 // implementation of `validate_cast_and_convert_metadata`, since
957 // `panic!` consumes a string literal, not an expression.
958 //
959 // It's important that these messages be in a separate module. If they
960 // were at the function's top level, we'd pass them to `test!` as, e.g.,
961 // `Err(TRAILING)`, which would run into a subtle Rust footgun - the
962 // `TRAILING` identifier would be treated as a pattern to match rather
963 // than a value to check for equality.
964 mod msgs {
965 pub(super) const TRAILING: &str =
966 "attempted to cast to slice type with zero-sized element";
967 pub(super) const OVERFLOW: &str = "`addr` + `bytes_len` > usize::MAX";
968 }
969
970 // casts with ZST trailing element types are unsupported
971 test!(layout((_, [0]), _).validate(_, _, _), Err(Some(msgs::TRAILING) | None),);
972
973 // addr + bytes_len must not overflow usize
974 test!(layout(_, _).validate([usize::MAX], (1..100), _), Err(Some(msgs::OVERFLOW) | None));
975 test!(layout(_, _).validate((1..100), [usize::MAX], _), Err(Some(msgs::OVERFLOW) | None));
976 test!(
977 layout(_, _).validate(
978 [usize::MAX / 2 + 1, usize::MAX],
979 [usize::MAX / 2 + 1, usize::MAX],
980 _
981 ),
982 Err(Some(msgs::OVERFLOW) | None)
983 );
984
985 // Validates that `validate_cast_and_convert_metadata` satisfies its own
986 // documented safety postconditions, and also a few other properties
987 // that aren't documented but we want to guarantee anyway.
988 fn validate_behavior(
989 (layout, addr, bytes_len, cast_type): (DstLayout, usize, usize, CastType),
990 ) {
991 if let Ok((elems, split_at)) =
992 layout.validate_cast_and_convert_metadata(addr, bytes_len, cast_type)
993 {
994 let (size_info, align) = (layout.size_info, layout.align);
995 let debug_str = format!(
996 "layout({:?}, {}).validate_cast_and_convert_metadata({}, {}, {:?}) => ({}, {})",
997 size_info, align, addr, bytes_len, cast_type, elems, split_at
998 );
999
1000 // If this is a sized type (no trailing slice), then `elems` is
1001 // meaningless, but in practice we set it to 0. Callers are not
1002 // allowed to rely on this, but a lot of math is nicer if
1003 // they're able to, and some callers might accidentally do that.
1004 let sized = matches!(layout.size_info, SizeInfo::Sized { .. });
1005 assert!(!(sized && elems != 0), "{}", debug_str);
1006
1007 let resulting_size = match layout.size_info {
1008 SizeInfo::Sized { size } => size,
1009 SizeInfo::SliceDst(TrailingSliceLayout { offset, elem_size }) => {
1010 let padded_size = |elems| {
1011 let without_padding = offset + elems * elem_size;
1012 without_padding + util::padding_needed_for(without_padding, align)
1013 };
1014
1015 let resulting_size = padded_size(elems);
1016 // Test that `validate_cast_and_convert_metadata`
1017 // computed the largest possible value that fits in the
1018 // given range.
1019 assert!(padded_size(elems + 1) > bytes_len, "{}", debug_str);
1020 resulting_size
1021 }
1022 };
1023
1024 // Test safety postconditions guaranteed by
1025 // `validate_cast_and_convert_metadata`.
1026 assert!(resulting_size <= bytes_len, "{}", debug_str);
1027 match cast_type {
1028 CastType::Prefix => {
1029 assert_eq!(addr % align, 0, "{}", debug_str);
1030 assert_eq!(resulting_size, split_at, "{}", debug_str);
1031 }
1032 CastType::Suffix => {
1033 assert_eq!(split_at, bytes_len - resulting_size, "{}", debug_str);
1034 assert_eq!((addr + split_at) % align, 0, "{}", debug_str);
1035 }
1036 }
1037 } else {
1038 let min_size = match layout.size_info {
1039 SizeInfo::Sized { size } => size,
1040 SizeInfo::SliceDst(TrailingSliceLayout { offset, .. }) => {
1041 offset + util::padding_needed_for(offset, layout.align)
1042 }
1043 };
1044
1045 // If a cast is invalid, it is either because...
1046 // 1. there are insufficent bytes at the given region for type:
1047 let insufficient_bytes = bytes_len < min_size;
1048 // 2. performing the cast would misalign type:
1049 let base = match cast_type {
1050 CastType::Prefix => 0,
1051 CastType::Suffix => bytes_len,
1052 };
1053 let misaligned = (base + addr) % layout.align != 0;
1054
1055 assert!(insufficient_bytes || misaligned);
1056 }
1057 }
1058
1059 let sizes = 0..8;
1060 let elem_sizes = 1..8;
1061 let size_infos = sizes
1062 .clone()
1063 .map(Into::<SizeInfo>::into)
1064 .chain(itertools::iproduct!(sizes, elem_sizes).map(Into::<SizeInfo>::into));
1065 let layouts = itertools::iproduct!(size_infos, [1, 2, 4, 8, 16, 32])
1066 .filter(|(size_info, align)| !matches!(size_info, SizeInfo::Sized { size } if size % align != 0))
1067 .map(|(size_info, align)| layout(size_info, align));
1068 itertools::iproduct!(layouts, 0..8, 0..8, [CastType::Prefix, CastType::Suffix])
1069 .for_each(validate_behavior);
1070 }
1071
1072 #[test]
1073 #[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
1074 fn test_validate_rust_layout() {
1075 use crate::util::testutil::*;
1076 use core::{
1077 convert::TryInto as _,
1078 ptr::{self, NonNull},
1079 };
1080
1081 // This test synthesizes pointers with various metadata and uses Rust's
1082 // built-in APIs to confirm that Rust makes decisions about type layout
1083 // which are consistent with what we believe is guaranteed by the
1084 // language. If this test fails, it doesn't just mean our code is wrong
1085 // - it means we're misunderstanding the language's guarantees.
1086
1087 #[derive(Debug)]
1088 struct MacroArgs {
1089 offset: usize,
1090 align: NonZeroUsize,
1091 elem_size: Option<usize>,
1092 }
1093
1094 /// # Safety
1095 ///
1096 /// `test` promises to only call `addr_of_slice_field` on a `NonNull<T>`
1097 /// which points to a valid `T`.
1098 ///
1099 /// `with_elems` must produce a pointer which points to a valid `T`.
1100 fn test<T: ?Sized, W: Fn(usize) -> NonNull<T>>(
1101 args: MacroArgs,
1102 with_elems: W,
1103 addr_of_slice_field: Option<fn(NonNull<T>) -> NonNull<u8>>,
1104 ) {
1105 let dst = args.elem_size.is_some();
1106 let layout = {
1107 let size_info = match args.elem_size {
1108 Some(elem_size) => {
1109 SizeInfo::SliceDst(TrailingSliceLayout { offset: args.offset, elem_size })
1110 }
1111 None => SizeInfo::Sized {
1112 // Rust only supports types whose sizes are a multiple
1113 // of their alignment. If the macro created a type like
1114 // this:
1115 //
1116 // #[repr(C, align(2))]
1117 // struct Foo([u8; 1]);
1118 //
1119 // ...then Rust will automatically round the type's size
1120 // up to 2.
1121 size: args.offset + util::padding_needed_for(args.offset, args.align),
1122 },
1123 };
1124 DstLayout { size_info, align: args.align }
1125 };
1126
1127 for elems in 0..128 {
1128 let ptr = with_elems(elems);
1129
1130 if let Some(addr_of_slice_field) = addr_of_slice_field {
1131 let slc_field_ptr = addr_of_slice_field(ptr).as_ptr();
1132 // SAFETY: Both `slc_field_ptr` and `ptr` are pointers to
1133 // the same valid Rust object.
1134 #[allow(clippy::incompatible_msrv)]
1135 // Work around https://github.com/rust-lang/rust-clippy/issues/12280
1136 let offset: usize =
1137 unsafe { slc_field_ptr.byte_offset_from(ptr.as_ptr()).try_into().unwrap() };
1138 assert_eq!(offset, args.offset);
1139 }
1140
1141 // SAFETY: `ptr` points to a valid `T`.
1142 let (size, align) = unsafe {
1143 (mem::size_of_val_raw(ptr.as_ptr()), mem::align_of_val_raw(ptr.as_ptr()))
1144 };
1145
1146 // Avoid expensive allocation when running under Miri.
1147 let assert_msg = if !cfg!(miri) {
1148 format!("\n{:?}\nsize:{}, align:{}", args, size, align)
1149 } else {
1150 String::new()
1151 };
1152
1153 let without_padding =
1154 args.offset + args.elem_size.map(|elem_size| elems * elem_size).unwrap_or(0);
1155 assert!(size >= without_padding, "{}", assert_msg);
1156 assert_eq!(align, args.align.get(), "{}", assert_msg);
1157
1158 // This encodes the most important part of the test: our
1159 // understanding of how Rust determines the layout of repr(C)
1160 // types. Sized repr(C) types are trivial, but DST types have
1161 // some subtlety. Note that:
1162 // - For sized types, `without_padding` is just the size of the
1163 // type that we constructed for `Foo`. Since we may have
1164 // requested a larger alignment, `Foo` may actually be larger
1165 // than this, hence `padding_needed_for`.
1166 // - For unsized types, `without_padding` is dynamically
1167 // computed from the offset, the element size, and element
1168 // count. We expect that the size of the object should be
1169 // `offset + elem_size * elems` rounded up to the next
1170 // alignment.
1171 let expected_size =
1172 without_padding + util::padding_needed_for(without_padding, args.align);
1173 assert_eq!(expected_size, size, "{}", assert_msg);
1174
1175 // For zero-sized element types,
1176 // `validate_cast_and_convert_metadata` just panics, so we skip
1177 // testing those types.
1178 if args.elem_size.map(|elem_size| elem_size > 0).unwrap_or(true) {
1179 let addr = ptr.addr().get();
1180 let (got_elems, got_split_at) = layout
1181 .validate_cast_and_convert_metadata(addr, size, CastType::Prefix)
1182 .unwrap();
1183 // Avoid expensive allocation when running under Miri.
1184 let assert_msg = if !cfg!(miri) {
1185 format!(
1186 "{}\nvalidate_cast_and_convert_metadata({}, {})",
1187 assert_msg, addr, size,
1188 )
1189 } else {
1190 String::new()
1191 };
1192 assert_eq!(got_split_at, size, "{}", assert_msg);
1193 if dst {
1194 assert!(got_elems >= elems, "{}", assert_msg);
1195 if got_elems != elems {
1196 // If `validate_cast_and_convert_metadata`
1197 // returned more elements than `elems`, that
1198 // means that `elems` is not the maximum number
1199 // of elements that can fit in `size` - in other
1200 // words, there is enough padding at the end of
1201 // the value to fit at least one more element.
1202 // If we use this metadata to synthesize a
1203 // pointer, despite having a different element
1204 // count, we still expect it to have the same
1205 // size.
1206 let got_ptr = with_elems(got_elems);
1207 // SAFETY: `got_ptr` is a pointer to a valid `T`.
1208 let size_of_got_ptr = unsafe { mem::size_of_val_raw(got_ptr.as_ptr()) };
1209 assert_eq!(size_of_got_ptr, size, "{}", assert_msg);
1210 }
1211 } else {
1212 // For sized casts, the returned element value is
1213 // technically meaningless, and we don't guarantee any
1214 // particular value. In practice, it's always zero.
1215 assert_eq!(got_elems, 0, "{}", assert_msg)
1216 }
1217 }
1218 }
1219 }
1220
1221 macro_rules! validate_against_rust {
1222 ($offset:literal, $align:literal $(, $elem_size:literal)?) => {{
1223 #[repr(C, align($align))]
1224 struct Foo([u8; $offset]$(, [[u8; $elem_size]])?);
1225
1226 let args = MacroArgs {
1227 offset: $offset,
1228 align: $align.try_into().unwrap(),
1229 elem_size: {
1230 #[allow(unused)]
1231 let ret = None::<usize>;
1232 $(let ret = Some($elem_size);)?
1233 ret
1234 }
1235 };
1236
1237 #[repr(C, align($align))]
1238 struct FooAlign;
1239 // Create an aligned buffer to use in order to synthesize
1240 // pointers to `Foo`. We don't ever load values from these
1241 // pointers - we just do arithmetic on them - so having a "real"
1242 // block of memory as opposed to a validly-aligned-but-dangling
1243 // pointer is only necessary to make Miri happy since we run it
1244 // with "strict provenance" checking enabled.
1245 let aligned_buf = Align::<_, FooAlign>::new([0u8; 1024]);
1246 let with_elems = |elems| {
1247 let slc = NonNull::slice_from_raw_parts(NonNull::from(&aligned_buf.t), elems);
1248 #[allow(clippy::as_conversions)]
1249 NonNull::new(slc.as_ptr() as *mut Foo).unwrap()
1250 };
1251 let addr_of_slice_field = {
1252 #[allow(unused)]
1253 let f = None::<fn(NonNull<Foo>) -> NonNull<u8>>;
1254 $(
1255 // SAFETY: `test` promises to only call `f` with a `ptr`
1256 // to a valid `Foo`.
1257 let f: Option<fn(NonNull<Foo>) -> NonNull<u8>> = Some(|ptr: NonNull<Foo>| unsafe {
1258 NonNull::new(ptr::addr_of_mut!((*ptr.as_ptr()).1)).unwrap().cast::<u8>()
1259 });
1260 let _ = $elem_size;
1261 )?
1262 f
1263 };
1264
1265 test::<Foo, _>(args, with_elems, addr_of_slice_field);
1266 }};
1267 }
1268
1269 // Every permutation of:
1270 // - offset in [0, 4]
1271 // - align in [1, 16]
1272 // - elem_size in [0, 4] (plus no elem_size)
1273 validate_against_rust!(0, 1);
1274 validate_against_rust!(0, 1, 0);
1275 validate_against_rust!(0, 1, 1);
1276 validate_against_rust!(0, 1, 2);
1277 validate_against_rust!(0, 1, 3);
1278 validate_against_rust!(0, 1, 4);
1279 validate_against_rust!(0, 2);
1280 validate_against_rust!(0, 2, 0);
1281 validate_against_rust!(0, 2, 1);
1282 validate_against_rust!(0, 2, 2);
1283 validate_against_rust!(0, 2, 3);
1284 validate_against_rust!(0, 2, 4);
1285 validate_against_rust!(0, 4);
1286 validate_against_rust!(0, 4, 0);
1287 validate_against_rust!(0, 4, 1);
1288 validate_against_rust!(0, 4, 2);
1289 validate_against_rust!(0, 4, 3);
1290 validate_against_rust!(0, 4, 4);
1291 validate_against_rust!(0, 8);
1292 validate_against_rust!(0, 8, 0);
1293 validate_against_rust!(0, 8, 1);
1294 validate_against_rust!(0, 8, 2);
1295 validate_against_rust!(0, 8, 3);
1296 validate_against_rust!(0, 8, 4);
1297 validate_against_rust!(0, 16);
1298 validate_against_rust!(0, 16, 0);
1299 validate_against_rust!(0, 16, 1);
1300 validate_against_rust!(0, 16, 2);
1301 validate_against_rust!(0, 16, 3);
1302 validate_against_rust!(0, 16, 4);
1303 validate_against_rust!(1, 1);
1304 validate_against_rust!(1, 1, 0);
1305 validate_against_rust!(1, 1, 1);
1306 validate_against_rust!(1, 1, 2);
1307 validate_against_rust!(1, 1, 3);
1308 validate_against_rust!(1, 1, 4);
1309 validate_against_rust!(1, 2);
1310 validate_against_rust!(1, 2, 0);
1311 validate_against_rust!(1, 2, 1);
1312 validate_against_rust!(1, 2, 2);
1313 validate_against_rust!(1, 2, 3);
1314 validate_against_rust!(1, 2, 4);
1315 validate_against_rust!(1, 4);
1316 validate_against_rust!(1, 4, 0);
1317 validate_against_rust!(1, 4, 1);
1318 validate_against_rust!(1, 4, 2);
1319 validate_against_rust!(1, 4, 3);
1320 validate_against_rust!(1, 4, 4);
1321 validate_against_rust!(1, 8);
1322 validate_against_rust!(1, 8, 0);
1323 validate_against_rust!(1, 8, 1);
1324 validate_against_rust!(1, 8, 2);
1325 validate_against_rust!(1, 8, 3);
1326 validate_against_rust!(1, 8, 4);
1327 validate_against_rust!(1, 16);
1328 validate_against_rust!(1, 16, 0);
1329 validate_against_rust!(1, 16, 1);
1330 validate_against_rust!(1, 16, 2);
1331 validate_against_rust!(1, 16, 3);
1332 validate_against_rust!(1, 16, 4);
1333 validate_against_rust!(2, 1);
1334 validate_against_rust!(2, 1, 0);
1335 validate_against_rust!(2, 1, 1);
1336 validate_against_rust!(2, 1, 2);
1337 validate_against_rust!(2, 1, 3);
1338 validate_against_rust!(2, 1, 4);
1339 validate_against_rust!(2, 2);
1340 validate_against_rust!(2, 2, 0);
1341 validate_against_rust!(2, 2, 1);
1342 validate_against_rust!(2, 2, 2);
1343 validate_against_rust!(2, 2, 3);
1344 validate_against_rust!(2, 2, 4);
1345 validate_against_rust!(2, 4);
1346 validate_against_rust!(2, 4, 0);
1347 validate_against_rust!(2, 4, 1);
1348 validate_against_rust!(2, 4, 2);
1349 validate_against_rust!(2, 4, 3);
1350 validate_against_rust!(2, 4, 4);
1351 validate_against_rust!(2, 8);
1352 validate_against_rust!(2, 8, 0);
1353 validate_against_rust!(2, 8, 1);
1354 validate_against_rust!(2, 8, 2);
1355 validate_against_rust!(2, 8, 3);
1356 validate_against_rust!(2, 8, 4);
1357 validate_against_rust!(2, 16);
1358 validate_against_rust!(2, 16, 0);
1359 validate_against_rust!(2, 16, 1);
1360 validate_against_rust!(2, 16, 2);
1361 validate_against_rust!(2, 16, 3);
1362 validate_against_rust!(2, 16, 4);
1363 validate_against_rust!(3, 1);
1364 validate_against_rust!(3, 1, 0);
1365 validate_against_rust!(3, 1, 1);
1366 validate_against_rust!(3, 1, 2);
1367 validate_against_rust!(3, 1, 3);
1368 validate_against_rust!(3, 1, 4);
1369 validate_against_rust!(3, 2);
1370 validate_against_rust!(3, 2, 0);
1371 validate_against_rust!(3, 2, 1);
1372 validate_against_rust!(3, 2, 2);
1373 validate_against_rust!(3, 2, 3);
1374 validate_against_rust!(3, 2, 4);
1375 validate_against_rust!(3, 4);
1376 validate_against_rust!(3, 4, 0);
1377 validate_against_rust!(3, 4, 1);
1378 validate_against_rust!(3, 4, 2);
1379 validate_against_rust!(3, 4, 3);
1380 validate_against_rust!(3, 4, 4);
1381 validate_against_rust!(3, 8);
1382 validate_against_rust!(3, 8, 0);
1383 validate_against_rust!(3, 8, 1);
1384 validate_against_rust!(3, 8, 2);
1385 validate_against_rust!(3, 8, 3);
1386 validate_against_rust!(3, 8, 4);
1387 validate_against_rust!(3, 16);
1388 validate_against_rust!(3, 16, 0);
1389 validate_against_rust!(3, 16, 1);
1390 validate_against_rust!(3, 16, 2);
1391 validate_against_rust!(3, 16, 3);
1392 validate_against_rust!(3, 16, 4);
1393 validate_against_rust!(4, 1);
1394 validate_against_rust!(4, 1, 0);
1395 validate_against_rust!(4, 1, 1);
1396 validate_against_rust!(4, 1, 2);
1397 validate_against_rust!(4, 1, 3);
1398 validate_against_rust!(4, 1, 4);
1399 validate_against_rust!(4, 2);
1400 validate_against_rust!(4, 2, 0);
1401 validate_against_rust!(4, 2, 1);
1402 validate_against_rust!(4, 2, 2);
1403 validate_against_rust!(4, 2, 3);
1404 validate_against_rust!(4, 2, 4);
1405 validate_against_rust!(4, 4);
1406 validate_against_rust!(4, 4, 0);
1407 validate_against_rust!(4, 4, 1);
1408 validate_against_rust!(4, 4, 2);
1409 validate_against_rust!(4, 4, 3);
1410 validate_against_rust!(4, 4, 4);
1411 validate_against_rust!(4, 8);
1412 validate_against_rust!(4, 8, 0);
1413 validate_against_rust!(4, 8, 1);
1414 validate_against_rust!(4, 8, 2);
1415 validate_against_rust!(4, 8, 3);
1416 validate_against_rust!(4, 8, 4);
1417 validate_against_rust!(4, 16);
1418 validate_against_rust!(4, 16, 0);
1419 validate_against_rust!(4, 16, 1);
1420 validate_against_rust!(4, 16, 2);
1421 validate_against_rust!(4, 16, 3);
1422 validate_against_rust!(4, 16, 4);
1423 }
1424}
1425