1 //! Implements "Stacked Borrows". See <https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md>
2 //! for further information.
4 use std::cell::RefCell;
6 use std::num::NonZeroU64;
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
12 use rustc_middle::mir::RetagKind;
13 use rustc_middle::ty::{self, layout::Size};
14 use rustc_hir::Mutability;
18 pub type PtrId = NonZeroU64;
19 pub type CallId = NonZeroU64;
20 pub type AllocExtra = Stacks;
22 /// Tracking pointer provenance
23 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
29 impl fmt::Debug for Tag {
30 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
32 Tag::Tagged(id) => write!(f, "<{}>", id),
33 Tag::Untagged => write!(f, "<untagged>"),
38 /// Indicates which permission is granted (by this item to some pointers)
39 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
41 /// Grants unique mutable access.
43 /// Grants shared mutable access.
45 /// Grants shared read-only access.
47 /// Grants no access, but separates two groups of SharedReadWrite so they are not
48 /// all considered mutually compatible.
52 /// An item in the per-location borrow stack.
53 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
55 /// The permission this item grants.
57 /// The pointers the permission is granted to.
59 /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
60 protector: Option<CallId>,
63 impl fmt::Debug for Item {
64 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
65 write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
66 if let Some(call) = self.protector {
67 write!(f, " (call {})", call)?;
74 /// Extra per-location state.
75 #[derive(Clone, Debug, PartialEq, Eq)]
77 /// Used *mostly* as a stack; never empty.
79 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
80 /// * Except for `Untagged`, no tag occurs in the stack more than once.
84 /// Extra per-allocation state.
85 #[derive(Clone, Debug)]
87 // Even reading memory can have effects on the stack, so we need a `RefCell` here.
88 stacks: RefCell<RangeMap<Stack>>,
89 // Pointer to global state
93 /// Extra global state, available to the memory access hooks.
95 pub struct GlobalState {
96 /// Next unused pointer ID (tag).
98 /// Table storing the "base" tag for each allocation.
99 /// The base tag is the one used for the initial pointer.
100 /// We need this in a separate table to handle cyclic statics.
101 base_ptr_ids: FxHashMap<AllocId, Tag>,
102 /// Next unused call ID (for protectors).
103 next_call_id: CallId,
104 /// Those call IDs corresponding to functions that are still running.
105 active_calls: FxHashSet<CallId>,
106 /// The id to trace in this execution run
107 tracked_pointer_tag: Option<PtrId>,
109 /// Memory extra state gives us interior mutable access to the global state.
110 pub type MemoryExtra = Rc<RefCell<GlobalState>>;
112 /// Indicates which kind of access is being performed.
113 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
114 pub enum AccessKind {
119 impl fmt::Display for AccessKind {
120 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
122 AccessKind::Read => write!(f, "read access"),
123 AccessKind::Write => write!(f, "write access"),
128 /// Indicates which kind of reference is being created.
129 /// Used by high-level `reborrow` to compute which permissions to grant to the
131 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
133 /// `&mut` and `Box`.
134 Unique { two_phase: bool },
135 /// `&` with or without interior mutability.
137 /// `*mut`/`*const` (raw pointers).
138 Raw { mutable: bool },
141 impl fmt::Display for RefKind {
142 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
144 RefKind::Unique { two_phase: false } => write!(f, "unique"),
145 RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
146 RefKind::Shared => write!(f, "shared"),
147 RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
148 RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
153 /// Utilities for initialization and ID generation
155 pub fn new(tracked_pointer_tag: Option<PtrId>) -> Self {
157 next_ptr_id: NonZeroU64::new(1).unwrap(),
158 base_ptr_ids: FxHashMap::default(),
159 next_call_id: NonZeroU64::new(1).unwrap(),
160 active_calls: FxHashSet::default(),
165 fn new_ptr(&mut self) -> PtrId {
166 let id = self.next_ptr_id;
167 self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
171 pub fn new_call(&mut self) -> CallId {
172 let id = self.next_call_id;
173 trace!("new_call: Assigning ID {}", id);
174 assert!(self.active_calls.insert(id));
175 self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
179 pub fn end_call(&mut self, id: CallId) {
180 assert!(self.active_calls.remove(&id));
183 fn is_active(&self, id: CallId) -> bool {
184 self.active_calls.contains(&id)
187 pub fn global_base_ptr(&mut self, id: AllocId) -> Tag {
188 self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| {
189 let tag = Tag::Tagged(self.new_ptr());
190 trace!("New allocation {:?} has base tag {:?}", id, tag);
191 self.base_ptr_ids.insert(id, tag).unwrap_none();
198 fn err_sb_ub(msg: String) -> InterpError<'static> {
199 err_machine_stop!(TerminationInfo::ExperimentalUb {
201 url: format!("https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md"),
205 // # Stacked Borrows Core Begin
207 /// We need to make at least the following things true:
209 /// U1: After creating a `Uniq`, it is at the top.
210 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
211 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
213 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
214 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
215 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
217 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
218 /// F3: If an access happens with an `&` outside `UnsafeCell`,
219 /// it requires the `SharedReadOnly` to still be in the stack.
221 /// Core relation on `Permission` to define which accesses are allowed
223 /// This defines for a given permission, whether it permits the given kind of access.
224 fn grants(self, access: AccessKind) -> bool {
225 // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
226 self != Permission::Disabled
227 && (access == AccessKind::Read || self != Permission::SharedReadOnly)
231 /// Core per-location operations: access, dealloc, reborrow.
233 /// Find the item granting the given kind of access to the given tag, and return where
234 /// it is on the stack.
235 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
238 .enumerate() // we also need to know *where* in the stack
239 .rev() // search top-to-bottom
240 // Return permission of first item that grants access.
241 // We require a permission with the right tag, ensuring U3 and F3.
244 if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
249 /// Find the first write-incompatible item above the given one --
250 /// i.e, find the height to which the stack will be truncated when writing to `granting`.
251 fn find_first_write_incompatible(&self, granting: usize) -> usize {
252 let perm = self.borrows[granting].perm;
254 Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
255 Permission::Disabled => bug!("Cannot use Disabled for anything"),
256 // On a write, everything above us is incompatible.
257 Permission::Unique => granting + 1,
258 Permission::SharedReadWrite => {
259 // The SharedReadWrite *just* above us are compatible, to skip those.
260 let mut idx = granting + 1;
261 while let Some(item) = self.borrows.get(idx) {
262 if item.perm == Permission::SharedReadWrite {
266 // Found first incompatible!
275 /// Check if the given item is protected.
276 fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
277 if let Tag::Tagged(id) = item.tag {
278 if Some(id) == global.tracked_pointer_tag {
279 register_diagnostic(NonHaltingDiagnostic::PoppedTrackedPointerTag(item.clone()));
282 if let Some(call) = item.protector {
283 if global.is_active(call) {
284 if let Some(tag) = tag {
285 Err(err_sb_ub(format!(
286 "not granting access to tag {:?} because incompatible item is protected: {:?}",
290 Err(err_sb_ub(format!(
291 "deallocating while item is protected: {:?}",
300 /// Test if a memory `access` using pointer tagged `tag` is granted.
301 /// If yes, return the index of the item that granted it.
302 fn access(&mut self, access: AccessKind, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
303 // Two main steps: Find granting item, remove incompatible items above.
305 // Step 1: Find granting item.
306 let granting_idx = self.find_granting(access, tag).ok_or_else(|| {
308 "no item granting {} to tag {:?} found in borrow stack.",
313 // Step 2: Remove incompatible items above them. Make sure we do not remove protected
314 // items. Behavior differs for reads and writes.
315 if access == AccessKind::Write {
316 // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
317 // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
318 let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
319 for item in self.borrows.drain(first_incompatible_idx..).rev() {
320 trace!("access: popping item {:?}", item);
321 Stack::check_protector(&item, Some(tag), global)?;
324 // On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
325 // The reason this is not following the stack discipline (by removing the first Unique and
326 // everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
327 // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
328 // `SharedReadWrite` for `raw`.
329 // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
330 // reference and use that.
331 // We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
332 for idx in ((granting_idx + 1)..self.borrows.len()).rev() {
333 let item = &mut self.borrows[idx];
334 if item.perm == Permission::Unique {
335 trace!("access: disabling item {:?}", item);
336 Stack::check_protector(item, Some(tag), global)?;
337 item.perm = Permission::Disabled;
346 /// Deallocate a location: Like a write access, but also there must be no
347 /// active protectors at all because we will remove all items.
348 fn dealloc(&mut self, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
349 // Step 1: Find granting item.
350 self.find_granting(AccessKind::Write, tag).ok_or_else(|| {
352 "no item granting write access for deallocation to tag {:?} found in borrow stack",
357 // Step 2: Remove all items. Also checks for protectors.
358 for item in self.borrows.drain(..).rev() {
359 Stack::check_protector(&item, None, global)?;
365 /// Derived a new pointer from one with the given tag.
366 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
367 /// an access, and they add the new item directly on top of the one it is derived
368 /// from instead of all the way at the top of the stack.
369 fn grant(&mut self, derived_from: Tag, new: Item, global: &GlobalState) -> InterpResult<'tcx> {
370 // Figure out which access `perm` corresponds to.
372 if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
373 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
374 // We use that to determine where to put the new item.
375 let granting_idx = self.find_granting(access, derived_from)
376 .ok_or_else(|| err_sb_ub(format!(
377 "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack",
378 new.perm, derived_from,
381 // Compute where to put the new item.
382 // Either way, we ensure that we insert the new item in a way such that between
383 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
384 let new_idx = if new.perm == Permission::SharedReadWrite {
386 access == AccessKind::Write,
387 "this case only makes sense for stack-like accesses"
389 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
390 // access. Instead of popping the stack, we insert the item at the place the stack would
391 // be popped to (i.e., we insert it above all the write-compatible items).
392 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
393 self.find_first_write_incompatible(granting_idx)
395 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
396 // Here, creating a reference actually counts as an access.
397 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
398 self.access(access, derived_from, global)?;
400 // We insert "as far up as possible": We know only compatible items are remaining
401 // on top of `derived_from`, and we want the new item at the top so that we
402 // get the strongest possible guarantees.
403 // This ensures U1 and F1.
407 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
408 if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) {
409 // Optimization applies, done.
410 trace!("reborrow: avoiding adding redundant item {:?}", new);
412 trace!("reborrow: adding item {:?}", new);
413 self.borrows.insert(new_idx, new);
419 // # Stacked Borrows Core End
421 /// Map per-stack operations to higher-level per-location-range operations.
423 /// Creates new stack with initial tag.
424 fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self {
425 let item = Item { perm, tag, protector: None };
426 let stack = Stack { borrows: vec![item] };
428 Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra }
431 /// Call `f` on every stack in the range.
436 f: impl Fn(&mut Stack, &GlobalState) -> InterpResult<'tcx>,
437 ) -> InterpResult<'tcx> {
438 let global = self.global.borrow();
439 let mut stacks = self.stacks.borrow_mut();
440 for stack in stacks.iter_mut(ptr.offset, size) {
447 /// Glue code to connect with Miri Machine Hooks
449 pub fn new_allocation(
453 kind: MemoryKind<MiriMemoryKind>,
455 let (tag, perm) = match kind {
456 // New unique borrow. This tag is not accessible by the program,
457 // so it will only ever be used when using the local directly (i.e.,
458 // not through a pointer). That is, whenever we directly write to a local, this will pop
459 // everything else off the stack, invalidating all previous pointers,
460 // and in particular, *all* raw pointers.
461 MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
462 // Global memory can be referenced by global pointers from `tcx`.
463 // Thus we call `global_base_ptr` such that the global pointers get the same tag
464 // as what we use here.
465 // `Machine` is used for extern statics, and thus must also be listed here.
466 // `Env` we list because we can get away with precise tracking there.
467 // The base pointer is not unique, so the base permission is `SharedReadWrite`.
468 MemoryKind::Machine(MiriMemoryKind::Global | MiriMemoryKind::Machine | MiriMemoryKind::Env) =>
469 (extra.borrow_mut().global_base_ptr(id), Permission::SharedReadWrite),
470 // Everything else we handle entirely untagged for now.
471 // FIXME: experiment with more precise tracking.
472 _ => (Tag::Untagged, Permission::SharedReadWrite),
474 (Stacks::new(size, perm, tag, extra), tag)
478 pub fn memory_read<'tcx>(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
479 trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
480 self.for_each(ptr, size, |stack, global| {
481 stack.access(AccessKind::Read, ptr.tag, global)?;
487 pub fn memory_written<'tcx>(&mut self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
488 trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
489 self.for_each(ptr, size, |stack, global| {
490 stack.access(AccessKind::Write, ptr.tag, global)?;
496 pub fn memory_deallocated<'tcx>(
500 ) -> InterpResult<'tcx> {
501 trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
502 self.for_each(ptr, size, |stack, global| stack.dealloc(ptr.tag, global))
506 /// Retagging/reborrowing. There is some policy in here, such as which permissions
507 /// to grant for which references, and when to add protectors.
508 impl<'mir, 'tcx> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
509 trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
512 place: MPlaceTy<'tcx, Tag>,
517 ) -> InterpResult<'tcx> {
518 let this = self.eval_context_mut();
519 let protector = if protect { Some(this.frame().extra.call_id) } else { None };
520 let ptr = place.ptr.assert_ptr();
522 "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
531 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
532 let extra = &this.memory.get_raw(ptr.alloc_id)?.extra;
533 let stacked_borrows =
534 extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data");
535 // Update the stacks.
536 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
537 // There could be existing unique pointers reborrowed from them that should remain valid!
538 let perm = match kind {
539 RefKind::Unique { two_phase: false } => Permission::Unique,
540 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
541 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
542 RefKind::Shared | RefKind::Raw { mutable: false } => {
543 // Shared references and *const are a whole different kind of game, the
544 // permission is not uniform across the entire range!
545 // We need a frozen-sensitive reborrow.
546 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
547 // We are only ever `SharedReadOnly` inside the frozen bits.
548 let perm = if frozen {
549 Permission::SharedReadOnly
551 Permission::SharedReadWrite
553 let item = Item { perm, tag: new_tag, protector };
554 stacked_borrows.for_each(cur_ptr, size, |stack, global| {
555 stack.grant(cur_ptr.tag, item, global)
560 let item = Item { perm, tag: new_tag, protector };
561 stacked_borrows.for_each(ptr, size, |stack, global| stack.grant(ptr.tag, item, global))
564 /// Retags an indidual pointer, returning the retagged version.
565 /// `mutbl` can be `None` to make this a raw pointer.
568 val: ImmTy<'tcx, Tag>,
571 ) -> InterpResult<'tcx, Immediate<Tag>> {
572 let this = self.eval_context_mut();
573 // We want a place for where the ptr *points to*, so we get one.
574 let place = this.ref_to_mplace(val)?;
576 .size_and_align_of_mplace(place)?
577 .map(|(size, _)| size)
578 .unwrap_or_else(|| place.layout.size);
579 // We can see dangling ptrs in here e.g. after a Box's `Unique` was
580 // updated using "self.0 = ..." (can happen in Box::from_raw); see miri#1050.
581 let place = this.mplace_access_checked(place)?;
582 if size == Size::ZERO {
583 // Nothing to do for ZSTs.
587 // Compute new borrow.
588 let new_tag = match kind {
589 // Give up tracking for raw pointers.
590 // FIXME: Experiment with more precise tracking. Blocked on `&raw`
591 // because `Rc::into_raw` currently creates intermediate references,
592 // breaking `Rc::from_raw`.
593 RefKind::Raw { .. } => Tag::Untagged,
594 // All other pointesr are properly tracked.
596 this.memory.extra.stacked_borrows.as_ref().unwrap().borrow_mut().new_ptr(),
601 this.reborrow(place, size, kind, new_tag, protect)?;
602 let new_place = place.replace_tag(new_tag);
604 // Return new pointer.
605 Ok(new_place.to_ref())
609 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
610 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
611 fn retag(&mut self, kind: RetagKind, place: PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> {
612 let this = self.eval_context_mut();
613 // Determine mutability and whether to add a protector.
614 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
615 // making it useless.
616 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
618 // References are simple.
619 ty::Ref(_, _, Mutability::Mut) => Some((
620 RefKind::Unique { two_phase: kind == RetagKind::TwoPhase },
621 kind == RetagKind::FnEntry,
623 ty::Ref(_, _, Mutability::Not) =>
624 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
625 // Raw pointers need to be enabled.
626 ty::RawPtr(tym) if kind == RetagKind::Raw =>
627 Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
628 // Boxes do not get a protector: protectors reflect that references outlive the call
629 // they were passed in to; that's just not the case for boxes.
630 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
635 // We only reborrow "bare" references/boxes.
636 // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
637 // but might also cost us optimization and analyses. We will have to experiment more with this.
638 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
640 let val = this.read_immediate(this.place_to_op(place)?)?;
641 let val = this.retag_reference(val, mutbl, protector)?;
642 this.write_immediate(val, place)?;