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;
5 use std::collections::{HashMap, HashSet};
7 use std::num::NonZeroU64;
10 use rustc_hir::Mutability;
11 use rustc::mir::RetagKind;
12 use rustc::ty::{self, layout::Size};
13 use rustc_mir::interpret::InterpError;
17 pub type PtrId = NonZeroU64;
18 pub type CallId = NonZeroU64;
19 pub type AllocExtra = Stacks;
21 /// Tracking pointer provenance
22 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
28 impl fmt::Debug for Tag {
29 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
31 Tag::Tagged(id) => write!(f, "<{}>", id),
32 Tag::Untagged => write!(f, "<untagged>"),
37 /// Indicates which permission is granted (by this item to some pointers)
38 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
40 /// Grants unique mutable access.
42 /// Grants shared mutable access.
44 /// Grants shared read-only access.
46 /// Grants no access, but separates two groups of SharedReadWrite so they are not
47 /// all considered mutually compatible.
51 /// An item in the per-location borrow stack.
52 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
54 /// The permission this item grants.
56 /// The pointers the permission is granted to.
58 /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
59 protector: Option<CallId>,
62 impl fmt::Debug for Item {
63 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
64 write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
65 if let Some(call) = self.protector {
66 write!(f, " (call {})", call)?;
73 /// Extra per-location state.
74 #[derive(Clone, Debug, PartialEq, Eq)]
76 /// Used *mostly* as a stack; never empty.
78 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
79 /// * Except for `Untagged`, no tag occurs in the stack more than once.
83 /// Extra per-allocation state.
84 #[derive(Clone, Debug)]
86 // Even reading memory can have effects on the stack, so we need a `RefCell` here.
87 stacks: RefCell<RangeMap<Stack>>,
88 // Pointer to global state
92 /// Extra global state, available to the memory access hooks.
94 pub struct GlobalState {
95 /// Next unused pointer ID (tag).
97 /// Table storing the "base" tag for each allocation.
98 /// The base tag is the one used for the initial pointer.
99 /// We need this in a separate table to handle cyclic statics.
100 base_ptr_ids: HashMap<AllocId, Tag>,
101 /// Next unused call ID (for protectors).
102 next_call_id: CallId,
103 /// Those call IDs corresponding to functions that are still running.
104 active_calls: HashSet<CallId>,
105 /// The id to trace in this execution run
106 tracked_pointer_tag: Option<PtrId>,
108 /// Memory extra state gives us interior mutable access to the global state.
109 pub type MemoryExtra = Rc<RefCell<GlobalState>>;
111 /// Indicates which kind of access is being performed.
112 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
113 pub enum AccessKind {
118 impl fmt::Display for AccessKind {
119 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
121 AccessKind::Read => write!(f, "read access"),
122 AccessKind::Write => write!(f, "write access"),
127 /// Indicates which kind of reference is being created.
128 /// Used by high-level `reborrow` to compute which permissions to grant to the
130 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
132 /// `&mut` and `Box`.
133 Unique { two_phase: bool },
134 /// `&` with or without interior mutability.
136 /// `*mut`/`*const` (raw pointers).
137 Raw { mutable: bool },
140 impl fmt::Display for RefKind {
141 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
143 RefKind::Unique { two_phase: false } => write!(f, "unique"),
144 RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
145 RefKind::Shared => write!(f, "shared"),
146 RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
147 RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
152 /// Utilities for initialization and ID generation
154 pub fn new(tracked_pointer_tag: Option<PtrId>) -> Self {
156 next_ptr_id: NonZeroU64::new(1).unwrap(),
157 base_ptr_ids: HashMap::default(),
158 next_call_id: NonZeroU64::new(1).unwrap(),
159 active_calls: HashSet::default(),
164 fn new_ptr(&mut self) -> PtrId {
165 let id = self.next_ptr_id;
166 self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
170 pub fn new_call(&mut self) -> CallId {
171 let id = self.next_call_id;
172 trace!("new_call: Assigning ID {}", id);
173 assert!(self.active_calls.insert(id));
174 self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
178 pub fn end_call(&mut self, id: CallId) {
179 assert!(self.active_calls.remove(&id));
182 fn is_active(&self, id: CallId) -> bool {
183 self.active_calls.contains(&id)
186 pub fn static_base_ptr(&mut self, id: AllocId) -> Tag {
187 self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| {
188 let tag = Tag::Tagged(self.new_ptr());
189 trace!("New allocation {:?} has base tag {:?}", id, tag);
190 self.base_ptr_ids.insert(id, tag).unwrap_none();
196 // # Stacked Borrows Core Begin
198 /// We need to make at least the following things true:
200 /// U1: After creating a `Uniq`, it is at the top.
201 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
202 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
204 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
205 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
206 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
208 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
209 /// F3: If an access happens with an `&` outside `UnsafeCell`,
210 /// it requires the `SharedReadOnly` to still be in the stack.
212 /// Core relation on `Permission` to define which accesses are allowed
214 /// This defines for a given permission, whether it permits the given kind of access.
215 fn grants(self, access: AccessKind) -> bool {
216 // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
217 self != Permission::Disabled
218 && (access == AccessKind::Read || self != Permission::SharedReadOnly)
222 /// Core per-location operations: access, dealloc, reborrow.
224 /// Find the item granting the given kind of access to the given tag, and return where
225 /// it is on the stack.
226 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
229 .enumerate() // we also need to know *where* in the stack
230 .rev() // search top-to-bottom
231 // Return permission of first item that grants access.
232 // We require a permission with the right tag, ensuring U3 and F3.
235 if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
240 /// Find the first write-incompatible item above the given one --
241 /// i.e, find the height to which the stack will be truncated when writing to `granting`.
242 fn find_first_write_incompatible(&self, granting: usize) -> usize {
243 let perm = self.borrows[granting].perm;
245 Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
246 Permission::Disabled => bug!("Cannot use Disabled for anything"),
247 // On a write, everything above us is incompatible.
248 Permission::Unique => granting + 1,
249 Permission::SharedReadWrite => {
250 // The SharedReadWrite *just* above us are compatible, to skip those.
251 let mut idx = granting + 1;
252 while let Some(item) = self.borrows.get(idx) {
253 if item.perm == Permission::SharedReadWrite {
257 // Found first incompatible!
266 /// Check if the given item is protected.
267 fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
268 if let Tag::Tagged(id) = item.tag {
269 if Some(id) == global.tracked_pointer_tag {
271 InterpError::MachineStop(Box::new(TerminationInfo::PoppedTrackedPointerTag(
278 if let Some(call) = item.protector {
279 if global.is_active(call) {
280 if let Some(tag) = tag {
281 throw_ub!(UbExperimental(format!(
282 "not granting access to tag {:?} because incompatible item is protected: {:?}",
286 throw_ub!(UbExperimental(format!(
287 "deallocating while item is protected: {:?}",
296 /// Test if a memory `access` using pointer tagged `tag` is granted.
297 /// If yes, return the index of the item that granted it.
298 fn access(&mut self, access: AccessKind, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
299 // Two main steps: Find granting item, remove incompatible items above.
301 // Step 1: Find granting item.
302 let granting_idx = self.find_granting(access, tag).ok_or_else(|| err_ub!(UbExperimental(
303 format!("no item granting {} to tag {:?} found in borrow stack.", access, tag),
306 // Step 2: Remove incompatible items above them. Make sure we do not remove protected
307 // items. Behavior differs for reads and writes.
308 if access == AccessKind::Write {
309 // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
310 // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
311 let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
312 for item in self.borrows.drain(first_incompatible_idx..).rev() {
313 trace!("access: popping item {:?}", item);
314 Stack::check_protector(&item, Some(tag), global)?;
317 // On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
318 // The reason this is not following the stack discipline (by removing the first Unique and
319 // everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
320 // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
321 // `SharedReadWrite` for `raw`.
322 // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
323 // reference and use that.
324 // We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
325 for idx in ((granting_idx + 1)..self.borrows.len()).rev() {
326 let item = &mut self.borrows[idx];
327 if item.perm == Permission::Unique {
328 trace!("access: disabling item {:?}", item);
329 Stack::check_protector(item, Some(tag), global)?;
330 item.perm = Permission::Disabled;
339 /// Deallocate a location: Like a write access, but also there must be no
340 /// active protectors at all because we will remove all items.
341 fn dealloc(&mut self, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
342 // Step 1: Find granting item.
343 self.find_granting(AccessKind::Write, tag).ok_or_else(|| err_ub!(UbExperimental(format!(
344 "no item granting write access for deallocation to tag {:?} found in borrow stack",
348 // Step 2: Remove all items. Also checks for protectors.
349 for item in self.borrows.drain(..).rev() {
350 Stack::check_protector(&item, None, global)?;
356 /// Derived a new pointer from one with the given tag.
357 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
358 /// an access, and they add the new item directly on top of the one it is derived
359 /// from instead of all the way at the top of the stack.
360 fn grant(&mut self, derived_from: Tag, new: Item, global: &GlobalState) -> InterpResult<'tcx> {
361 // Figure out which access `perm` corresponds to.
363 if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
364 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
365 // We use that to determine where to put the new item.
366 let granting_idx = self.find_granting(access, derived_from)
367 .ok_or_else(|| err_ub!(UbExperimental(format!(
368 "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack",
369 new.perm, derived_from,
372 // Compute where to put the new item.
373 // Either way, we ensure that we insert the new item in a way such that between
374 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
375 let new_idx = if new.perm == Permission::SharedReadWrite {
377 access == AccessKind::Write,
378 "this case only makes sense for stack-like accesses"
380 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
381 // access. Instead of popping the stack, we insert the item at the place the stack would
382 // be popped to (i.e., we insert it above all the write-compatible items).
383 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
384 self.find_first_write_incompatible(granting_idx)
386 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
387 // Here, creating a reference actually counts as an access.
388 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
389 self.access(access, derived_from, global)?;
391 // We insert "as far up as possible": We know only compatible items are remaining
392 // on top of `derived_from`, and we want the new item at the top so that we
393 // get the strongest possible guarantees.
394 // This ensures U1 and F1.
398 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
399 if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) {
400 // Optimization applies, done.
401 trace!("reborrow: avoiding adding redundant item {:?}", new);
403 trace!("reborrow: adding item {:?}", new);
404 self.borrows.insert(new_idx, new);
410 // # Stacked Borrows Core End
412 /// Map per-stack operations to higher-level per-location-range operations.
414 /// Creates new stack with initial tag.
415 fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self {
416 let item = Item { perm, tag, protector: None };
417 let stack = Stack { borrows: vec![item] };
419 Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra }
422 /// Call `f` on every stack in the range.
427 f: impl Fn(&mut Stack, &GlobalState) -> InterpResult<'tcx>,
428 ) -> InterpResult<'tcx> {
429 let global = self.global.borrow();
430 let mut stacks = self.stacks.borrow_mut();
431 for stack in stacks.iter_mut(ptr.offset, size) {
438 /// Glue code to connect with Miri Machine Hooks
440 pub fn new_allocation(
444 kind: MemoryKind<MiriMemoryKind>,
446 let (tag, perm) = match kind {
447 // New unique borrow. This tag is not accessible by the program,
448 // so it will only ever be used when using the local directly (i.e.,
449 // not through a pointer). That is, whenever we directly write to a local, this will pop
450 // everything else off the stack, invalidating all previous pointers,
451 // and in particular, *all* raw pointers.
452 MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
453 // Static memory can be referenced by "global" pointers from `tcx`.
454 // Thus we call `static_base_ptr` such that the global pointers get the same tag
455 // as what we use here.
456 // The base pointer is not unique, so the base permission is `SharedReadWrite`.
457 MemoryKind::Machine(MiriMemoryKind::Static) =>
458 (extra.borrow_mut().static_base_ptr(id), Permission::SharedReadWrite),
459 // Everything else we handle entirely untagged for now.
460 // FIXME: experiment with more precise tracking.
461 _ => (Tag::Untagged, Permission::SharedReadWrite),
463 (Stacks::new(size, perm, tag, extra), tag)
467 pub fn memory_read<'tcx>(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
468 trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
469 self.for_each(ptr, size, |stack, global| {
470 stack.access(AccessKind::Read, ptr.tag, global)?;
476 pub fn memory_written<'tcx>(&mut self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
477 trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
478 self.for_each(ptr, size, |stack, global| {
479 stack.access(AccessKind::Write, ptr.tag, global)?;
485 pub fn memory_deallocated<'tcx>(
489 ) -> InterpResult<'tcx> {
490 trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
491 self.for_each(ptr, size, |stack, global| stack.dealloc(ptr.tag, global))
495 /// Retagging/reborrowing. There is some policy in here, such as which permissions
496 /// to grant for which references, and when to add protectors.
497 impl<'mir, 'tcx> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
498 trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
501 place: MPlaceTy<'tcx, Tag>,
506 ) -> InterpResult<'tcx> {
507 let this = self.eval_context_mut();
508 let protector = if protect { Some(this.frame().extra.call_id) } else { None };
509 let ptr = place.ptr.assert_ptr();
511 "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
520 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
521 let extra = &this.memory.get_raw(ptr.alloc_id)?.extra;
522 let stacked_borrows =
523 extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data");
524 // Update the stacks.
525 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
526 // There could be existing unique pointers reborrowed from them that should remain valid!
527 let perm = match kind {
528 RefKind::Unique { two_phase: false } => Permission::Unique,
529 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
530 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
531 RefKind::Shared | RefKind::Raw { mutable: false } => {
532 // Shared references and *const are a whole different kind of game, the
533 // permission is not uniform across the entire range!
534 // We need a frozen-sensitive reborrow.
535 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
536 // We are only ever `SharedReadOnly` inside the frozen bits.
537 let perm = if frozen {
538 Permission::SharedReadOnly
540 Permission::SharedReadWrite
542 let item = Item { perm, tag: new_tag, protector };
543 stacked_borrows.for_each(cur_ptr, size, |stack, global| {
544 stack.grant(cur_ptr.tag, item, global)
549 let item = Item { perm, tag: new_tag, protector };
550 stacked_borrows.for_each(ptr, size, |stack, global| stack.grant(ptr.tag, item, global))
553 /// Retags an indidual pointer, returning the retagged version.
554 /// `mutbl` can be `None` to make this a raw pointer.
557 val: ImmTy<'tcx, Tag>,
560 ) -> InterpResult<'tcx, Immediate<Tag>> {
561 let this = self.eval_context_mut();
562 // We want a place for where the ptr *points to*, so we get one.
563 let place = this.ref_to_mplace(val)?;
565 .size_and_align_of_mplace(place)?
566 .map(|(size, _)| size)
567 .unwrap_or_else(|| place.layout.size);
568 // We can see dangling ptrs in here e.g. after a Box's `Unique` was
569 // updated using "self.0 = ..." (can happen in Box::from_raw); see miri#1050.
570 let place = this.mplace_access_checked(place)?;
571 if size == Size::ZERO {
572 // Nothing to do for ZSTs.
576 // Compute new borrow.
577 let new_tag = match kind {
578 // Give up tracking for raw pointers.
579 // FIXME: Experiment with more precise tracking. Blocked on `&raw`
580 // because `Rc::into_raw` currently creates intermediate references,
581 // breaking `Rc::from_raw`.
582 RefKind::Raw { .. } => Tag::Untagged,
583 // All other pointesr are properly tracked.
584 _ => Tag::Tagged(this.memory.extra.stacked_borrows.borrow_mut().new_ptr()),
588 this.reborrow(place, size, kind, new_tag, protect)?;
589 let new_place = place.replace_tag(new_tag);
591 // Return new pointer.
592 Ok(new_place.to_ref())
596 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
597 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
598 fn retag(&mut self, kind: RetagKind, place: PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> {
599 let this = self.eval_context_mut();
600 // Determine mutability and whether to add a protector.
601 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
602 // making it useless.
603 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
605 // References are simple.
606 ty::Ref(_, _, Mutability::Mut) => Some((
607 RefKind::Unique { two_phase: kind == RetagKind::TwoPhase },
608 kind == RetagKind::FnEntry,
610 ty::Ref(_, _, Mutability::Not) =>
611 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
612 // Raw pointers need to be enabled.
613 ty::RawPtr(tym) if kind == RetagKind::Raw =>
614 Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
615 // Boxes do not get a protector: protectors reflect that references outlive the call
616 // they were passed in to; that's just not the case for boxes.
617 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
622 // We only reborrow "bare" references/boxes.
623 // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
624 // but might also cost us optimization and analyses. We will have to experiment more with this.
625 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
627 let val = this.read_immediate(this.place_to_op(place)?)?;
628 let val = this.retag_reference(val, mutbl, protector)?;
629 this.write_immediate(val, place)?;