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};
8 use std::num::NonZeroU64;
11 use rustc_hir::Mutability;
12 use rustc::mir::RetagKind;
13 use rustc::ty::{self, layout::Size};
14 use rustc_mir::interpret::InterpError;
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: HashMap<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: HashSet<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: HashMap::default(),
159 next_call_id: NonZeroU64::new(1).unwrap(),
160 active_calls: HashSet::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 static_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();
197 // # Stacked Borrows Core Begin
199 /// We need to make at least the following things true:
201 /// U1: After creating a `Uniq`, it is at the top.
202 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
203 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
205 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
206 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
207 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
209 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
210 /// F3: If an access happens with an `&` outside `UnsafeCell`,
211 /// it requires the `SharedReadOnly` to still be in the stack.
213 /// Core relation on `Permission` to define which accesses are allowed
215 /// This defines for a given permission, whether it permits the given kind of access.
216 fn grants(self, access: AccessKind) -> bool {
217 // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
218 self != Permission::Disabled
219 && (access == AccessKind::Read || self != Permission::SharedReadOnly)
223 /// Core per-location operations: access, dealloc, reborrow.
225 /// Find the item granting the given kind of access to the given tag, and return where
226 /// it is on the stack.
227 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
230 .enumerate() // we also need to know *where* in the stack
231 .rev() // search top-to-bottom
232 // Return permission of first item that grants access.
233 // We require a permission with the right tag, ensuring U3 and F3.
236 if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
241 /// Find the first write-incompatible item above the given one --
242 /// i.e, find the height to which the stack will be truncated when writing to `granting`.
243 fn find_first_write_incompatible(&self, granting: usize) -> usize {
244 let perm = self.borrows[granting].perm;
246 Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
247 Permission::Disabled => bug!("Cannot use Disabled for anything"),
248 // On a write, everything above us is incompatible.
249 Permission::Unique => granting + 1,
250 Permission::SharedReadWrite => {
251 // The SharedReadWrite *just* above us are compatible, to skip those.
252 let mut idx = granting + 1;
253 while let Some(item) = self.borrows.get(idx) {
254 if item.perm == Permission::SharedReadWrite {
258 // Found first incompatible!
267 /// Check if the given item is protected.
268 fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
269 if let Tag::Tagged(id) = item.tag {
270 if Some(id) == global.tracked_pointer_tag {
272 InterpError::MachineStop(Box::new(TerminationInfo::PoppedTrackedPointerTag(
279 if let Some(call) = item.protector {
280 if global.is_active(call) {
281 if let Some(tag) = tag {
282 return Err(err_ub_experimental(
285 "not granting access to tag {:?} because incompatible item is protected: {:?}",
290 throw_ub!(UbExperimental(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(|| {
309 format!("no item granting {} to tag {:?} found in borrow stack.", access, tag),
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(|| {
353 "no item granting write access for deallocation to tag {:?} found in borrow stack",
358 // Step 2: Remove all items. Also checks for protectors.
359 for item in self.borrows.drain(..).rev() {
360 Stack::check_protector(&item, None, global)?;
366 /// Derived a new pointer from one with the given tag.
367 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
368 /// an access, and they add the new item directly on top of the one it is derived
369 /// from instead of all the way at the top of the stack.
370 fn grant(&mut self, derived_from: Tag, new: Item, global: &GlobalState) -> InterpResult<'tcx> {
371 // Figure out which access `perm` corresponds to.
373 if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
374 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
375 // We use that to determine where to put the new item.
376 let granting_idx = self.find_granting(access, derived_from)
381 "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack",
382 new.perm, derived_from,
386 // Compute where to put the new item.
387 // Either way, we ensure that we insert the new item in a way such that between
388 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
389 let new_idx = if new.perm == Permission::SharedReadWrite {
391 access == AccessKind::Write,
392 "this case only makes sense for stack-like accesses"
394 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
395 // access. Instead of popping the stack, we insert the item at the place the stack would
396 // be popped to (i.e., we insert it above all the write-compatible items).
397 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
398 self.find_first_write_incompatible(granting_idx)
400 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
401 // Here, creating a reference actually counts as an access.
402 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
403 self.access(access, derived_from, global)?;
405 // We insert "as far up as possible": We know only compatible items are remaining
406 // on top of `derived_from`, and we want the new item at the top so that we
407 // get the strongest possible guarantees.
408 // This ensures U1 and F1.
412 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
413 if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) {
414 // Optimization applies, done.
415 trace!("reborrow: avoiding adding redundant item {:?}", new);
417 trace!("reborrow: adding item {:?}", new);
418 self.borrows.insert(new_idx, new);
424 // # Stacked Borrows Core End
426 /// Map per-stack operations to higher-level per-location-range operations.
428 /// Creates new stack with initial tag.
429 fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self {
430 let item = Item { perm, tag, protector: None };
431 let stack = Stack { borrows: vec![item] };
433 Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra }
436 /// Call `f` on every stack in the range.
441 f: impl Fn(&mut Stack, &GlobalState) -> InterpResult<'tcx>,
442 ) -> InterpResult<'tcx> {
443 let global = self.global.borrow();
444 let mut stacks = self.stacks.borrow_mut();
445 for stack in stacks.iter_mut(ptr.offset, size) {
452 /// Glue code to connect with Miri Machine Hooks
454 pub fn new_allocation(
458 kind: MemoryKind<MiriMemoryKind>,
460 let (tag, perm) = match kind {
461 // New unique borrow. This tag is not accessible by the program,
462 // so it will only ever be used when using the local directly (i.e.,
463 // not through a pointer). That is, whenever we directly write to a local, this will pop
464 // everything else off the stack, invalidating all previous pointers,
465 // and in particular, *all* raw pointers.
466 MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
467 // Static memory can be referenced by "global" pointers from `tcx`.
468 // Thus we call `static_base_ptr` such that the global pointers get the same tag
469 // as what we use here.
470 // The base pointer is not unique, so the base permission is `SharedReadWrite`.
471 MemoryKind::Machine(MiriMemoryKind::Static) =>
472 (extra.borrow_mut().static_base_ptr(id), Permission::SharedReadWrite),
473 // Everything else we handle entirely untagged for now.
474 // FIXME: experiment with more precise tracking.
475 _ => (Tag::Untagged, Permission::SharedReadWrite),
477 (Stacks::new(size, perm, tag, extra), tag)
481 pub fn memory_read<'tcx>(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
482 trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
483 self.for_each(ptr, size, |stack, global| {
484 stack.access(AccessKind::Read, ptr.tag, global)?;
490 pub fn memory_written<'tcx>(&mut self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
491 trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
492 self.for_each(ptr, size, |stack, global| {
493 stack.access(AccessKind::Write, ptr.tag, global)?;
499 pub fn memory_deallocated<'tcx>(
503 ) -> InterpResult<'tcx> {
504 trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
505 self.for_each(ptr, size, |stack, global| stack.dealloc(ptr.tag, global))
509 /// Retagging/reborrowing. There is some policy in here, such as which permissions
510 /// to grant for which references, and when to add protectors.
511 impl<'mir, 'tcx> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
512 trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
515 place: MPlaceTy<'tcx, Tag>,
520 ) -> InterpResult<'tcx> {
521 let this = self.eval_context_mut();
522 let protector = if protect { Some(this.frame().extra.call_id) } else { None };
523 let ptr = place.ptr.assert_ptr();
525 "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
534 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
535 let extra = &this.memory.get_raw(ptr.alloc_id)?.extra;
536 let stacked_borrows =
537 extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data");
538 // Update the stacks.
539 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
540 // There could be existing unique pointers reborrowed from them that should remain valid!
541 let perm = match kind {
542 RefKind::Unique { two_phase: false } => Permission::Unique,
543 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
544 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
545 RefKind::Shared | RefKind::Raw { mutable: false } => {
546 // Shared references and *const are a whole different kind of game, the
547 // permission is not uniform across the entire range!
548 // We need a frozen-sensitive reborrow.
549 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
550 // We are only ever `SharedReadOnly` inside the frozen bits.
551 let perm = if frozen {
552 Permission::SharedReadOnly
554 Permission::SharedReadWrite
556 let item = Item { perm, tag: new_tag, protector };
557 stacked_borrows.for_each(cur_ptr, size, |stack, global| {
558 stack.grant(cur_ptr.tag, item, global)
563 let item = Item { perm, tag: new_tag, protector };
564 stacked_borrows.for_each(ptr, size, |stack, global| stack.grant(ptr.tag, item, global))
567 /// Retags an indidual pointer, returning the retagged version.
568 /// `mutbl` can be `None` to make this a raw pointer.
571 val: ImmTy<'tcx, Tag>,
574 ) -> InterpResult<'tcx, Immediate<Tag>> {
575 let this = self.eval_context_mut();
576 // We want a place for where the ptr *points to*, so we get one.
577 let place = this.ref_to_mplace(val)?;
579 .size_and_align_of_mplace(place)?
580 .map(|(size, _)| size)
581 .unwrap_or_else(|| place.layout.size);
582 // We can see dangling ptrs in here e.g. after a Box's `Unique` was
583 // updated using "self.0 = ..." (can happen in Box::from_raw); see miri#1050.
584 let place = this.mplace_access_checked(place)?;
585 if size == Size::ZERO {
586 // Nothing to do for ZSTs.
590 // Compute new borrow.
591 let new_tag = match kind {
592 // Give up tracking for raw pointers.
593 // FIXME: Experiment with more precise tracking. Blocked on `&raw`
594 // because `Rc::into_raw` currently creates intermediate references,
595 // breaking `Rc::from_raw`.
596 RefKind::Raw { .. } => Tag::Untagged,
597 // All other pointesr are properly tracked.
598 _ => Tag::Tagged(this.memory.extra.stacked_borrows.borrow_mut().new_ptr()),
602 this.reborrow(place, size, kind, new_tag, protect)?;
603 let new_place = place.replace_tag(new_tag);
605 // Return new pointer.
606 Ok(new_place.to_ref())
610 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
611 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
612 fn retag(&mut self, kind: RetagKind, place: PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> {
613 let this = self.eval_context_mut();
614 // Determine mutability and whether to add a protector.
615 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
616 // making it useless.
617 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
619 // References are simple.
620 ty::Ref(_, _, Mutability::Mut) => Some((
621 RefKind::Unique { two_phase: kind == RetagKind::TwoPhase },
622 kind == RetagKind::FnEntry,
624 ty::Ref(_, _, Mutability::Not) =>
625 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
626 // Raw pointers need to be enabled.
627 ty::RawPtr(tym) if kind == RetagKind::Raw =>
628 Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
629 // Boxes do not get a protector: protectors reflect that references outlive the call
630 // they were passed in to; that's just not the case for boxes.
631 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
636 // We only reborrow "bare" references/boxes.
637 // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
638 // but might also cost us optimization and analyses. We will have to experiment more with this.
639 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
641 let val = this.read_immediate(this.place_to_op(place)?)?;
642 let val = this.retag_reference(val, mutbl, protector)?;
643 this.write_immediate(val, place)?;
650 fn err_ub_experimental(tag: Tag, mut msg: String) -> InterpErrorInfo<'static> {
651 if let Tag::Tagged(id) = tag {
652 // FIXME: do not add this message when the flag is already set
653 write!(msg, " Rerun with `-Zmiri-track-pointer-tag={}` for more information", id).unwrap();
655 err_ub!(UbExperimental(msg)).into()