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};
16 pub type PtrId = NonZeroU64;
17 pub type CallId = NonZeroU64;
18 pub type AllocExtra = Stacks;
20 /// Tracking pointer provenance
21 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
27 impl fmt::Debug for Tag {
28 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
30 Tag::Tagged(id) => write!(f, "<{}>", id),
31 Tag::Untagged => write!(f, "<untagged>"),
36 /// Indicates which permission is granted (by this item to some pointers)
37 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
39 /// Grants unique mutable access.
41 /// Grants shared mutable access.
43 /// Grants shared read-only access.
45 /// Grants no access, but separates two groups of SharedReadWrite so they are not
46 /// all considered mutually compatible.
50 /// An item in the per-location borrow stack.
51 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
53 /// The permission this item grants.
55 /// The pointers the permission is granted to.
57 /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
58 protector: Option<CallId>,
61 impl fmt::Debug for Item {
62 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
63 write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
64 if let Some(call) = self.protector {
65 write!(f, " (call {})", call)?;
72 /// Extra per-location state.
73 #[derive(Clone, Debug, PartialEq, Eq)]
75 /// Used *mostly* as a stack; never empty.
77 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
78 /// * Except for `Untagged`, no tag occurs in the stack more than once.
82 /// Extra per-allocation state.
83 #[derive(Clone, Debug)]
85 // Even reading memory can have effects on the stack, so we need a `RefCell` here.
86 stacks: RefCell<RangeMap<Stack>>,
87 // Pointer to global state
91 /// Extra global state, available to the memory access hooks.
93 pub struct GlobalState {
94 /// Next unused pointer ID (tag).
96 /// Table storing the "base" tag for each allocation.
97 /// The base tag is the one used for the initial pointer.
98 /// We need this in a separate table to handle cyclic statics.
99 base_ptr_ids: HashMap<AllocId, Tag>,
100 /// Next unused call ID (for protectors).
101 next_call_id: CallId,
102 /// Those call IDs corresponding to functions that are still running.
103 active_calls: HashSet<CallId>,
104 /// The id to trace in this execution run
105 tracked_pointer_tag: Option<PtrId>,
107 /// Memory extra state gives us interior mutable access to the global state.
108 pub type MemoryExtra = Rc<RefCell<GlobalState>>;
110 /// Indicates which kind of access is being performed.
111 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
112 pub enum AccessKind {
117 impl fmt::Display for AccessKind {
118 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
120 AccessKind::Read => write!(f, "read access"),
121 AccessKind::Write => write!(f, "write access"),
126 /// Indicates which kind of reference is being created.
127 /// Used by high-level `reborrow` to compute which permissions to grant to the
129 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
131 /// `&mut` and `Box`.
132 Unique { two_phase: bool },
133 /// `&` with or without interior mutability.
135 /// `*mut`/`*const` (raw pointers).
136 Raw { mutable: bool },
139 impl fmt::Display for RefKind {
140 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
142 RefKind::Unique { two_phase: false } => write!(f, "unique"),
143 RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
144 RefKind::Shared => write!(f, "shared"),
145 RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
146 RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
151 /// Utilities for initialization and ID generation
153 pub fn new(tracked_pointer_tag: Option<PtrId>) -> Self {
155 next_ptr_id: NonZeroU64::new(1).unwrap(),
156 base_ptr_ids: HashMap::default(),
157 next_call_id: NonZeroU64::new(1).unwrap(),
158 active_calls: HashSet::default(),
163 fn new_ptr(&mut self) -> PtrId {
164 let id = self.next_ptr_id;
165 self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
169 pub fn new_call(&mut self) -> CallId {
170 let id = self.next_call_id;
171 trace!("new_call: Assigning ID {}", id);
172 assert!(self.active_calls.insert(id));
173 self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
177 pub fn end_call(&mut self, id: CallId) {
178 assert!(self.active_calls.remove(&id));
181 fn is_active(&self, id: CallId) -> bool {
182 self.active_calls.contains(&id)
185 pub fn static_base_ptr(&mut self, id: AllocId) -> Tag {
186 self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| {
187 let tag = Tag::Tagged(self.new_ptr());
188 trace!("New allocation {:?} has base tag {:?}", id, tag);
189 self.base_ptr_ids.insert(id, tag).unwrap_none();
195 // # Stacked Borrows Core Begin
197 /// We need to make at least the following things true:
199 /// U1: After creating a `Uniq`, it is at the top.
200 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
201 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
203 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
204 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
205 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
207 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
208 /// F3: If an access happens with an `&` outside `UnsafeCell`,
209 /// it requires the `SharedReadOnly` to still be in the stack.
211 /// Core relation on `Permission` to define which accesses are allowed
213 /// This defines for a given permission, whether it permits the given kind of access.
214 fn grants(self, access: AccessKind) -> bool {
215 // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
216 self != Permission::Disabled
217 && (access == AccessKind::Read || self != Permission::SharedReadOnly)
221 /// Core per-location operations: access, dealloc, reborrow.
223 /// Find the item granting the given kind of access to the given tag, and return where
224 /// it is on the stack.
225 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
228 .enumerate() // we also need to know *where* in the stack
229 .rev() // search top-to-bottom
230 // Return permission of first item that grants access.
231 // We require a permission with the right tag, ensuring U3 and F3.
234 if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
239 /// Find the first write-incompatible item above the given one --
240 /// i.e, find the height to which the stack will be truncated when writing to `granting`.
241 fn find_first_write_incompatible(&self, granting: usize) -> usize {
242 let perm = self.borrows[granting].perm;
244 Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
245 Permission::Disabled => bug!("Cannot use Disabled for anything"),
246 // On a write, everything above us is incompatible.
247 Permission::Unique => granting + 1,
248 Permission::SharedReadWrite => {
249 // The SharedReadWrite *just* above us are compatible, to skip those.
250 let mut idx = granting + 1;
251 while let Some(item) = self.borrows.get(idx) {
252 if item.perm == Permission::SharedReadWrite {
256 // Found first incompatible!
265 /// Check if the given item is protected.
266 fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
267 if let Tag::Tagged(id) = item.tag {
268 if Some(id) == global.tracked_pointer_tag {
269 register_diagnostic(NonHaltingDiagnostic::PoppedTrackedPointerTag(item.clone()));
272 if let Some(call) = item.protector {
273 if global.is_active(call) {
274 if let Some(tag) = tag {
275 throw_ub!(UbExperimental(format!(
276 "not granting access to tag {:?} because incompatible item is protected: {:?}",
280 throw_ub!(UbExperimental(format!(
281 "deallocating while item is protected: {:?}",
290 /// Test if a memory `access` using pointer tagged `tag` is granted.
291 /// If yes, return the index of the item that granted it.
292 fn access(&mut self, access: AccessKind, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
293 // Two main steps: Find granting item, remove incompatible items above.
295 // Step 1: Find granting item.
296 let granting_idx = self.find_granting(access, tag).ok_or_else(|| err_ub!(UbExperimental(
297 format!("no item granting {} to tag {:?} found in borrow stack.", access, tag),
300 // Step 2: Remove incompatible items above them. Make sure we do not remove protected
301 // items. Behavior differs for reads and writes.
302 if access == AccessKind::Write {
303 // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
304 // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
305 let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
306 for item in self.borrows.drain(first_incompatible_idx..).rev() {
307 trace!("access: popping item {:?}", item);
308 Stack::check_protector(&item, Some(tag), global)?;
311 // On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
312 // The reason this is not following the stack discipline (by removing the first Unique and
313 // everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
314 // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
315 // `SharedReadWrite` for `raw`.
316 // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
317 // reference and use that.
318 // We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
319 for idx in ((granting_idx + 1)..self.borrows.len()).rev() {
320 let item = &mut self.borrows[idx];
321 if item.perm == Permission::Unique {
322 trace!("access: disabling item {:?}", item);
323 Stack::check_protector(item, Some(tag), global)?;
324 item.perm = Permission::Disabled;
333 /// Deallocate a location: Like a write access, but also there must be no
334 /// active protectors at all because we will remove all items.
335 fn dealloc(&mut self, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
336 // Step 1: Find granting item.
337 self.find_granting(AccessKind::Write, tag).ok_or_else(|| err_ub!(UbExperimental(format!(
338 "no item granting write access for deallocation to tag {:?} found in borrow stack",
342 // Step 2: Remove all items. Also checks for protectors.
343 for item in self.borrows.drain(..).rev() {
344 Stack::check_protector(&item, None, global)?;
350 /// Derived a new pointer from one with the given tag.
351 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
352 /// an access, and they add the new item directly on top of the one it is derived
353 /// from instead of all the way at the top of the stack.
354 fn grant(&mut self, derived_from: Tag, new: Item, global: &GlobalState) -> InterpResult<'tcx> {
355 // Figure out which access `perm` corresponds to.
357 if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
358 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
359 // We use that to determine where to put the new item.
360 let granting_idx = self.find_granting(access, derived_from)
361 .ok_or_else(|| err_ub!(UbExperimental(format!(
362 "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack",
363 new.perm, derived_from,
366 // Compute where to put the new item.
367 // Either way, we ensure that we insert the new item in a way such that between
368 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
369 let new_idx = if new.perm == Permission::SharedReadWrite {
371 access == AccessKind::Write,
372 "this case only makes sense for stack-like accesses"
374 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
375 // access. Instead of popping the stack, we insert the item at the place the stack would
376 // be popped to (i.e., we insert it above all the write-compatible items).
377 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
378 self.find_first_write_incompatible(granting_idx)
380 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
381 // Here, creating a reference actually counts as an access.
382 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
383 self.access(access, derived_from, global)?;
385 // We insert "as far up as possible": We know only compatible items are remaining
386 // on top of `derived_from`, and we want the new item at the top so that we
387 // get the strongest possible guarantees.
388 // This ensures U1 and F1.
392 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
393 if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) {
394 // Optimization applies, done.
395 trace!("reborrow: avoiding adding redundant item {:?}", new);
397 trace!("reborrow: adding item {:?}", new);
398 self.borrows.insert(new_idx, new);
404 // # Stacked Borrows Core End
406 /// Map per-stack operations to higher-level per-location-range operations.
408 /// Creates new stack with initial tag.
409 fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self {
410 let item = Item { perm, tag, protector: None };
411 let stack = Stack { borrows: vec![item] };
413 Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra }
416 /// Call `f` on every stack in the range.
421 f: impl Fn(&mut Stack, &GlobalState) -> InterpResult<'tcx>,
422 ) -> InterpResult<'tcx> {
423 let global = self.global.borrow();
424 let mut stacks = self.stacks.borrow_mut();
425 for stack in stacks.iter_mut(ptr.offset, size) {
432 /// Glue code to connect with Miri Machine Hooks
434 pub fn new_allocation(
438 kind: MemoryKind<MiriMemoryKind>,
440 let (tag, perm) = match kind {
441 // New unique borrow. This tag is not accessible by the program,
442 // so it will only ever be used when using the local directly (i.e.,
443 // not through a pointer). That is, whenever we directly write to a local, this will pop
444 // everything else off the stack, invalidating all previous pointers,
445 // and in particular, *all* raw pointers.
446 MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
447 // Static memory can be referenced by "global" pointers from `tcx`.
448 // Thus we call `static_base_ptr` such that the global pointers get the same tag
449 // as what we use here.
450 // The base pointer is not unique, so the base permission is `SharedReadWrite`.
451 MemoryKind::Machine(MiriMemoryKind::Static) =>
452 (extra.borrow_mut().static_base_ptr(id), Permission::SharedReadWrite),
453 // Everything else we handle entirely untagged for now.
454 // FIXME: experiment with more precise tracking.
455 _ => (Tag::Untagged, Permission::SharedReadWrite),
457 (Stacks::new(size, perm, tag, extra), tag)
461 pub fn memory_read<'tcx>(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
462 trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
463 self.for_each(ptr, size, |stack, global| {
464 stack.access(AccessKind::Read, ptr.tag, global)?;
470 pub fn memory_written<'tcx>(&mut self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
471 trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
472 self.for_each(ptr, size, |stack, global| {
473 stack.access(AccessKind::Write, ptr.tag, global)?;
479 pub fn memory_deallocated<'tcx>(
483 ) -> InterpResult<'tcx> {
484 trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
485 self.for_each(ptr, size, |stack, global| stack.dealloc(ptr.tag, global))
489 /// Retagging/reborrowing. There is some policy in here, such as which permissions
490 /// to grant for which references, and when to add protectors.
491 impl<'mir, 'tcx> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
492 trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
495 place: MPlaceTy<'tcx, Tag>,
500 ) -> InterpResult<'tcx> {
501 let this = self.eval_context_mut();
502 let protector = if protect { Some(this.frame().extra.call_id) } else { None };
503 let ptr = place.ptr.assert_ptr();
505 "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
514 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
515 let extra = &this.memory.get_raw(ptr.alloc_id)?.extra;
516 let stacked_borrows =
517 extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data");
518 // Update the stacks.
519 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
520 // There could be existing unique pointers reborrowed from them that should remain valid!
521 let perm = match kind {
522 RefKind::Unique { two_phase: false } => Permission::Unique,
523 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
524 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
525 RefKind::Shared | RefKind::Raw { mutable: false } => {
526 // Shared references and *const are a whole different kind of game, the
527 // permission is not uniform across the entire range!
528 // We need a frozen-sensitive reborrow.
529 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
530 // We are only ever `SharedReadOnly` inside the frozen bits.
531 let perm = if frozen {
532 Permission::SharedReadOnly
534 Permission::SharedReadWrite
536 let item = Item { perm, tag: new_tag, protector };
537 stacked_borrows.for_each(cur_ptr, size, |stack, global| {
538 stack.grant(cur_ptr.tag, item, global)
543 let item = Item { perm, tag: new_tag, protector };
544 stacked_borrows.for_each(ptr, size, |stack, global| stack.grant(ptr.tag, item, global))
547 /// Retags an indidual pointer, returning the retagged version.
548 /// `mutbl` can be `None` to make this a raw pointer.
551 val: ImmTy<'tcx, Tag>,
554 ) -> InterpResult<'tcx, Immediate<Tag>> {
555 let this = self.eval_context_mut();
556 // We want a place for where the ptr *points to*, so we get one.
557 let place = this.ref_to_mplace(val)?;
559 .size_and_align_of_mplace(place)?
560 .map(|(size, _)| size)
561 .unwrap_or_else(|| place.layout.size);
562 // We can see dangling ptrs in here e.g. after a Box's `Unique` was
563 // updated using "self.0 = ..." (can happen in Box::from_raw); see miri#1050.
564 let place = this.mplace_access_checked(place)?;
565 if size == Size::ZERO {
566 // Nothing to do for ZSTs.
570 // Compute new borrow.
571 let new_tag = match kind {
572 // Give up tracking for raw pointers.
573 // FIXME: Experiment with more precise tracking. Blocked on `&raw`
574 // because `Rc::into_raw` currently creates intermediate references,
575 // breaking `Rc::from_raw`.
576 RefKind::Raw { .. } => Tag::Untagged,
577 // All other pointesr are properly tracked.
578 _ => Tag::Tagged(this.memory.extra.stacked_borrows.borrow_mut().new_ptr()),
582 this.reborrow(place, size, kind, new_tag, protect)?;
583 let new_place = place.replace_tag(new_tag);
585 // Return new pointer.
586 Ok(new_place.to_ref())
590 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
591 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
592 fn retag(&mut self, kind: RetagKind, place: PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> {
593 let this = self.eval_context_mut();
594 // Determine mutability and whether to add a protector.
595 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
596 // making it useless.
597 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
599 // References are simple.
600 ty::Ref(_, _, Mutability::Mut) => Some((
601 RefKind::Unique { two_phase: kind == RetagKind::TwoPhase },
602 kind == RetagKind::FnEntry,
604 ty::Ref(_, _, Mutability::Not) =>
605 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
606 // Raw pointers need to be enabled.
607 ty::RawPtr(tym) if kind == RetagKind::Raw =>
608 Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
609 // Boxes do not get a protector: protectors reflect that references outlive the call
610 // they were passed in to; that's just not the case for boxes.
611 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
616 // We only reborrow "bare" references/boxes.
617 // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
618 // but might also cost us optimization and analyses. We will have to experiment more with this.
619 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
621 let val = this.read_immediate(this.place_to_op(place)?)?;
622 let val = this.retag_reference(val, mutbl, protector)?;
623 this.write_immediate(val, place)?;