//! Implements "Stacked Borrows". See //! for further information. use std::cell::RefCell; use std::fmt; use std::num::NonZeroU64; use std::rc::Rc; use log::trace; use rustc_data_structures::fx::{FxHashMap, FxHashSet}; use rustc_middle::mir::RetagKind; use rustc_middle::ty; use rustc_target::abi::{Align, LayoutOf, Size}; use rustc_hir::Mutability; use crate::*; pub type PtrId = NonZeroU64; pub type CallId = NonZeroU64; pub type AllocExtra = Stacks; /// Tracking pointer provenance #[derive(Copy, Clone, Hash, PartialEq, Eq)] pub enum Tag { Tagged(PtrId), Untagged, } impl fmt::Debug for Tag { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { Tag::Tagged(id) => write!(f, "<{}>", id), Tag::Untagged => write!(f, ""), } } } /// Indicates which permission is granted (by this item to some pointers) #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)] pub enum Permission { /// Grants unique mutable access. Unique, /// Grants shared mutable access. SharedReadWrite, /// Grants shared read-only access. SharedReadOnly, /// Grants no access, but separates two groups of SharedReadWrite so they are not /// all considered mutually compatible. Disabled, } /// An item in the per-location borrow stack. #[derive(Copy, Clone, Hash, PartialEq, Eq)] pub struct Item { /// The permission this item grants. perm: Permission, /// The pointers the permission is granted to. tag: Tag, /// An optional protector, ensuring the item cannot get popped until `CallId` is over. protector: Option, } impl fmt::Debug for Item { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "[{:?} for {:?}", self.perm, self.tag)?; if let Some(call) = self.protector { write!(f, " (call {})", call)?; } write!(f, "]")?; Ok(()) } } /// Extra per-location state. #[derive(Clone, Debug, PartialEq, Eq)] pub struct Stack { /// Used *mostly* as a stack; never empty. /// Invariants: /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`. /// * Except for `Untagged`, no tag occurs in the stack more than once. borrows: Vec, } /// Extra per-allocation state. #[derive(Clone, Debug)] pub struct Stacks { // Even reading memory can have effects on the stack, so we need a `RefCell` here. stacks: RefCell>, // Pointer to global state global: MemoryExtra, } /// Extra global state, available to the memory access hooks. #[derive(Debug)] pub struct GlobalState { /// Next unused pointer ID (tag). next_ptr_id: PtrId, /// Table storing the "base" tag for each allocation. /// The base tag is the one used for the initial pointer. /// We need this in a separate table to handle cyclic statics. base_ptr_ids: FxHashMap, /// Next unused call ID (for protectors). next_call_id: CallId, /// Those call IDs corresponding to functions that are still running. active_calls: FxHashSet, /// The pointer id to trace tracked_pointer_tag: Option, /// The call id to trace tracked_call_id: Option, /// Whether to track raw pointers. track_raw: bool, } /// Memory extra state gives us interior mutable access to the global state. pub type MemoryExtra = Rc>; /// Indicates which kind of access is being performed. #[derive(Copy, Clone, Hash, PartialEq, Eq)] pub enum AccessKind { Read, Write, } impl fmt::Display for AccessKind { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { AccessKind::Read => write!(f, "read access"), AccessKind::Write => write!(f, "write access"), } } } /// Indicates which kind of reference is being created. /// Used by high-level `reborrow` to compute which permissions to grant to the /// new pointer. #[derive(Copy, Clone, Hash, PartialEq, Eq)] pub enum RefKind { /// `&mut` and `Box`. Unique { two_phase: bool }, /// `&` with or without interior mutability. Shared, /// `*mut`/`*const` (raw pointers). Raw { mutable: bool }, } impl fmt::Display for RefKind { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { RefKind::Unique { two_phase: false } => write!(f, "unique"), RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"), RefKind::Shared => write!(f, "shared"), RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"), RefKind::Raw { mutable: false } => write!(f, "raw (constant)"), } } } /// Utilities for initialization and ID generation impl GlobalState { pub fn new(tracked_pointer_tag: Option, tracked_call_id: Option, track_raw: bool) -> Self { GlobalState { next_ptr_id: NonZeroU64::new(1).unwrap(), base_ptr_ids: FxHashMap::default(), next_call_id: NonZeroU64::new(1).unwrap(), active_calls: FxHashSet::default(), tracked_pointer_tag, tracked_call_id, track_raw, } } fn new_ptr(&mut self) -> PtrId { let id = self.next_ptr_id; if Some(id) == self.tracked_pointer_tag { register_diagnostic(NonHaltingDiagnostic::CreatedPointerTag(id)); } self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap(); id } pub fn new_call(&mut self) -> CallId { let id = self.next_call_id; trace!("new_call: Assigning ID {}", id); if Some(id) == self.tracked_call_id { register_diagnostic(NonHaltingDiagnostic::CreatedCallId(id)); } assert!(self.active_calls.insert(id)); self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap(); id } pub fn end_call(&mut self, id: CallId) { assert!(self.active_calls.remove(&id)); } fn is_active(&self, id: CallId) -> bool { self.active_calls.contains(&id) } pub fn global_base_ptr(&mut self, id: AllocId) -> Tag { self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| { let tag = Tag::Tagged(self.new_ptr()); trace!("New allocation {:?} has base tag {:?}", id, tag); self.base_ptr_ids.insert(id, tag).unwrap_none(); tag }) } } /// Error reporting fn err_sb_ub(msg: String) -> InterpError<'static> { err_machine_stop!(TerminationInfo::ExperimentalUb { msg, url: format!("https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md"), }) } // # Stacked Borrows Core Begin /// We need to make at least the following things true: /// /// U1: After creating a `Uniq`, it is at the top. /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it. /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack. /// /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top. /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces: /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly` /// gets popped. /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`. /// F3: If an access happens with an `&` outside `UnsafeCell`, /// it requires the `SharedReadOnly` to still be in the stack. /// Core relation on `Permission` to define which accesses are allowed impl Permission { /// This defines for a given permission, whether it permits the given kind of access. fn grants(self, access: AccessKind) -> bool { // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access. self != Permission::Disabled && (access == AccessKind::Read || self != Permission::SharedReadOnly) } } /// Core per-location operations: access, dealloc, reborrow. impl<'tcx> Stack { /// Find the item granting the given kind of access to the given tag, and return where /// it is on the stack. fn find_granting(&self, access: AccessKind, tag: Tag) -> Option { self.borrows .iter() .enumerate() // we also need to know *where* in the stack .rev() // search top-to-bottom // Return permission of first item that grants access. // We require a permission with the right tag, ensuring U3 and F3. .find_map( |(idx, item)| { if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None } }, ) } /// Find the first write-incompatible item above the given one -- /// i.e, find the height to which the stack will be truncated when writing to `granting`. fn find_first_write_incompatible(&self, granting: usize) -> usize { let perm = self.borrows[granting].perm; match perm { Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"), Permission::Disabled => bug!("Cannot use Disabled for anything"), // On a write, everything above us is incompatible. Permission::Unique => granting + 1, Permission::SharedReadWrite => { // The SharedReadWrite *just* above us are compatible, to skip those. let mut idx = granting + 1; while let Some(item) = self.borrows.get(idx) { if item.perm == Permission::SharedReadWrite { // Go on. idx += 1; } else { // Found first incompatible! break; } } idx } } } /// Check if the given item is protected. fn check_protector(item: &Item, tag: Option, global: &GlobalState) -> InterpResult<'tcx> { if let Tag::Tagged(id) = item.tag { if Some(id) == global.tracked_pointer_tag { register_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(item.clone())); } } if let Some(call) = item.protector { if global.is_active(call) { if let Some(tag) = tag { Err(err_sb_ub(format!( "not granting access to tag {:?} because incompatible item is protected: {:?}", tag, item )))? } else { Err(err_sb_ub(format!( "deallocating while item is protected: {:?}", item )))? } } } Ok(()) } /// Test if a memory `access` using pointer tagged `tag` is granted. /// If yes, return the index of the item that granted it. fn access(&mut self, access: AccessKind, ptr: Pointer, global: &GlobalState) -> InterpResult<'tcx> { // Two main steps: Find granting item, remove incompatible items above. // Step 1: Find granting item. let granting_idx = self.find_granting(access, ptr.tag).ok_or_else(|| { err_sb_ub(format!( "no item granting {} to tag {:?} at {} found in borrow stack.", access, ptr.tag, ptr.erase_tag(), )) })?; // Step 2: Remove incompatible items above them. Make sure we do not remove protected // items. Behavior differs for reads and writes. if access == AccessKind::Write { // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses). let first_incompatible_idx = self.find_first_write_incompatible(granting_idx); for item in self.borrows.drain(first_incompatible_idx..).rev() { trace!("access: popping item {:?}", item); Stack::check_protector(&item, Some(ptr.tag), global)?; } } else { // On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses. // The reason this is not following the stack discipline (by removing the first Unique and // everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the // `SharedReadWrite` for `raw`. // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared // reference and use that. // We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs. for idx in ((granting_idx + 1)..self.borrows.len()).rev() { let item = &mut self.borrows[idx]; if item.perm == Permission::Unique { trace!("access: disabling item {:?}", item); Stack::check_protector(item, Some(ptr.tag), global)?; item.perm = Permission::Disabled; } } } // Done. Ok(()) } /// Deallocate a location: Like a write access, but also there must be no /// active protectors at all because we will remove all items. fn dealloc(&mut self, ptr: Pointer, global: &GlobalState) -> InterpResult<'tcx> { // Step 1: Find granting item. self.find_granting(AccessKind::Write, ptr.tag).ok_or_else(|| { err_sb_ub(format!( "no item granting write access for deallocation to tag {:?} at {} found in borrow stack", ptr.tag, ptr.erase_tag(), )) })?; // Step 2: Remove all items. Also checks for protectors. for item in self.borrows.drain(..).rev() { Stack::check_protector(&item, None, global)?; } Ok(()) } /// Derive a new pointer from one with the given tag. /// `weak` controls whether this operation is weak or strong: weak granting does not act as /// an access, and they add the new item directly on top of the one it is derived /// from instead of all the way at the top of the stack. fn grant(&mut self, derived_from: Pointer, new: Item, global: &GlobalState) -> InterpResult<'tcx> { // Figure out which access `perm` corresponds to. let access = if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read }; // Now we figure out which item grants our parent (`derived_from`) this kind of access. // We use that to determine where to put the new item. let granting_idx = self.find_granting(access, derived_from.tag) .ok_or_else(|| err_sb_ub(format!( "trying to reborrow for {:?} at {}, but parent tag {:?} does not have an appropriate item in the borrow stack", new.perm, derived_from.erase_tag(), derived_from.tag, )))?; // Compute where to put the new item. // Either way, we ensure that we insert the new item in a way such that between // `derived_from` and the new one, there are only items *compatible with* `derived_from`. let new_idx = if new.perm == Permission::SharedReadWrite { assert!( access == AccessKind::Write, "this case only makes sense for stack-like accesses" ); // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write // access. Instead of popping the stack, we insert the item at the place the stack would // be popped to (i.e., we insert it above all the write-compatible items). // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`. self.find_first_write_incompatible(granting_idx) } else { // A "safe" reborrow for a pointer that actually expects some aliasing guarantees. // Here, creating a reference actually counts as an access. // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`. self.access(access, derived_from, global)?; // We insert "as far up as possible": We know only compatible items are remaining // on top of `derived_from`, and we want the new item at the top so that we // get the strongest possible guarantees. // This ensures U1 and F1. self.borrows.len() }; // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors. if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) { // Optimization applies, done. trace!("reborrow: avoiding adding redundant item {:?}", new); } else { trace!("reborrow: adding item {:?}", new); self.borrows.insert(new_idx, new); } Ok(()) } } // # Stacked Borrows Core End /// Map per-stack operations to higher-level per-location-range operations. impl<'tcx> Stacks { /// Creates new stack with initial tag. fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self { let item = Item { perm, tag, protector: None }; let stack = Stack { borrows: vec![item] }; Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra } } /// Call `f` on every stack in the range. fn for_each( &self, ptr: Pointer, size: Size, f: impl Fn(Pointer, &mut Stack, &GlobalState) -> InterpResult<'tcx>, ) -> InterpResult<'tcx> { let global = self.global.borrow(); let mut stacks = self.stacks.borrow_mut(); for (offset, stack) in stacks.iter_mut(ptr.offset, size) { let mut cur_ptr = ptr; cur_ptr.offset = offset; f(cur_ptr, stack, &*global)?; } Ok(()) } } /// Glue code to connect with Miri Machine Hooks impl Stacks { pub fn new_allocation( id: AllocId, size: Size, extra: MemoryExtra, kind: MemoryKind, ) -> (Self, Tag) { let (tag, perm) = match kind { // New unique borrow. This tag is not accessible by the program, // so it will only ever be used when using the local directly (i.e., // not through a pointer). That is, whenever we directly write to a local, this will pop // everything else off the stack, invalidating all previous pointers, // and in particular, *all* raw pointers. MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique), // `Global` memory can be referenced by global pointers from `tcx`. // Thus we call `global_base_ptr` such that the global pointers get the same tag // as what we use here. // `ExternStatic` is used for extern statics, and thus must also be listed here. // `Env` we list because we can get away with precise tracking there. // The base pointer is not unique, so the base permission is `SharedReadWrite`. MemoryKind::Machine(MiriMemoryKind::Global | MiriMemoryKind::ExternStatic | MiriMemoryKind::Tls | MiriMemoryKind::Env) => (extra.borrow_mut().global_base_ptr(id), Permission::SharedReadWrite), // Everything else we handle like raw pointers for now. _ => { let mut extra = extra.borrow_mut(); let tag = if extra.track_raw { Tag::Tagged(extra.new_ptr()) } else { Tag::Untagged }; (tag, Permission::SharedReadWrite) } }; (Stacks::new(size, perm, tag, extra), tag) } #[inline(always)] pub fn memory_read<'tcx>(&self, ptr: Pointer, size: Size) -> InterpResult<'tcx> { trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes()); self.for_each(ptr, size, |ptr, stack, global| stack.access(AccessKind::Read, ptr, global)) } #[inline(always)] pub fn memory_written<'tcx>(&mut self, ptr: Pointer, size: Size) -> InterpResult<'tcx> { trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes()); self.for_each(ptr, size, |ptr, stack, global| stack.access(AccessKind::Write, ptr, global)) } #[inline(always)] pub fn memory_deallocated<'tcx>( &mut self, ptr: Pointer, size: Size, ) -> InterpResult<'tcx> { trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes()); self.for_each(ptr, size, |ptr, stack, global| stack.dealloc(ptr, global)) } } /// Retagging/reborrowing. There is some policy in here, such as which permissions /// to grant for which references, and when to add protectors. impl<'mir, 'tcx: 'mir> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {} trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> { fn reborrow( &mut self, place: MPlaceTy<'tcx, Tag>, size: Size, kind: RefKind, new_tag: Tag, protect: bool, ) -> InterpResult<'tcx> { let this = self.eval_context_mut(); let protector = if protect { Some(this.frame().extra.call_id) } else { None }; let ptr = place.ptr.assert_ptr(); trace!( "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}", kind, new_tag, ptr.tag, place.layout.ty, ptr.erase_tag(), size.bytes() ); // Get the allocation. It might not be mutable, so we cannot use `get_mut`. let extra = &this.memory.get_raw(ptr.alloc_id)?.extra; let stacked_borrows = extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data"); // Update the stacks. // Make sure that raw pointers and mutable shared references are reborrowed "weak": // There could be existing unique pointers reborrowed from them that should remain valid! let perm = match kind { RefKind::Unique { two_phase: false } => Permission::Unique, RefKind::Unique { two_phase: true } => Permission::SharedReadWrite, RefKind::Raw { mutable: true } => Permission::SharedReadWrite, RefKind::Shared | RefKind::Raw { mutable: false } => { // Shared references and *const are a whole different kind of game, the // permission is not uniform across the entire range! // We need a frozen-sensitive reborrow. return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| { // We are only ever `SharedReadOnly` inside the frozen bits. let perm = if frozen { Permission::SharedReadOnly } else { Permission::SharedReadWrite }; let item = Item { perm, tag: new_tag, protector }; stacked_borrows.for_each(cur_ptr, size, |cur_ptr, stack, global| { stack.grant(cur_ptr, item, global) }) }); } }; let item = Item { perm, tag: new_tag, protector }; stacked_borrows.for_each(ptr, size, |ptr, stack, global| stack.grant(ptr, item, global)) } /// Retags an indidual pointer, returning the retagged version. /// `mutbl` can be `None` to make this a raw pointer. fn retag_reference( &mut self, val: ImmTy<'tcx, Tag>, kind: RefKind, protect: bool, ) -> InterpResult<'tcx, ImmTy<'tcx, Tag>> { let this = self.eval_context_mut(); // We want a place for where the ptr *points to*, so we get one. let place = this.ref_to_mplace(val)?; let size = this .size_and_align_of_mplace(place)? .map(|(size, _)| size) .unwrap_or_else(|| place.layout.size); // `reborrow` relies on getting a `Pointer` and everything being in-bounds, // so let's ensure that. However, we do not care about alignment. // We can see dangling ptrs in here e.g. after a Box's `Unique` was // updated using "self.0 = ..." (can happen in Box::from_raw) so we cannot ICE; see miri#1050. let place = this.mplace_access_checked(place, Some(Align::from_bytes(1).unwrap()))?; // Nothing to do for ZSTs. if size == Size::ZERO { return Ok(val); } // Compute new borrow. let new_tag = { let mut mem_extra = this.memory.extra.stacked_borrows.as_ref().unwrap().borrow_mut(); match kind { // Give up tracking for raw pointers. RefKind::Raw { .. } if !mem_extra.track_raw => Tag::Untagged, // All other pointers are properly tracked. _ => Tag::Tagged(mem_extra.new_ptr()), } }; // Reborrow. this.reborrow(place, size, kind, new_tag, protect)?; let new_place = place.replace_tag(new_tag); // Return new pointer. Ok(ImmTy::from_immediate(new_place.to_ref(), val.layout)) } } impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {} pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> { fn retag(&mut self, kind: RetagKind, place: PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> { let this = self.eval_context_mut(); // Determine mutability and whether to add a protector. // Cannot use `builtin_deref` because that reports *immutable* for `Box`, // making it useless. fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> { match ty.kind() { // References are simple. ty::Ref(_, _, Mutability::Mut) => Some(( RefKind::Unique { two_phase: kind == RetagKind::TwoPhase }, kind == RetagKind::FnEntry, )), ty::Ref(_, _, Mutability::Not) => Some((RefKind::Shared, kind == RetagKind::FnEntry)), // Raw pointers need to be enabled. ty::RawPtr(tym) if kind == RetagKind::Raw => Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)), // Boxes do not get a protector: protectors reflect that references outlive the call // they were passed in to; that's just not the case for boxes. ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)), _ => None, } } // We only reborrow "bare" references/boxes. // Not traversing into fields helps with , // but might also cost us optimization and analyses. We will have to experiment more with this. if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) { // Fast path. let val = this.read_immediate(this.place_to_op(place)?)?; let val = this.retag_reference(val, mutbl, protector)?; this.write_immediate(*val, place)?; } Ok(()) } /// After a stack frame got pushed, retag the return place so that we are sure /// it does not alias with anything. /// /// This is a HACK because there is nothing in MIR that would make the retag /// explicit. Also see https://github.com/rust-lang/rust/issues/71117. fn retag_return_place(&mut self) -> InterpResult<'tcx> { let this = self.eval_context_mut(); let return_place = if let Some(return_place) = this.frame_mut().return_place { return_place } else { // No return place, nothing to do. return Ok(()); }; if return_place.layout.is_zst() { // There may not be any memory here, nothing to do. return Ok(()); } // We need this to be in-memory to use tagged pointers. let return_place = this.force_allocation(return_place)?; // We have to turn the place into a pointer to use the existing code. // (The pointer type does not matter, so we use a raw pointer.) let ptr_layout = this.layout_of(this.tcx.mk_mut_ptr(return_place.layout.ty))?; let val = ImmTy::from_immediate(return_place.to_ref(), ptr_layout); // Reborrow it. let val = this.retag_reference(val, RefKind::Unique { two_phase: false }, /*protector*/ true)?; // And use reborrowed pointer for return place. let return_place = this.ref_to_mplace(val)?; this.frame_mut().return_place = Some(return_place.into()); Ok(()) } }