miri/borrow_tracker/

mod.rs

use std::cell::RefCell;
use std::fmt;
use std::num::NonZero;

use rustc_abi::Size;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_middle::mir::RetagKind;
use smallvec::SmallVec;

use crate::*;
pub mod stacked_borrows;
pub mod tree_borrows;

/// Tracking pointer provenance
#[derive(Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
pub struct BorTag(NonZero<u64>);

impl BorTag {
    pub fn new(i: u64) -> Option<Self> {
        NonZero::new(i).map(BorTag)
    }

    pub fn get(&self) -> u64 {
        self.0.get()
    }

    pub fn inner(&self) -> NonZero<u64> {
        self.0
    }

    pub fn succ(self) -> Option<Self> {
        self.0.checked_add(1).map(Self)
    }

    /// The minimum representable tag
    pub fn one() -> Self {
        Self::new(1).unwrap()
    }
}

impl std::default::Default for BorTag {
    /// The default to be used when borrow tracking is disabled
    fn default() -> Self {
        Self::one()
    }
}

impl fmt::Debug for BorTag {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "<{}>", self.0)
    }
}

/// Per-call-stack-frame data for borrow tracking
#[derive(Debug)]
pub struct FrameState {
    /// If this frame is protecting any tags, they are listed here. We use this list to do
    /// incremental updates of the global list of protected tags stored in the
    /// `stacked_borrows::GlobalState` upon function return, and if we attempt to pop a protected
    /// tag, to identify which call is responsible for protecting the tag.
    /// See `Stack::item_invalidated` for more explanation.
    /// Tree Borrows also needs to know which allocation these tags
    /// belong to so that it can perform a read through them immediately before
    /// the frame gets popped.
    ///
    /// This will contain one tag per reference passed to the function, so
    /// a size of 2 is enough for the vast majority of functions.
    protected_tags: SmallVec<[(AllocId, BorTag); 2]>,
}

impl VisitProvenance for FrameState {
    fn visit_provenance(&self, visit: &mut VisitWith<'_>) {
        // Visit all protected tags. At least in Tree Borrows,
        // protected tags can not be GC'd because they still have
        // an access coming when the protector ends. Additionally,
        // the tree compacting mechanism of TB's GC relies on the fact
        // that all protected tags are marked as live for correctness,
        // so we _have_ to visit them here.
        for (id, tag) in &self.protected_tags {
            visit(Some(*id), Some(*tag));
        }
    }
}

/// Extra global state, available to the memory access hooks.
#[derive(Debug)]
pub struct GlobalStateInner {
    /// Borrow tracker method currently in use.
    borrow_tracker_method: BorrowTrackerMethod,
    /// Next unused pointer ID (tag).
    next_ptr_tag: BorTag,
    /// Table storing the "root" tag for each allocation.
    /// The root tag is the one used for the initial pointer.
    /// We need this in a separate table to handle cyclic statics.
    root_ptr_tags: FxHashMap<AllocId, BorTag>,
    /// All currently protected tags.
    /// We add tags to this when they are created with a protector in `reborrow`, and
    /// we remove tags from this when the call which is protecting them returns, in
    /// `GlobalStateInner::end_call`. See `Stack::item_invalidated` for more details.
    protected_tags: FxHashMap<BorTag, ProtectorKind>,
    /// The pointer ids to trace
    tracked_pointer_tags: FxHashSet<BorTag>,
    /// Whether to recurse into datatypes when searching for pointers to retag.
    retag_fields: RetagFields,
}

impl VisitProvenance for GlobalStateInner {
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {
        // All the provenance in protected_tags is also stored in FrameState, and visited there.
        // The only other candidate is base_ptr_tags, and that does not need visiting since we don't ever
        // GC the bottommost/root tag.
    }
}

/// We need interior mutable access to the global state.
pub type GlobalState = RefCell<GlobalStateInner>;

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"),
        }
    }
}

/// Policy on whether to recurse into fields to retag
#[derive(Copy, Clone, Debug)]
pub enum RetagFields {
    /// Don't retag any fields.
    No,
    /// Retag all fields.
    Yes,
    /// Only retag fields of types with Scalar and ScalarPair layout,
    /// to match the LLVM `noalias` we generate.
    OnlyScalar,
}

/// The flavor of the protector.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum ProtectorKind {
    /// Protected against aliasing violations from other pointers.
    ///
    /// Items protected like this cause UB when they are invalidated, *but* the pointer itself may
    /// still be used to issue a deallocation.
    ///
    /// This is required for LLVM IR pointers that are `noalias` but *not* `dereferenceable`.
    WeakProtector,

    /// Protected against any kind of invalidation.
    ///
    /// Items protected like this cause UB when they are invalidated or the memory is deallocated.
    /// This is strictly stronger protection than `WeakProtector`.
    ///
    /// This is required for LLVM IR pointers that are `dereferenceable` (and also allows `noalias`).
    StrongProtector,
}

/// Utilities for initialization and ID generation
impl GlobalStateInner {
    pub fn new(
        borrow_tracker_method: BorrowTrackerMethod,
        tracked_pointer_tags: FxHashSet<BorTag>,
        retag_fields: RetagFields,
    ) -> Self {
        GlobalStateInner {
            borrow_tracker_method,
            next_ptr_tag: BorTag::one(),
            root_ptr_tags: FxHashMap::default(),
            protected_tags: FxHashMap::default(),
            tracked_pointer_tags,
            retag_fields,
        }
    }

    /// Generates a new pointer tag. Remember to also check track_pointer_tags and log its creation!
    fn new_ptr(&mut self) -> BorTag {
        let id = self.next_ptr_tag;
        self.next_ptr_tag = id.succ().unwrap();
        id
    }

    pub fn new_frame(&mut self) -> FrameState {
        FrameState { protected_tags: SmallVec::new() }
    }

    fn end_call(&mut self, frame: &machine::FrameExtra<'_>) {
        for (_, tag) in &frame
            .borrow_tracker
            .as_ref()
            .expect("we should have borrow tracking data")
            .protected_tags
        {
            self.protected_tags.remove(tag);
        }
    }

    pub fn root_ptr_tag(&mut self, id: AllocId, machine: &MiriMachine<'_>) -> BorTag {
        self.root_ptr_tags.get(&id).copied().unwrap_or_else(|| {
            let tag = self.new_ptr();
            if self.tracked_pointer_tags.contains(&tag) {
                machine.emit_diagnostic(NonHaltingDiagnostic::CreatedPointerTag(
                    tag.inner(),
                    None,
                    None,
                ));
            }
            trace!("New allocation {:?} has rpot tag {:?}", id, tag);
            self.root_ptr_tags.try_insert(id, tag).unwrap();
            tag
        })
    }

    pub fn remove_unreachable_allocs(&mut self, allocs: &LiveAllocs<'_, '_>) {
        self.root_ptr_tags.retain(|id, _| allocs.is_live(*id));
    }

    pub fn borrow_tracker_method(&self) -> BorrowTrackerMethod {
        self.borrow_tracker_method
    }
}

/// Which borrow tracking method to use
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum BorrowTrackerMethod {
    /// Stacked Borrows, as implemented in borrow_tracker/stacked_borrows
    StackedBorrows,
    /// Tree borrows, as implemented in borrow_tracker/tree_borrows
    TreeBorrows,
}

impl BorrowTrackerMethod {
    pub fn instantiate_global_state(self, config: &MiriConfig) -> GlobalState {
        RefCell::new(GlobalStateInner::new(
            self,
            config.tracked_pointer_tags.clone(),
            config.retag_fields,
        ))
    }
}

impl GlobalStateInner {
    pub fn new_allocation(
        &mut self,
        id: AllocId,
        alloc_size: Size,
        kind: MemoryKind,
        machine: &MiriMachine<'_>,
    ) -> AllocState {
        match self.borrow_tracker_method {
            BorrowTrackerMethod::StackedBorrows =>
                AllocState::StackedBorrows(Box::new(RefCell::new(Stacks::new_allocation(
                    id, alloc_size, self, kind, machine,
                )))),
            BorrowTrackerMethod::TreeBorrows =>
                AllocState::TreeBorrows(Box::new(RefCell::new(Tree::new_allocation(
                    id, alloc_size, self, kind, machine,
                )))),
        }
    }
}

impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
    fn retag_ptr_value(
        &mut self,
        kind: RetagKind,
        val: &ImmTy<'tcx>,
    ) -> InterpResult<'tcx, ImmTy<'tcx>> {
        let this = self.eval_context_mut();
        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => this.sb_retag_ptr_value(kind, val),
            BorrowTrackerMethod::TreeBorrows => this.tb_retag_ptr_value(kind, val),
        }
    }

    fn retag_place_contents(
        &mut self,
        kind: RetagKind,
        place: &PlaceTy<'tcx>,
    ) -> InterpResult<'tcx> {
        let this = self.eval_context_mut();
        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => this.sb_retag_place_contents(kind, place),
            BorrowTrackerMethod::TreeBorrows => this.tb_retag_place_contents(kind, place),
        }
    }

    fn protect_place(&mut self, place: &MPlaceTy<'tcx>) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
        let this = self.eval_context_mut();
        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => this.sb_protect_place(place),
            BorrowTrackerMethod::TreeBorrows => this.tb_protect_place(place),
        }
    }

    fn expose_tag(&self, alloc_id: AllocId, tag: BorTag) -> InterpResult<'tcx> {
        let this = self.eval_context_ref();
        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => this.sb_expose_tag(alloc_id, tag),
            BorrowTrackerMethod::TreeBorrows => this.tb_expose_tag(alloc_id, tag),
        }
    }

    fn give_pointer_debug_name(
        &mut self,
        ptr: Pointer,
        nth_parent: u8,
        name: &str,
    ) -> InterpResult<'tcx> {
        let this = self.eval_context_mut();
        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => {
                this.tcx.tcx.dcx().warn("Stacked Borrows does not support named pointers; `miri_pointer_name` is a no-op");
                interp_ok(())
            }
            BorrowTrackerMethod::TreeBorrows =>
                this.tb_give_pointer_debug_name(ptr, nth_parent, name),
        }
    }

    fn print_borrow_state(&mut self, alloc_id: AllocId, show_unnamed: bool) -> InterpResult<'tcx> {
        let this = self.eval_context_mut();
        let Some(borrow_tracker) = &this.machine.borrow_tracker else {
            eprintln!("attempted to print borrow state, but no borrow state is being tracked");
            return interp_ok(());
        };
        let method = borrow_tracker.borrow().borrow_tracker_method;
        match method {
            BorrowTrackerMethod::StackedBorrows => this.print_stacks(alloc_id),
            BorrowTrackerMethod::TreeBorrows => this.print_tree(alloc_id, show_unnamed),
        }
    }

    fn on_stack_pop(
        &self,
        frame: &Frame<'tcx, Provenance, FrameExtra<'tcx>>,
    ) -> InterpResult<'tcx> {
        let this = self.eval_context_ref();
        let borrow_tracker = this.machine.borrow_tracker.as_ref().unwrap();
        // The body of this loop needs `borrow_tracker` immutably
        // so we can't move this code inside the following `end_call`.
        for (alloc_id, tag) in &frame
            .extra
            .borrow_tracker
            .as_ref()
            .expect("we should have borrow tracking data")
            .protected_tags
        {
            // Just because the tag is protected doesn't guarantee that
            // the allocation still exists (weak protectors allow deallocations)
            // so we must check that the allocation exists.
            // If it does exist, then we have the guarantee that the
            // pointer is readable, and the implicit read access inserted
            // will never cause UB on the pointer itself.
            let kind = this.get_alloc_info(*alloc_id).kind;
            if matches!(kind, AllocKind::LiveData) {
                let alloc_extra = this.get_alloc_extra(*alloc_id)?; // can still fail for `extern static`
                let alloc_borrow_tracker = &alloc_extra.borrow_tracker.as_ref().unwrap();
                alloc_borrow_tracker.release_protector(
                    &this.machine,
                    borrow_tracker,
                    *tag,
                    *alloc_id,
                )?;
            }
        }
        borrow_tracker.borrow_mut().end_call(&frame.extra);
        interp_ok(())
    }
}

/// Extra per-allocation data for borrow tracking
#[derive(Debug, Clone)]
pub enum AllocState {
    /// Data corresponding to Stacked Borrows
    StackedBorrows(Box<RefCell<stacked_borrows::AllocState>>),
    /// Data corresponding to Tree Borrows
    TreeBorrows(Box<RefCell<tree_borrows::AllocState>>),
}

impl machine::AllocExtra<'_> {
    #[track_caller]
    pub fn borrow_tracker_sb(&self) -> &RefCell<stacked_borrows::AllocState> {
        match self.borrow_tracker {
            Some(AllocState::StackedBorrows(ref sb)) => sb,
            _ => panic!("expected Stacked Borrows borrow tracking, got something else"),
        }
    }

    #[track_caller]
    pub fn borrow_tracker_sb_mut(&mut self) -> &mut RefCell<stacked_borrows::AllocState> {
        match self.borrow_tracker {
            Some(AllocState::StackedBorrows(ref mut sb)) => sb,
            _ => panic!("expected Stacked Borrows borrow tracking, got something else"),
        }
    }

    #[track_caller]
    pub fn borrow_tracker_tb(&self) -> &RefCell<tree_borrows::AllocState> {
        match self.borrow_tracker {
            Some(AllocState::TreeBorrows(ref tb)) => tb,
            _ => panic!("expected Tree Borrows borrow tracking, got something else"),
        }
    }
}

impl AllocState {
    pub fn before_memory_read<'tcx>(
        &self,
        alloc_id: AllocId,
        prov_extra: ProvenanceExtra,
        range: AllocRange,
        machine: &MiriMachine<'tcx>,
    ) -> InterpResult<'tcx> {
        match self {
            AllocState::StackedBorrows(sb) =>
                sb.borrow_mut().before_memory_read(alloc_id, prov_extra, range, machine),
            AllocState::TreeBorrows(tb) =>
                tb.borrow_mut().before_memory_access(
                    AccessKind::Read,
                    alloc_id,
                    prov_extra,
                    range,
                    machine,
                ),
        }
    }

    pub fn before_memory_write<'tcx>(
        &mut self,
        alloc_id: AllocId,
        prov_extra: ProvenanceExtra,
        range: AllocRange,
        machine: &MiriMachine<'tcx>,
    ) -> InterpResult<'tcx> {
        match self {
            AllocState::StackedBorrows(sb) =>
                sb.get_mut().before_memory_write(alloc_id, prov_extra, range, machine),
            AllocState::TreeBorrows(tb) =>
                tb.get_mut().before_memory_access(
                    AccessKind::Write,
                    alloc_id,
                    prov_extra,
                    range,
                    machine,
                ),
        }
    }

    pub fn before_memory_deallocation<'tcx>(
        &mut self,
        alloc_id: AllocId,
        prov_extra: ProvenanceExtra,
        size: Size,
        machine: &MiriMachine<'tcx>,
    ) -> InterpResult<'tcx> {
        match self {
            AllocState::StackedBorrows(sb) =>
                sb.get_mut().before_memory_deallocation(alloc_id, prov_extra, size, machine),
            AllocState::TreeBorrows(tb) =>
                tb.get_mut().before_memory_deallocation(alloc_id, prov_extra, size, machine),
        }
    }

    pub fn remove_unreachable_tags(&self, tags: &FxHashSet<BorTag>) {
        match self {
            AllocState::StackedBorrows(sb) => sb.borrow_mut().remove_unreachable_tags(tags),
            AllocState::TreeBorrows(tb) => tb.borrow_mut().remove_unreachable_tags(tags),
        }
    }

    /// Tree Borrows needs to be told when a tag stops being protected.
    pub fn release_protector<'tcx>(
        &self,
        machine: &MiriMachine<'tcx>,
        global: &GlobalState,
        tag: BorTag,
        alloc_id: AllocId, // diagnostics
    ) -> InterpResult<'tcx> {
        match self {
            AllocState::StackedBorrows(_sb) => interp_ok(()),
            AllocState::TreeBorrows(tb) =>
                tb.borrow_mut().release_protector(machine, global, tag, alloc_id),
        }
    }
}

impl VisitProvenance for AllocState {
    fn visit_provenance(&self, visit: &mut VisitWith<'_>) {
        match self {
            AllocState::StackedBorrows(sb) => sb.visit_provenance(visit),
            AllocState::TreeBorrows(tb) => tb.visit_provenance(visit),
        }
    }
}