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//! In this file we handle the "Tree" part of Tree Borrows, i.e. all tree
//! traversal functions, optimizations to trim branches, and keeping track of
//! the relative position of the access to each node being updated. This of course
//! also includes the definition of the tree structure.
//!
//! Functions here manipulate permissions but are oblivious to them: as
//! the internals of `Permission` are private, the update process is a black
//! box. All we need to know here are
//! - the fact that updates depend only on the old state, the status of protectors,
//! and the relative position of the access;
//! - idempotency properties asserted in `perms.rs` (for optimizations)
use std::fmt;
use smallvec::SmallVec;
use rustc_const_eval::interpret::InterpResult;
use rustc_data_structures::fx::FxHashSet;
use rustc_span::Span;
use rustc_target::abi::Size;
use crate::borrow_tracker::tree_borrows::{
diagnostics::{self, NodeDebugInfo, TbError, TransitionError},
perms::PermTransition,
unimap::{UniEntry, UniIndex, UniKeyMap, UniValMap},
Permission,
};
use crate::borrow_tracker::{AccessKind, GlobalState, ProtectorKind};
use crate::*;
mod tests;
/// Data for a single *location*.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub(super) struct LocationState {
/// A location is initialized when it is child-accessed for the first time (and the initial
/// retag initializes the location for the range covered by the type), and it then stays
/// initialized forever.
/// For initialized locations, "permission" is the current permission. However, for
/// uninitialized locations, we still need to track the "future initial permission": this will
/// start out to be `default_initial_perm`, but foreign accesses need to be taken into account.
/// Crucially however, while transitions to `Disabled` would usually be UB if this location is
/// protected, that is *not* the case for uninitialized locations. Instead we just have a latent
/// "future initial permission" of `Disabled`, causing UB only if an access is ever actually
/// performed.
initialized: bool,
/// This pointer's current permission / future initial permission.
permission: Permission,
/// Strongest foreign access whose effects have already been applied to
/// this node and all its children since the last child access.
/// This is `None` if the most recent access is a child access,
/// `Some(Write)` if at least one foreign write access has been applied
/// since the previous child access, and `Some(Read)` if at least one
/// foreign read and no foreign write have occurred since the last child access.
latest_foreign_access: Option<AccessKind>,
}
impl LocationState {
/// Default initial state has never been accessed and has been subjected to no
/// foreign access.
fn new(permission: Permission) -> Self {
Self { permission, initialized: false, latest_foreign_access: None }
}
/// Record that this location was accessed through a child pointer by
/// marking it as initialized
fn with_access(mut self) -> Self {
self.initialized = true;
self
}
/// Check if the location has been initialized, i.e. if it has
/// ever been accessed through a child pointer.
pub fn is_initialized(&self) -> bool {
self.initialized
}
/// Check if the state can exist as the initial permission of a pointer.
///
/// Do not confuse with `is_initialized`, the two are almost orthogonal
/// as apart from `Active` which is not initial and must be initialized,
/// any other permission can have an arbitrary combination of being
/// initial/initialized.
/// FIXME: when the corresponding `assert` in `tree_borrows/mod.rs` finally
/// passes and can be uncommented, remove this `#[allow(dead_code)]`.
#[cfg_attr(not(test), allow(dead_code))]
pub fn is_initial(&self) -> bool {
self.permission.is_initial()
}
pub fn permission(&self) -> Permission {
self.permission
}
/// Apply the effect of an access to one location, including
/// - applying `Permission::perform_access` to the inner `Permission`,
/// - emitting protector UB if the location is initialized,
/// - updating the initialized status (child accesses produce initialized locations).
fn perform_access(
&mut self,
access_kind: AccessKind,
rel_pos: AccessRelatedness,
protected: bool,
) -> Result<PermTransition, TransitionError> {
let old_perm = self.permission;
let transition = Permission::perform_access(access_kind, rel_pos, old_perm, protected)
.ok_or(TransitionError::ChildAccessForbidden(old_perm))?;
// Why do only initialized locations cause protector errors?
// Consider two mutable references `x`, `y` into disjoint parts of
// the same allocation. A priori, these may actually both be used to
// access the entire allocation, as long as only reads occur. However,
// a write to `y` needs to somehow record that `x` can no longer be used
// on that location at all. For these uninitialized locations (i.e., locations
// that haven't been accessed with `x` yet), we track the "future initial state":
// it defaults to whatever the initial state of the tag is,
// but the access to `y` moves that "future initial state" of `x` to `Disabled`.
// However, usually a `Reserved -> Disabled` transition would be UB due to the protector!
// So clearly protectors shouldn't fire for such "future initial state" transitions.
//
// See the test `two_mut_protected_same_alloc` in `tests/pass/tree_borrows/tree-borrows.rs`
// for an example of safe code that would be UB if we forgot to check `self.initialized`.
if protected && self.initialized && transition.produces_disabled() {
return Err(TransitionError::ProtectedDisabled(old_perm));
}
self.permission = transition.applied(old_perm).unwrap();
self.initialized |= !rel_pos.is_foreign();
Ok(transition)
}
// Helper to optimize the tree traversal.
// The optimization here consists of observing thanks to the tests
// `foreign_read_is_noop_after_foreign_write` and `all_transitions_idempotent`,
// that there are actually just three possible sequences of events that can occur
// in between two child accesses that produce different results.
//
// Indeed,
// - applying any number of foreign read accesses is the same as applying
// exactly one foreign read,
// - applying any number of foreign read or write accesses is the same
// as applying exactly one foreign write.
// therefore the three sequences of events that can produce different
// outcomes are
// - an empty sequence (`self.latest_foreign_access = None`)
// - a nonempty read-only sequence (`self.latest_foreign_access = Some(Read)`)
// - a nonempty sequence with at least one write (`self.latest_foreign_access = Some(Write)`)
//
// This function not only determines if skipping the propagation right now
// is possible, it also updates the internal state to keep track of whether
// the propagation can be skipped next time.
// It is a performance loss not to call this function when a foreign access occurs.
// It is unsound not to call this function when a child access occurs.
fn skip_if_known_noop(
&mut self,
access_kind: AccessKind,
rel_pos: AccessRelatedness,
) -> ContinueTraversal {
if rel_pos.is_foreign() {
let new_access_noop = match (self.latest_foreign_access, access_kind) {
// Previously applied transition makes the new one a guaranteed
// noop in the two following cases:
// (1) justified by `foreign_read_is_noop_after_foreign_write`
(Some(AccessKind::Write), AccessKind::Read) => true,
// (2) justified by `all_transitions_idempotent`
(Some(old), new) if old == new => true,
// In all other cases there has been a recent enough
// child access that the effects of the new foreign access
// need to be applied to this subtree.
_ => false,
};
if new_access_noop {
// Abort traversal if the new transition is indeed guaranteed
// to be noop.
// No need to update `self.latest_foreign_access`,
// the type of the current streak among nonempty read-only
// or nonempty with at least one write has not changed.
ContinueTraversal::SkipChildren
} else {
// Otherwise propagate this time, and also record the
// access that just occurred so that we can skip the propagation
// next time.
self.latest_foreign_access = Some(access_kind);
ContinueTraversal::Recurse
}
} else {
// A child access occurred, this breaks the streak of foreign
// accesses in a row and the sequence since the previous child access
// is now empty.
self.latest_foreign_access = None;
ContinueTraversal::Recurse
}
}
}
impl fmt::Display for LocationState {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{}", self.permission)?;
if !self.initialized {
write!(f, "?")?;
}
Ok(())
}
}
/// Tree structure with both parents and children since we want to be
/// able to traverse the tree efficiently in both directions.
#[derive(Clone, Debug)]
pub struct Tree {
/// Mapping from tags to keys. The key obtained can then be used in
/// any of the `UniValMap` relative to this allocation, i.e. both the
/// `nodes` and `rperms` of the same `Tree`.
/// The parent-child relationship in `Node` is encoded in terms of these same
/// keys, so traversing the entire tree needs exactly one access to
/// `tag_mapping`.
pub(super) tag_mapping: UniKeyMap<BorTag>,
/// All nodes of this tree.
pub(super) nodes: UniValMap<Node>,
/// Maps a tag and a location to a perm, with possible lazy
/// initialization.
///
/// NOTE: not all tags registered in `nodes` are necessarily in all
/// ranges of `rperms`, because `rperms` is in part lazily initialized.
/// Just because `nodes.get(key)` is `Some(_)` does not mean you can safely
/// `unwrap` any `perm.get(key)`.
///
/// We do uphold the fact that `keys(perms)` is a subset of `keys(nodes)`
pub(super) rperms: RangeMap<UniValMap<LocationState>>,
/// The index of the root node.
pub(super) root: UniIndex,
}
/// A node in the borrow tree. Each node is uniquely identified by a tag via
/// the `nodes` map of `Tree`.
#[derive(Clone, Debug)]
pub(super) struct Node {
/// The tag of this node.
pub tag: BorTag,
/// All tags except the root have a parent tag.
pub parent: Option<UniIndex>,
/// If the pointer was reborrowed, it has children.
// FIXME: bench to compare this to FxHashSet and to other SmallVec sizes
pub children: SmallVec<[UniIndex; 4]>,
/// Either `Reserved` or `Frozen`, the permission this tag will be lazily initialized
/// to on the first access.
default_initial_perm: Permission,
/// Some extra information useful only for debugging purposes
pub debug_info: NodeDebugInfo,
}
/// Data given to the transition function
struct NodeAppArgs<'node> {
/// Node on which the transition is currently being applied
node: &'node mut Node,
/// Mutable access to its permissions
perm: UniEntry<'node, LocationState>,
/// Relative position of the access
rel_pos: AccessRelatedness,
}
/// Data given to the error handler
struct ErrHandlerArgs<'node, InErr> {
/// Kind of error that occurred
error_kind: InErr,
/// Tag that triggered the error (not the tag that was accessed,
/// rather the parent tag that had insufficient permissions or the
/// non-parent tag that had a protector).
conflicting_info: &'node NodeDebugInfo,
/// Information about the tag that was accessed just before the
/// error was triggered.
accessed_info: &'node NodeDebugInfo,
}
/// Internal contents of `Tree` with the minimum of mutable access for
/// the purposes of the tree traversal functions: the permissions (`perms`) can be
/// updated but not the tree structure (`tag_mapping` and `nodes`)
struct TreeVisitor<'tree> {
tag_mapping: &'tree UniKeyMap<BorTag>,
nodes: &'tree mut UniValMap<Node>,
perms: &'tree mut UniValMap<LocationState>,
}
/// Whether to continue exploring the children recursively or not.
enum ContinueTraversal {
Recurse,
SkipChildren,
}
impl<'tree> TreeVisitor<'tree> {
// Applies `f_propagate` to every vertex of the tree top-down in the following order: first
// all ancestors of `start`, then `start` itself, then children of `start`, then the rest.
// This ensures that errors are triggered in the following order
// - first invalid accesses with insufficient permissions, closest to the root first,
// - then protector violations, closest to `start` first.
//
// `f_propagate` should follow the following format: for a given `Node` it updates its
// `Permission` depending on the position relative to `start` (given by an
// `AccessRelatedness`).
// It outputs whether the tree traversal for this subree should continue or not.
fn traverse_parents_this_children_others<InnErr, OutErr>(
mut self,
start: BorTag,
f_propagate: impl Fn(NodeAppArgs<'_>) -> Result<ContinueTraversal, InnErr>,
err_builder: impl Fn(ErrHandlerArgs<'_, InnErr>) -> OutErr,
) -> Result<(), OutErr>
where {
struct TreeVisitAux<NodeApp, ErrHandler> {
accessed_tag: UniIndex,
f_propagate: NodeApp,
err_builder: ErrHandler,
stack: Vec<(UniIndex, AccessRelatedness)>,
}
impl<NodeApp, InnErr, OutErr, ErrHandler> TreeVisitAux<NodeApp, ErrHandler>
where
NodeApp: Fn(NodeAppArgs<'_>) -> Result<ContinueTraversal, InnErr>,
ErrHandler: Fn(ErrHandlerArgs<'_, InnErr>) -> OutErr,
{
fn pop(&mut self) -> Option<(UniIndex, AccessRelatedness)> {
self.stack.pop()
}
/// Apply the function to the current `tag`, and push its children
/// to the stack of future tags to visit.
fn exec_and_visit(
&mut self,
this: &mut TreeVisitor<'_>,
tag: UniIndex,
exclude: Option<UniIndex>,
rel_pos: AccessRelatedness,
) -> Result<(), OutErr> {
// 1. apply the propagation function
let node = this.nodes.get_mut(tag).unwrap();
let recurse =
(self.f_propagate)(NodeAppArgs { node, perm: this.perms.entry(tag), rel_pos })
.map_err(|error_kind| {
(self.err_builder)(ErrHandlerArgs {
error_kind,
conflicting_info: &this.nodes.get(tag).unwrap().debug_info,
accessed_info: &this
.nodes
.get(self.accessed_tag)
.unwrap()
.debug_info,
})
})?;
let node = this.nodes.get(tag).unwrap();
// 2. add the children to the stack for future traversal
if matches!(recurse, ContinueTraversal::Recurse) {
let child_rel = rel_pos.for_child();
for &child in node.children.iter() {
// some child might be excluded from here and handled separately
if Some(child) != exclude {
self.stack.push((child, child_rel));
}
}
}
Ok(())
}
}
let start_idx = self.tag_mapping.get(&start).unwrap();
let mut stack =
TreeVisitAux { accessed_tag: start_idx, f_propagate, err_builder, stack: Vec::new() };
{
let mut path_ascend = Vec::new();
// First climb to the root while recording the path
let mut curr = start_idx;
while let Some(ancestor) = self.nodes.get(curr).unwrap().parent {
path_ascend.push((ancestor, curr));
curr = ancestor;
}
// Then descend:
// - execute f_propagate on each node
// - record children in visit
while let Some((ancestor, next_in_path)) = path_ascend.pop() {
// Explore ancestors in descending order.
// `next_in_path` is excluded from the recursion because it
// will be the `ancestor` of the next iteration.
// It also needs a different `AccessRelatedness` than the other
// children of `ancestor`.
stack.exec_and_visit(
&mut self,
ancestor,
Some(next_in_path),
AccessRelatedness::StrictChildAccess,
)?;
}
};
// All (potentially zero) ancestors have been explored, call f_propagate on start
stack.exec_and_visit(&mut self, start_idx, None, AccessRelatedness::This)?;
// up to this point we have never popped from `stack`, hence if the
// path to the root is `root = p(n) <- p(n-1)... <- p(1) <- p(0) = start`
// then now `stack` contains
// `[<children(p(n)) except p(n-1)> ... <children(p(1)) except p(0)> <children(p(0))>]`,
// all of which are for now unexplored.
// This is the starting point of a standard DFS which will thus
// explore all non-ancestors of `start` in the following order:
// - all descendants of `start`;
// - then the unexplored descendants of `parent(start)`;
// ...
// - until finally the unexplored descendants of `root`.
while let Some((tag, rel_pos)) = stack.pop() {
stack.exec_and_visit(&mut self, tag, None, rel_pos)?;
}
Ok(())
}
}
impl Tree {
/// Create a new tree, with only a root pointer.
pub fn new(root_tag: BorTag, size: Size, span: Span) -> Self {
let root_perm = Permission::new_active();
let mut tag_mapping = UniKeyMap::default();
let root_idx = tag_mapping.insert(root_tag);
let nodes = {
let mut nodes = UniValMap::<Node>::default();
let mut debug_info = NodeDebugInfo::new(root_tag, root_perm, span);
// name the root so that all allocations contain one named pointer
debug_info.add_name("root of the allocation");
nodes.insert(
root_idx,
Node {
tag: root_tag,
parent: None,
children: SmallVec::default(),
default_initial_perm: root_perm,
debug_info,
},
);
nodes
};
let rperms = {
let mut perms = UniValMap::default();
perms.insert(root_idx, LocationState::new(root_perm).with_access());
RangeMap::new(size, perms)
};
Self { root: root_idx, nodes, rperms, tag_mapping }
}
}
impl<'tcx> Tree {
/// Insert a new tag in the tree
pub fn new_child(
&mut self,
parent_tag: BorTag,
new_tag: BorTag,
default_initial_perm: Permission,
reborrow_range: AllocRange,
span: Span,
) -> InterpResult<'tcx> {
assert!(!self.tag_mapping.contains_key(&new_tag));
let idx = self.tag_mapping.insert(new_tag);
let parent_idx = self.tag_mapping.get(&parent_tag).unwrap();
// Create the node
self.nodes.insert(
idx,
Node {
tag: new_tag,
parent: Some(parent_idx),
children: SmallVec::default(),
default_initial_perm,
debug_info: NodeDebugInfo::new(new_tag, default_initial_perm, span),
},
);
// Register new_tag as a child of parent_tag
self.nodes.get_mut(parent_idx).unwrap().children.push(idx);
// Initialize perms
let perm = LocationState::new(default_initial_perm).with_access();
for (_perms_range, perms) in self.rperms.iter_mut(reborrow_range.start, reborrow_range.size)
{
perms.insert(idx, perm);
}
Ok(())
}
/// Deallocation requires
/// - a pointer that permits write accesses
/// - the absence of Strong Protectors anywhere in the allocation
pub fn dealloc(
&mut self,
tag: BorTag,
access_range: AllocRange,
global: &GlobalState,
span: Span, // diagnostics
) -> InterpResult<'tcx> {
self.perform_access(
AccessKind::Write,
tag,
access_range,
global,
span,
diagnostics::AccessCause::Dealloc,
)?;
for (perms_range, perms) in self.rperms.iter_mut(access_range.start, access_range.size) {
TreeVisitor { nodes: &mut self.nodes, tag_mapping: &self.tag_mapping, perms }
.traverse_parents_this_children_others(
tag,
|args: NodeAppArgs<'_>| -> Result<ContinueTraversal, TransitionError> {
let NodeAppArgs { node, .. } = args;
if global.borrow().protected_tags.get(&node.tag)
== Some(&ProtectorKind::StrongProtector)
{
Err(TransitionError::ProtectedDealloc)
} else {
Ok(ContinueTraversal::Recurse)
}
},
|args: ErrHandlerArgs<'_, TransitionError>| -> InterpError<'tcx> {
let ErrHandlerArgs { error_kind, conflicting_info, accessed_info } = args;
TbError {
conflicting_info,
access_cause: diagnostics::AccessCause::Dealloc,
error_offset: perms_range.start,
error_kind,
accessed_info,
}
.build()
},
)?;
}
Ok(())
}
/// Map the per-node and per-location `LocationState::perform_access`
/// to each location of `access_range`, on every tag of the allocation.
///
/// `LocationState::perform_access` will take care of raising transition
/// errors and updating the `initialized` status of each location,
/// this traversal adds to that:
/// - inserting into the map locations that do not exist yet,
/// - trimming the traversal,
/// - recording the history.
pub fn perform_access(
&mut self,
access_kind: AccessKind,
tag: BorTag,
access_range: AllocRange,
global: &GlobalState,
span: Span, // diagnostics
access_cause: diagnostics::AccessCause, // diagnostics
) -> InterpResult<'tcx> {
for (perms_range, perms) in self.rperms.iter_mut(access_range.start, access_range.size) {
TreeVisitor { nodes: &mut self.nodes, tag_mapping: &self.tag_mapping, perms }
.traverse_parents_this_children_others(
tag,
|args: NodeAppArgs<'_>| -> Result<ContinueTraversal, TransitionError> {
let NodeAppArgs { node, mut perm, rel_pos } = args;
let old_state =
perm.or_insert_with(|| LocationState::new(node.default_initial_perm));
match old_state.skip_if_known_noop(access_kind, rel_pos) {
ContinueTraversal::SkipChildren =>
return Ok(ContinueTraversal::SkipChildren),
_ => {}
}
let protected = global.borrow().protected_tags.contains_key(&node.tag);
let transition =
old_state.perform_access(access_kind, rel_pos, protected)?;
// Record the event as part of the history
if !transition.is_noop() {
node.debug_info.history.push(diagnostics::Event {
transition,
is_foreign: rel_pos.is_foreign(),
access_cause,
access_range,
transition_range: perms_range.clone(),
span,
});
}
Ok(ContinueTraversal::Recurse)
},
|args: ErrHandlerArgs<'_, TransitionError>| -> InterpError<'tcx> {
let ErrHandlerArgs { error_kind, conflicting_info, accessed_info } = args;
TbError {
conflicting_info,
access_cause,
error_offset: perms_range.start,
error_kind,
accessed_info,
}
.build()
},
)?;
}
Ok(())
}
}
/// Integration with the BorTag garbage collector
impl Tree {
pub fn remove_unreachable_tags(&mut self, live_tags: &FxHashSet<BorTag>) {
let root_is_needed = self.keep_only_needed(self.root, live_tags); // root can't be removed
assert!(root_is_needed);
// Right after the GC runs is a good moment to check if we can
// merge some adjacent ranges that were made equal by the removal of some
// tags (this does not necessarily mean that they have identical internal representations,
// see the `PartialEq` impl for `UniValMap`)
self.rperms.merge_adjacent_thorough();
}
/// Traverses the entire tree looking for useless tags.
/// Returns true iff the tag it was called on is still live or has live children,
/// and removes from the tree all tags that have no live children.
///
/// NOTE: This leaves in the middle of the tree tags that are unreachable but have
/// reachable children. There is a potential for compacting the tree by reassigning
/// children of dead tags to the nearest live parent, but it must be done with care
/// not to remove UB.
///
/// Example: Consider the tree `root - parent - child`, with `parent: Frozen` and
/// `child: Reserved`. This tree can exist. If we blindly delete `parent` and reassign
/// `child` to be a direct child of `root` then Writes to `child` are now permitted
/// whereas they were not when `parent` was still there.
fn keep_only_needed(&mut self, idx: UniIndex, live: &FxHashSet<BorTag>) -> bool {
let node = self.nodes.get(idx).unwrap();
// FIXME: this function does a lot of cloning, a 2-pass approach is possibly
// more efficient. It could consist of
// 1. traverse the Tree, collect all useless tags in a Vec
// 2. traverse the Vec, remove all tags previously selected
// Bench it.
let children: SmallVec<_> = node
.children
.clone()
.into_iter()
.filter(|child| self.keep_only_needed(*child, live))
.collect();
let no_children = children.is_empty();
let node = self.nodes.get_mut(idx).unwrap();
node.children = children;
if !live.contains(&node.tag) && no_children {
// All of the children and this node are unreachable, delete this tag
// from the tree (the children have already been deleted by recursive
// calls).
// Due to the API of UniMap we must absolutely call
// `UniValMap::remove` for the key of this tag on *all* maps that used it
// (which are `self.nodes` and every range of `self.rperms`)
// before we can safely apply `UniValMap::forget` to truly remove
// the tag from the mapping.
let tag = node.tag;
self.nodes.remove(idx);
for (_perms_range, perms) in self.rperms.iter_mut_all() {
perms.remove(idx);
}
self.tag_mapping.remove(&tag);
// The tag has been deleted, inform the caller
false
} else {
// The tag is still live or has live children, it must be kept
true
}
}
}
impl VisitTags for Tree {
fn visit_tags(&self, visit: &mut dyn FnMut(BorTag)) {
// To ensure that the root never gets removed, we visit it
// (the `root` node of `Tree` is not an `Option<_>`)
visit(self.nodes.get(self.root).unwrap().tag)
}
}
/// Relative position of the access
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum AccessRelatedness {
/// The accessed pointer is the current one
This,
/// The accessed pointer is a (transitive) child of the current one.
// Current pointer is excluded (unlike in some other places of this module
// where "child" is inclusive).
StrictChildAccess,
/// The accessed pointer is a (transitive) parent of the current one.
// Current pointer is excluded.
AncestorAccess,
/// The accessed pointer is neither of the above.
// It's a cousin/uncle/etc., something in a side branch.
// FIXME: find a better name ?
DistantAccess,
}
impl AccessRelatedness {
/// Check that access is either Ancestor or Distant, i.e. not
/// a transitive child (initial pointer included).
pub fn is_foreign(self) -> bool {
matches!(self, AccessRelatedness::AncestorAccess | AccessRelatedness::DistantAccess)
}
/// Given the AccessRelatedness for the parent node, compute the AccessRelatedness
/// for the child node. This function assumes that we propagate away from the initial
/// access.
pub fn for_child(self) -> Self {
use AccessRelatedness::*;
match self {
AncestorAccess | This => AncestorAccess,
StrictChildAccess | DistantAccess => DistantAccess,
}
}
}