648 lines
25 KiB
Rust
648 lines
25 KiB
Rust
//! Implements "Stacked Borrows". See <https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md>
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//! for further information.
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use std::cell::RefCell;
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use std::collections::{HashMap, HashSet};
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use std::rc::Rc;
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use std::fmt;
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use std::num::NonZeroU64;
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use rustc::ty::{self, layout::Size};
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use rustc::hir::{MutMutable, MutImmutable};
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use rustc::mir::RetagKind;
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use crate::{
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InterpResult, HelpersEvalContextExt,
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MemoryKind, MiriMemoryKind, RangeMap, AllocId, Pointer, Immediate, ImmTy, PlaceTy, MPlaceTy,
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};
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pub type PtrId = NonZeroU64;
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pub type CallId = NonZeroU64;
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pub type AllocExtra = Stacks;
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/// Tracking pointer provenance
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#[derive(Copy, Clone, Hash, PartialEq, Eq)]
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pub enum Tag {
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Tagged(PtrId),
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Untagged,
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}
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impl fmt::Debug for Tag {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self {
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Tag::Tagged(id) => write!(f, "<{}>", id),
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Tag::Untagged => write!(f, "<untagged>"),
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}
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}
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}
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/// Indicates which permission is granted (by this item to some pointers)
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#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
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pub enum Permission {
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/// Grants unique mutable access.
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Unique,
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/// Grants shared mutable access.
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SharedReadWrite,
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/// Grants shared read-only access.
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SharedReadOnly,
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/// Grants no access, but separates two groups of SharedReadWrite so they are not
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/// all considered mutually compatible.
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Disabled,
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}
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/// An item in the per-location borrow stack.
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#[derive(Copy, Clone, Hash, PartialEq, Eq)]
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pub struct Item {
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/// The permission this item grants.
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perm: Permission,
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/// The pointers the permission is granted to.
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tag: Tag,
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/// An optional protector, ensuring the item cannot get popped until `CallId` is over.
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protector: Option<CallId>,
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}
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impl fmt::Debug for Item {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
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if let Some(call) = self.protector {
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write!(f, " (call {})", call)?;
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}
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write!(f, "]")?;
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Ok(())
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}
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}
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/// Extra per-location state.
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#[derive(Clone, Debug, PartialEq, Eq)]
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pub struct Stack {
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/// Used *mostly* as a stack; never empty.
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/// Invariants:
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/// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
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/// * Except for `Untagged`, no tag occurs in the stack more than once.
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borrows: Vec<Item>,
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}
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/// Extra per-allocation state.
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#[derive(Clone, Debug)]
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pub struct Stacks {
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// Even reading memory can have effects on the stack, so we need a `RefCell` here.
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stacks: RefCell<RangeMap<Stack>>,
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// Pointer to global state
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global: MemoryExtra,
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}
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/// Extra global state, available to the memory access hooks.
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#[derive(Debug)]
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pub struct GlobalState {
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/// Next unused pointer ID (tag).
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next_ptr_id: PtrId,
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/// Table storing the "base" tag for each allocation.
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/// The base tag is the one used for the initial pointer.
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/// We need this in a separate table to handle cyclic statics.
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base_ptr_ids: HashMap<AllocId, Tag>,
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/// Next unused call ID (for protectors).
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next_call_id: CallId,
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/// Those call IDs corresponding to functions that are still running.
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active_calls: HashSet<CallId>,
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}
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/// Memory extra state gives us interior mutable access to the global state.
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pub type MemoryExtra = Rc<RefCell<GlobalState>>;
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/// Indicates which kind of access is being performed.
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#[derive(Copy, Clone, Hash, PartialEq, Eq)]
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pub enum AccessKind {
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Read,
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Write,
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}
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impl fmt::Display for AccessKind {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self {
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AccessKind::Read => write!(f, "read access"),
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AccessKind::Write => write!(f, "write access"),
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}
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}
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}
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/// Indicates which kind of reference is being created.
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/// Used by high-level `reborrow` to compute which permissions to grant to the
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/// new pointer.
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#[derive(Copy, Clone, Hash, PartialEq, Eq)]
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pub enum RefKind {
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/// `&mut` and `Box`.
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Unique { two_phase: bool },
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/// `&` with or without interior mutability.
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Shared,
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/// `*mut`/`*const` (raw pointers).
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Raw { mutable: bool },
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}
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impl fmt::Display for RefKind {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self {
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RefKind::Unique { two_phase: false } => write!(f, "unique"),
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RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
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RefKind::Shared => write!(f, "shared"),
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RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
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RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
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}
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}
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}
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/// Utilities for initialization and ID generation
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impl Default for GlobalState {
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fn default() -> Self {
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GlobalState {
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next_ptr_id: NonZeroU64::new(1).unwrap(),
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base_ptr_ids: HashMap::default(),
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next_call_id: NonZeroU64::new(1).unwrap(),
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active_calls: HashSet::default(),
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}
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}
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}
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impl GlobalState {
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fn new_ptr(&mut self) -> PtrId {
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let id = self.next_ptr_id;
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self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
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id
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}
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pub fn new_call(&mut self) -> CallId {
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let id = self.next_call_id;
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trace!("new_call: Assigning ID {}", id);
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assert!(self.active_calls.insert(id));
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self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
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id
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}
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pub fn end_call(&mut self, id: CallId) {
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assert!(self.active_calls.remove(&id));
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}
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fn is_active(&self, id: CallId) -> bool {
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self.active_calls.contains(&id)
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}
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pub fn static_base_ptr(&mut self, id: AllocId) -> Tag {
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self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| {
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let tag = Tag::Tagged(self.new_ptr());
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trace!("New allocation {:?} has base tag {:?}", id, tag);
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self.base_ptr_ids.insert(id, tag).unwrap_none();
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tag
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})
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}
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}
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// # Stacked Borrows Core Begin
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/// We need to make at least the following things true:
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///
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/// U1: After creating a `Uniq`, it is at the top.
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/// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
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/// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
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///
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/// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
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/// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
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/// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
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/// gets popped.
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/// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
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/// F3: If an access happens with an `&` outside `UnsafeCell`,
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/// it requires the `SharedReadOnly` to still be in the stack.
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/// Core relation on `Permission` to define which accesses are allowed
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impl Permission {
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/// This defines for a given permission, whether it permits the given kind of access.
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fn grants(self, access: AccessKind) -> bool {
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// Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
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self != Permission::Disabled && (access == AccessKind::Read || self != Permission::SharedReadOnly)
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}
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}
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/// Core per-location operations: access, dealloc, reborrow.
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impl<'tcx> Stack {
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/// Find the item granting the given kind of access to the given tag, and return where
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/// it is on the stack.
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fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
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self.borrows.iter()
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.enumerate() // we also need to know *where* in the stack
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.rev() // search top-to-bottom
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// Return permission of first item that grants access.
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// We require a permission with the right tag, ensuring U3 and F3.
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.find_map(|(idx, item)|
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if tag == item.tag && item.perm.grants(access) {
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Some(idx)
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} else {
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None
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}
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)
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}
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/// Find the first write-incompatible item above the given one --
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/// i.e, find the height to which the stack will be truncated when writing to `granting`.
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fn find_first_write_incompatible(&self, granting: usize) -> usize {
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let perm = self.borrows[granting].perm;
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match perm {
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Permission::SharedReadOnly =>
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bug!("Cannot use SharedReadOnly for writing"),
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Permission::Disabled =>
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bug!("Cannot use Disabled for anything"),
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Permission::Unique =>
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// On a write, everything above us is incompatible.
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granting + 1,
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Permission::SharedReadWrite => {
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// The SharedReadWrite *just* above us are compatible, to skip those.
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let mut idx = granting + 1;
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while let Some(item) = self.borrows.get(idx) {
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if item.perm == Permission::SharedReadWrite {
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// Go on.
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idx += 1;
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} else {
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// Found first incompatible!
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break;
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}
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}
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idx
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}
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}
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}
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/// Check if the given item is protected.
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fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
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if let Some(call) = item.protector {
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if global.is_active(call) {
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if let Some(tag) = tag {
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throw_ub!(UbExperimental(format!(
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"not granting access to tag {:?} because incompatible item is protected: {:?}",
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tag, item
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)));
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} else {
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throw_ub!(UbExperimental(format!(
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"deallocating while item is protected: {:?}", item
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)));
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}
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}
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}
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Ok(())
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}
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/// Test if a memory `access` using pointer tagged `tag` is granted.
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/// If yes, return the index of the item that granted it.
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fn access(
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&mut self,
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access: AccessKind,
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tag: Tag,
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global: &GlobalState,
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) -> InterpResult<'tcx> {
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// Two main steps: Find granting item, remove incompatible items above.
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// Step 1: Find granting item.
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let granting_idx = self.find_granting(access, tag)
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.ok_or_else(|| err_ub!(UbExperimental(format!(
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"no item granting {} to tag {:?} found in borrow stack",
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access, tag,
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))))?;
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// Step 2: Remove incompatible items above them. Make sure we do not remove protected
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// items. Behavior differs for reads and writes.
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if access == AccessKind::Write {
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// Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
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// pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
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let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
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for item in self.borrows.drain(first_incompatible_idx..).rev() {
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trace!("access: popping item {:?}", item);
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Stack::check_protector(&item, Some(tag), global)?;
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}
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} else {
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// On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
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// The reason this is not following the stack discipline (by removing the first Unique and
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// everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
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// would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
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// `SharedReadWrite` for `raw`.
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// This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
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// reference and use that.
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// We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
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for idx in (granting_idx+1 .. self.borrows.len()).rev() {
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let item = &mut self.borrows[idx];
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if item.perm == Permission::Unique {
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trace!("access: disabling item {:?}", item);
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Stack::check_protector(item, Some(tag), global)?;
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item.perm = Permission::Disabled;
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}
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}
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}
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// Done.
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Ok(())
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}
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/// Deallocate a location: Like a write access, but also there must be no
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/// active protectors at all because we will remove all items.
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fn dealloc(
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&mut self,
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tag: Tag,
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global: &GlobalState,
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) -> InterpResult<'tcx> {
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// Step 1: Find granting item.
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self.find_granting(AccessKind::Write, tag)
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.ok_or_else(|| err_ub!(UbExperimental(format!(
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"no item granting write access for deallocation to tag {:?} found in borrow stack",
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tag,
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))))?;
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// Step 2: Remove all items. Also checks for protectors.
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for item in self.borrows.drain(..).rev() {
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Stack::check_protector(&item, None, global)?;
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}
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Ok(())
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}
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/// Derived a new pointer from one with the given tag.
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/// `weak` controls whether this operation is weak or strong: weak granting does not act as
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/// an access, and they add the new item directly on top of the one it is derived
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/// from instead of all the way at the top of the stack.
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fn grant(
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&mut self,
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derived_from: Tag,
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new: Item,
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global: &GlobalState,
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) -> InterpResult<'tcx> {
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// Figure out which access `perm` corresponds to.
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let access = if new.perm.grants(AccessKind::Write) {
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AccessKind::Write
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} else {
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AccessKind::Read
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};
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// Now we figure out which item grants our parent (`derived_from`) this kind of access.
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// We use that to determine where to put the new item.
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let granting_idx = self.find_granting(access, derived_from)
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.ok_or_else(|| err_ub!(UbExperimental(format!(
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"trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack", new.perm, derived_from,
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))))?;
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// Compute where to put the new item.
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// Either way, we ensure that we insert the new item in a way such that between
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// `derived_from` and the new one, there are only items *compatible with* `derived_from`.
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let new_idx = if new.perm == Permission::SharedReadWrite {
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assert!(access == AccessKind::Write, "this case only makes sense for stack-like accesses");
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// SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
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// access. Instead of popping the stack, we insert the item at the place the stack would
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// be popped to (i.e., we insert it above all the write-compatible items).
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// This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
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self.find_first_write_incompatible(granting_idx)
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} else {
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// A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
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// Here, creating a reference actually counts as an access.
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// This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
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self.access(access, derived_from, global)?;
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// We insert "as far up as possible": We know only compatible items are remaining
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// on top of `derived_from`, and we want the new item at the top so that we
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// get the strongest possible guarantees.
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// This ensures U1 and F1.
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self.borrows.len()
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};
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// Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
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if self.borrows[new_idx-1] == new || self.borrows.get(new_idx) == Some(&new) {
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// Optimization applies, done.
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trace!("reborrow: avoiding adding redundant item {:?}", new);
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} else {
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trace!("reborrow: adding item {:?}", new);
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self.borrows.insert(new_idx, new);
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}
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Ok(())
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}
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}
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// # Stacked Borrows Core End
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/// Map per-stack operations to higher-level per-location-range operations.
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impl<'tcx> Stacks {
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/// Creates new stack with initial tag.
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fn new(
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size: Size,
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perm: Permission,
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tag: Tag,
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extra: MemoryExtra,
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) -> Self {
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let item = Item { perm, tag, protector: None };
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let stack = Stack {
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borrows: vec![item],
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};
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Stacks {
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stacks: RefCell::new(RangeMap::new(size, stack)),
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global: extra,
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}
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}
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/// Call `f` on every stack in the range.
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fn for_each(
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&self,
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ptr: Pointer<Tag>,
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size: Size,
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f: impl Fn(&mut Stack, &GlobalState) -> InterpResult<'tcx>,
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) -> InterpResult<'tcx> {
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let global = self.global.borrow();
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let mut stacks = self.stacks.borrow_mut();
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for stack in stacks.iter_mut(ptr.offset, size) {
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f(stack, &*global)?;
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}
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Ok(())
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}
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}
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/// Glue code to connect with Miri Machine Hooks
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impl Stacks {
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pub fn new_allocation(
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id: AllocId,
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size: Size,
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extra: MemoryExtra,
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kind: MemoryKind<MiriMemoryKind>,
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) -> (Self, Tag) {
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let (tag, perm) = match kind {
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MemoryKind::Stack =>
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// New unique borrow. This tag is not accessible by the program,
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// so it will only ever be used when using the local directly (i.e.,
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// not through a pointer). That is, whenever we directly write to a local, this will pop
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// everything else off the stack, invalidating all previous pointers,
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// and in particular, *all* raw pointers.
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(Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
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MemoryKind::Machine(MiriMemoryKind::Static) =>
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(extra.borrow_mut().static_base_ptr(id), Permission::SharedReadWrite),
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_ =>
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(Tag::Untagged, Permission::SharedReadWrite),
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};
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let stack = Stacks::new(size, perm, tag, extra);
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(stack, tag)
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}
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#[inline(always)]
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pub fn memory_read<'tcx>(
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&self,
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ptr: Pointer<Tag>,
|
|
size: Size,
|
|
) -> InterpResult<'tcx> {
|
|
trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
|
|
self.for_each(ptr, size, |stack, global| {
|
|
stack.access(AccessKind::Read, ptr.tag, global)?;
|
|
Ok(())
|
|
})
|
|
}
|
|
|
|
#[inline(always)]
|
|
pub fn memory_written<'tcx>(
|
|
&mut self,
|
|
ptr: Pointer<Tag>,
|
|
size: Size,
|
|
) -> InterpResult<'tcx> {
|
|
trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
|
|
self.for_each(ptr, size, |stack, global| {
|
|
stack.access(AccessKind::Write, ptr.tag, global)?;
|
|
Ok(())
|
|
})
|
|
}
|
|
|
|
#[inline(always)]
|
|
pub fn memory_deallocated<'tcx>(
|
|
&mut self,
|
|
ptr: Pointer<Tag>,
|
|
size: Size,
|
|
) -> InterpResult<'tcx> {
|
|
trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
|
|
self.for_each(ptr, size, |stack, global| {
|
|
stack.dealloc(ptr.tag, 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> 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) } else { None };
|
|
let ptr = this.memory.check_ptr_access(place.ptr, size, place.align)
|
|
.expect("validity checks should have excluded dangling/unaligned pointer")
|
|
.expect("we shouldn't get here for ZST");
|
|
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 alloc = this.memory.get(ptr.alloc_id)?;
|
|
let stacked_borrows = alloc.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, |stack, global| {
|
|
stack.grant(cur_ptr.tag, item, global)
|
|
})
|
|
});
|
|
}
|
|
};
|
|
let item = Item { perm, tag: new_tag, protector };
|
|
stacked_borrows.for_each(ptr, size, |stack, global| {
|
|
stack.grant(ptr.tag, 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, Immediate<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);
|
|
if size == Size::ZERO {
|
|
// Nothing to do for ZSTs.
|
|
return Ok(*val);
|
|
}
|
|
let place = this.force_mplace_ptr(place)?;
|
|
|
|
// Compute new borrow.
|
|
let new_tag = match kind {
|
|
RefKind::Raw { .. } => Tag::Untagged,
|
|
_ => Tag::Tagged(this.memory.extra.stacked_borrows.borrow_mut().new_ptr()),
|
|
};
|
|
|
|
// Reborrow.
|
|
this.reborrow(place, size, kind, new_tag, protect)?;
|
|
let new_place = place.replace_tag(new_tag);
|
|
|
|
// Return new pointer.
|
|
Ok(new_place.to_ref())
|
|
}
|
|
}
|
|
|
|
impl<'mir, 'tcx> 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(_, _, MutMutable) =>
|
|
Some((RefKind::Unique { two_phase: kind == RetagKind::TwoPhase}, kind == RetagKind::FnEntry)),
|
|
ty::Ref(_, _, MutImmutable) =>
|
|
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 == MutMutable }, 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 <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
|
|
// 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(())
|
|
}
|
|
}
|