rust/src/stacked_borrows.rs

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use std::cell::RefCell;
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use rustc::ty::{self, layout::Size};
use rustc::hir;
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use crate::{
EvalResult, MiriEvalContext, HelpersEvalContextExt,
MemoryKind, MiriMemoryKind, RangeMap, AllocId, Allocation, AllocationExtra,
Pointer, PlaceTy, MPlaceTy,
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};
pub type Timestamp = u64;
/// Information about which kind of borrow was used to create the reference this is tagged
/// with.
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub enum Borrow {
/// A unique (mutable) reference.
Uniq(Timestamp),
/// A shared reference. This is also used by raw pointers, which do not track details
/// of how or when they were created, hence the timestamp is optional.
/// Shr(Some(_)) does NOT mean that the destination of this reference is frozen;
/// that depends on the type! Only those parts outside of an `UnsafeCell` are actually
/// frozen.
Shr(Option<Timestamp>),
}
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impl Borrow {
#[inline(always)]
pub fn is_shr(self) -> bool {
match self {
Borrow::Shr(_) => true,
_ => false,
}
}
#[inline(always)]
pub fn is_uniq(self) -> bool {
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match self {
Borrow::Uniq(_) => true,
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_ => false,
}
}
}
impl Default for Borrow {
fn default() -> Self {
Borrow::Shr(None)
}
}
/// An item in the per-location borrow stack
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub enum BorStackItem {
/// Indicates the unique reference that may mutate.
Uniq(Timestamp),
/// Indicates that the location has been shared. Used for raw pointers, but
/// also for shared references. The latter *additionally* get frozen
/// when there is no `UnsafeCell`.
Shr,
/// A barrier, tracking the function it belongs to by its index on the call stack
#[allow(dead_code)] // for future use
FnBarrier(usize)
}
impl BorStackItem {
#[inline(always)]
pub fn is_fn_barrier(self) -> bool {
match self {
BorStackItem::FnBarrier(_) => true,
_ => false,
}
}
}
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/// Extra per-location state
#[derive(Clone, Debug)]
pub struct Stack {
borrows: Vec<BorStackItem>, // used as a stack; never empty
frozen_since: Option<Timestamp>, // virtual frozen "item" on top of the stack
}
impl Default for Stack {
fn default() -> Self {
Stack {
borrows: vec![BorStackItem::Shr],
frozen_since: None,
}
}
}
impl Stack {
#[inline(always)]
pub fn is_frozen(&self) -> bool {
self.frozen_since.is_some()
}
}
/// What kind of usage of the pointer are we talking about?
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub enum UsageKind {
/// Write, or create &mut
Write,
/// Read, or create &
Read,
/// Create * (raw ptr)
Raw,
}
impl From<Option<hir::Mutability>> for UsageKind {
fn from(mutbl: Option<hir::Mutability>) -> Self {
match mutbl {
None => UsageKind::Raw,
Some(hir::MutMutable) => UsageKind::Write,
Some(hir::MutImmutable) => UsageKind::Read,
}
}
}
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/// Extra global machine state
#[derive(Clone, Debug)]
pub struct State {
clock: Timestamp
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}
impl State {
pub fn new() -> State {
State { clock: 0 }
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}
}
/// Extra per-allocation state
#[derive(Clone, Debug, Default)]
pub struct Stacks {
// Even reading memory can have effects on the stack, so we need a `RefCell` here.
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stacks: RefCell<RangeMap<Stack>>,
}
/// Core operations
impl<'tcx> Stack {
/// Check if `bor` could be activated by unfreezing and popping.
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/// `is_write` indicates whether this is being used to write (or, equivalently, to
/// borrow as &mut).
/// Returns `Err` if the answer is "no"; otherwise the return value indicates what to
/// do: With `Some(n)` you need to unfreeze, and then additionally pop `n` items.
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fn reactivatable(&self, bor: Borrow, is_write: bool) -> Result<Option<usize>, String> {
// Check if we can match the frozen "item". Not possible on writes!
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if !is_write {
// For now, we do NOT check the timestamp. That might be surprising, but
// we cannot even notice when a location should be frozen but is not!
// Those checks are both done in `tag_dereference`, where we have type information.
// Either way, it is crucial that the frozen "item" matches raw pointers:
// Reading through a raw should not unfreeze.
match (self.frozen_since, bor) {
(Some(_), Borrow::Shr(_)) => {
return Ok(None)
}
_ => {},
}
}
// See if we can find this borrow.
for (idx, &itm) in self.borrows.iter().rev().enumerate() {
// Check borrow and stack item for compatibility.
match (itm, bor) {
(BorStackItem::FnBarrier(_), _) => {
return Err(format!("Trying to reactivate a borrow ({:?}) that lives \
behind a barrier", bor))
}
(BorStackItem::Uniq(itm_t), Borrow::Uniq(bor_t)) if itm_t == bor_t => {
// Found matching unique item.
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if !is_write {
// As a special case, if we are reading and since we *did* find the `Uniq`,
// we try to pop less: We are happy with making a `Shr` or `Frz` active;
// that one will not mind concurrent reads.
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match self.reactivatable(Borrow::default(), is_write) {
// If we got something better that `idx`, use that
Ok(None) => return Ok(None),
Ok(Some(shr_idx)) if shr_idx <= idx => return Ok(Some(shr_idx)),
// Otherwise just go on.
_ => {},
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}
}
return Ok(Some(idx))
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}
(BorStackItem::Shr, Borrow::Shr(_)) => {
// Found matching shared/raw item.
return Ok(Some(idx))
}
// Go on looking.
_ => {}
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}
}
// Nothing to be found.
Err(format!("Borrow-to-reactivate {:?} does not exist on the stack", bor))
}
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/// Reactive `bor` for this stack. `is_write` indicates whether this is being
/// used to write (or, equivalently, to borrow as &mut).
fn reactivate(&mut self, bor: Borrow, is_write: bool) -> EvalResult<'tcx> {
let mut pop = match self.reactivatable(bor, is_write) {
Ok(None) => return Ok(()),
Ok(Some(pop)) => pop,
Err(err) => return err!(MachineError(err)),
};
// Pop what `reactivatable` told us to pop. Always unfreeze.
if self.is_frozen() {
trace!("reactivate: Unfreezing");
}
self.frozen_since = None;
while pop > 0 {
let itm = self.borrows.pop().unwrap();
trace!("reactivate: Popping {:?}", itm);
pop -= 1;
}
Ok(())
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}
/// Initiate `bor`; mostly this means pushing.
/// This operation cannot fail; it is up to the caller to ensure that the precondition
/// is met: We cannot push onto frozen stacks.
fn initiate(&mut self, bor: Borrow) {
if let Some(_) = self.frozen_since {
// "Pushing" a Shr or Frz on top is redundant.
match bor {
Borrow::Uniq(_) => bug!("Trying to create unique ref to frozen location"),
Borrow::Shr(_) => trace!("initiate: New shared ref to frozen location is a NOP"),
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}
} else {
// Just push.
let itm = match bor {
Borrow::Uniq(t) => BorStackItem::Uniq(t),
Borrow::Shr(_) if *self.borrows.last().unwrap() == BorStackItem::Shr => {
// Optimization: Don't push a Shr onto a Shr.
trace!("initiate: New shared ref to already shared location is a NOP");
return
},
Borrow::Shr(_) => BorStackItem::Shr,
};
trace!("initiate: Pushing {:?}", itm);
self.borrows.push(itm)
}
}
/// Check if this location is "frozen enough".
fn check_frozen(&self, bor_t: Timestamp) -> EvalResult<'tcx> {
let frozen = self.frozen_since.map_or(false, |itm_t| itm_t <= bor_t);
if !frozen {
err!(MachineError(format!("Location is not frozen long enough")))
} else {
Ok(())
}
}
/// Freeze this location, since `bor_t`.
fn freeze(&mut self, bor_t: Timestamp) {
if let Some(itm_t) = self.frozen_since {
assert!(itm_t <= bor_t, "Trying to freeze shorter than it was frozen?");
} else {
trace!("Freezing");
self.frozen_since = Some(bor_t);
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}
}
}
impl State {
fn increment_clock(&mut self) -> Timestamp {
let val = self.clock;
self.clock = val + 1;
val
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}
}
/// Higher-level operations
impl<'tcx> Stacks {
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/// The single most important operation: Make sure that using `ptr` is okay,
/// and if `new_bor` is present then make that the new current borrow.
fn use_and_maybe_re_borrow(
&self,
ptr: Pointer<Borrow>,
size: Size,
usage: UsageKind,
new_bor: Option<Borrow>,
) -> EvalResult<'tcx> {
trace!("use_and_maybe_re_borrow of tag {:?} as {:?}, new {:?}: {:?}, size {}",
ptr.tag, usage, new_bor, ptr, size.bytes());
let mut stacks = self.stacks.borrow_mut();
for stack in stacks.iter_mut(ptr.offset, size) {
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stack.reactivate(ptr.tag, usage == UsageKind::Write)?;
if let Some(new_bor) = new_bor {
stack.initiate(new_bor);
}
}
Ok(())
}
/// Freeze the given memory range.
fn freeze(
&self,
ptr: Pointer<Borrow>,
size: Size,
bor_t: Timestamp
) -> EvalResult<'tcx> {
let mut stacks = self.stacks.borrow_mut();
for stack in stacks.iter_mut(ptr.offset, size) {
stack.freeze(bor_t);
}
Ok(())
}
/// Check that this stack is fine with being dereferenced
fn check_deref(
&self,
ptr: Pointer<Borrow>,
size: Size,
) -> EvalResult<'tcx> {
let mut stacks = self.stacks.borrow_mut();
// We need `iter_mut` because `iter` would skip gaps!
for stack in stacks.iter_mut(ptr.offset, size) {
// Conservatively assume we will just read
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if let Err(err) = stack.reactivatable(ptr.tag, /*is_write*/false) {
return err!(MachineError(format!(
"Encountered reference with non-reactivatable tag: {}",
err
)))
}
}
Ok(())
}
/// Check that this stack is appropriately frozen
fn check_frozen(
&self,
ptr: Pointer<Borrow>,
size: Size,
bor_t: Timestamp
) -> EvalResult<'tcx> {
let mut stacks = self.stacks.borrow_mut();
for stack in stacks.iter_mut(ptr.offset, size) {
stack.check_frozen(bor_t)?;
}
Ok(())
}
}
/// Hooks and glue
impl AllocationExtra<Borrow> for Stacks {
#[inline(always)]
fn memory_read<'tcx>(
alloc: &Allocation<Borrow, Stacks>,
ptr: Pointer<Borrow>,
size: Size,
) -> EvalResult<'tcx> {
// Reads behave exactly like the first half of a reborrow-to-shr
alloc.extra.use_and_maybe_re_borrow(ptr, size, UsageKind::Read, None)
}
#[inline(always)]
fn memory_written<'tcx>(
alloc: &mut Allocation<Borrow, Stacks>,
ptr: Pointer<Borrow>,
size: Size,
) -> EvalResult<'tcx> {
// Writes behave exactly like the first half of a reborrow-to-mut
alloc.extra.use_and_maybe_re_borrow(ptr, size, UsageKind::Write, None)
}
}
impl<'tcx> Stacks {
#[inline(always)]
pub fn memory_deallocated(
&mut self,
ptr: Pointer<Borrow>,
size: Size,
) -> EvalResult<'tcx> {
// This is like mutating
self.use_and_maybe_re_borrow(ptr, size, UsageKind::Write, None)
// FIXME: Error out of there are any barriers?
}
/// Pushes the first item to the stacks.
pub fn first_item(
&mut self,
itm: BorStackItem,
size: Size
) {
assert!(!itm.is_fn_barrier());
for stack in self.stacks.get_mut().iter_mut(Size::ZERO, size) {
assert!(stack.borrows.len() == 1);
assert_eq!(stack.borrows.pop().unwrap(), BorStackItem::Shr);
stack.borrows.push(itm);
}
}
}
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pub trait EvalContextExt<'tcx> {
fn tag_reference(
&mut self,
place: MPlaceTy<'tcx, Borrow>,
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size: Size,
usage: UsageKind,
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) -> EvalResult<'tcx, Borrow>;
fn tag_dereference(
&self,
place: MPlaceTy<'tcx, Borrow>,
size: Size,
usage: UsageKind,
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) -> EvalResult<'tcx, Borrow>;
fn tag_new_allocation(
&mut self,
id: AllocId,
kind: MemoryKind<MiriMemoryKind>,
) -> Borrow;
fn retag(
&mut self,
fn_entry: bool,
place: PlaceTy<'tcx, Borrow>
) -> EvalResult<'tcx>;
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}
impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
/// Called for place-to-value conversion.
fn tag_reference(
&mut self,
place: MPlaceTy<'tcx, Borrow>,
size: Size,
usage: UsageKind,
) -> EvalResult<'tcx, Borrow> {
let ptr = place.ptr.to_ptr()?;
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let time = self.machine.stacked_borrows.increment_clock();
let new_bor = match usage {
UsageKind::Write => Borrow::Uniq(time),
UsageKind::Read => Borrow::Shr(Some(time)),
UsageKind::Raw => Borrow::Shr(None),
};
trace!("tag_reference: Creating new reference ({:?}) for {:?} (pointee {}): {:?}",
usage, ptr, place.layout.ty, new_bor);
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// Update the stacks. First create the new ref as usual, then maybe freeze stuff.
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self.memory().check_bounds(ptr, size, false)?;
let alloc = self.memory().get(ptr.alloc_id).expect("We checked that the ptr is fine!");
alloc.extra.use_and_maybe_re_borrow(ptr, size, usage, Some(new_bor))?;
// Maybe freeze stuff
if let Borrow::Shr(Some(bor_t)) = new_bor {
self.visit_frozen(place, size, |frz_ptr, size| {
debug_assert_eq!(frz_ptr.alloc_id, ptr.alloc_id);
// Be frozen!
alloc.extra.freeze(frz_ptr, size, bor_t)
})?;
}
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Ok(new_bor)
}
/// Called for value-to-place conversion.
///
/// Note that this does NOT mean that all this memory will actually get accessed/referenced!
/// We could be in the middle of `&(*var).1`.
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fn tag_dereference(
&self,
place: MPlaceTy<'tcx, Borrow>,
size: Size,
usage: UsageKind,
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) -> EvalResult<'tcx, Borrow> {
let ptr = place.ptr.to_ptr()?;
trace!("tag_dereference: Accessing reference ({:?}) for {:?} (pointee {})",
usage, ptr, place.layout.ty);
// In principle we should not have to do anything here. However, with transmutes involved,
// it can happen that the tag of `ptr` does not actually match `usage`, and we
// should adjust for that.
// Notably, the compiler can introduce such transmutes by optimizing away `&[mut]*`.
// That can transmute a raw ptr to a (shared/mut) ref, and a mut ref to a shared one.
match (usage, ptr.tag) {
(UsageKind::Raw, _) => {
// Don't use the tag, this is a raw access! Even if there is a tag,
// that means transmute happened and we ignore the tag.
// Also don't do any further validation, this is raw after all.
return Ok(Borrow::default());
}
(UsageKind::Write, Borrow::Uniq(_)) |
(UsageKind::Read, Borrow::Shr(_)) => {
// Expected combinations. Nothing to do.
}
(UsageKind::Write, Borrow::Shr(None)) => {
// Raw transmuted to mut ref. Keep this as raw access.
// We cannot reborrow here; there might be a raw in `&(*var).1` where
// `var` is an `&mut`. The other field of the struct might be already frozen,
// also using `var`, and that would be okay.
}
(UsageKind::Read, Borrow::Uniq(_)) => {
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// A mut got transmuted to shr. Can happen even from compiler transformations:
// `&*x` gets optimized to `x` even when `x` is a `&mut`.
}
(UsageKind::Write, Borrow::Shr(Some(_))) => {
// This is just invalid: A shr got transmuted to a mut.
// If we ever allow this, we have to consider what we do when a turn a
// `Raw`-tagged `&mut` into a raw pointer pointing to a frozen location.
// We probably do not want to allow that, but we have to allow
// turning a `Raw`-tagged `&` into a raw ptr to a frozen location.
return err!(MachineError(format!("Encountered mutable reference with frozen tag {:?}", ptr.tag)))
}
}
// If we got here, we do some checking, *but* we leave the tag unchanged.
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self.memory().check_bounds(ptr, size, false)?;
let alloc = self.memory().get(ptr.alloc_id).expect("We checked that the ptr is fine!");
alloc.extra.check_deref(ptr, size)?;
// Maybe check frozen stuff
if let Borrow::Shr(Some(bor_t)) = ptr.tag {
self.visit_frozen(place, size, |frz_ptr, size| {
debug_assert_eq!(frz_ptr.alloc_id, ptr.alloc_id);
// Are you frozen?
alloc.extra.check_frozen(frz_ptr, size, bor_t)
})?;
}
// All is good, and do not change the tag
Ok(ptr.tag)
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}
fn tag_new_allocation(
&mut self,
id: AllocId,
kind: MemoryKind<MiriMemoryKind>,
) -> Borrow {
let time = match kind {
MemoryKind::Stack => {
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// New unique borrow. This `Uniq` is not accessible by the program,
// so it will only ever be used when using the local directly (i.e.,
// not through a pointer). IOW, whenever we directly use a local this will pop
// everything else off the stack, invalidating all previous pointers
// and, in particular, *all* raw pointers. This subsumes the explicit
// `reset` which the blog post [1] says to perform when accessing a local.
//
// [1] https://www.ralfj.de/blog/2018/08/07/stacked-borrows.html
self.machine.stacked_borrows.increment_clock()
}
_ => {
// Nothing to do for everything else
return Borrow::default()
}
};
// Make this the active borrow for this allocation
let alloc = self.memory_mut().get_mut(id).expect("This is a new allocation, it must still exist");
let size = Size::from_bytes(alloc.bytes.len() as u64);
alloc.extra.first_item(BorStackItem::Uniq(time), size);
Borrow::Uniq(time)
}
fn retag(
&mut self,
_fn_entry: bool,
place: PlaceTy<'tcx, Borrow>
) -> EvalResult<'tcx> {
// For now, we only retag if the toplevel type is a reference.
// TODO: Recurse into structs and enums, sharing code with validation.
let mutbl = match place.layout.ty.sty {
ty::Ref(_, _, mutbl) => mutbl, // go ahead
_ => return Ok(()), // don't do a thing
};
// We want to reborrow the reference stored there. This will call the hooks
// above. First deref, which will call `tag_dereference`.
// (This is somewhat redundant because validation already did the same thing,
// but what can you do.)
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let val = self.read_immediate(self.place_to_op(place)?)?;
let dest = self.ref_to_mplace(val)?;
// Now put a new ref into the old place, which will call `tag_reference`.
// FIXME: Honor `fn_entry`!
let val = self.create_ref(dest, Some(mutbl))?;
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self.write_immediate(val, place)?;
Ok(())
}
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}