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//! Implements "Stacked Borrows". See <https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md>
//! for further information.
pub mod diagnostics;
mod item;
mod stack;
use std::cell::RefCell;
use std::cmp;
use std::fmt::Write;
use std::mem;
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::mir::{Mutability, RetagKind};
use rustc_middle::ty::{self, layout::HasParamEnv, Ty};
use rustc_target::abi::{Abi, Size};
use crate::borrow_tracker::{
stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder},
GlobalStateInner, ProtectorKind,
};
use crate::concurrency::data_race::{NaReadType, NaWriteType};
use crate::*;
use diagnostics::{RetagCause, RetagInfo};
pub use item::{Item, Permission};
pub use stack::Stack;
pub type AllocState = Stacks;
/// Extra per-allocation state.
#[derive(Clone, Debug)]
pub struct Stacks {
// Even reading memory can have effects on the stack, so we need a `RefCell` here.
stacks: RangeMap<Stack>,
/// Stores past operations on this allocation
history: AllocHistory,
/// The set of tags that have been exposed inside this allocation.
exposed_tags: FxHashSet<BorTag>,
}
/// Indicates which permissions to grant to the retagged pointer.
#[derive(Clone, Debug)]
enum NewPermission {
Uniform {
perm: Permission,
access: Option<AccessKind>,
protector: Option<ProtectorKind>,
},
FreezeSensitive {
freeze_perm: Permission,
freeze_access: Option<AccessKind>,
freeze_protector: Option<ProtectorKind>,
nonfreeze_perm: Permission,
nonfreeze_access: Option<AccessKind>,
// nonfreeze_protector must always be None
},
}
impl NewPermission {
/// A key function: determine the permissions to grant at a retag for the given kind of
/// reference/pointer.
fn from_ref_ty<'tcx>(ty: Ty<'tcx>, kind: RetagKind, cx: &crate::MiriInterpCx<'tcx>) -> Self {
let protector = (kind == RetagKind::FnEntry).then_some(ProtectorKind::StrongProtector);
match ty.kind() {
ty::Ref(_, pointee, Mutability::Mut) => {
if kind == RetagKind::TwoPhase {
// We mostly just give up on 2phase-borrows, and treat these exactly like raw pointers.
assert!(protector.is_none()); // RetagKind can't be both FnEntry and TwoPhase.
NewPermission::Uniform {
perm: Permission::SharedReadWrite,
access: None,
protector: None,
}
} else if pointee.is_unpin(*cx.tcx, cx.param_env()) {
// A regular full mutable reference. On `FnEntry` this is `noalias` and `dereferenceable`.
NewPermission::Uniform {
perm: Permission::Unique,
access: Some(AccessKind::Write),
protector,
}
} else {
// `!Unpin` dereferences do not get `noalias` nor `dereferenceable`.
NewPermission::Uniform {
perm: Permission::SharedReadWrite,
access: None,
protector: None,
}
}
}
ty::RawPtr(_, Mutability::Mut) => {
assert!(protector.is_none()); // RetagKind can't be both FnEntry and Raw.
// Mutable raw pointer. No access, not protected.
NewPermission::Uniform {
perm: Permission::SharedReadWrite,
access: None,
protector: None,
}
}
ty::Ref(_, _pointee, Mutability::Not) => {
// Shared references. If frozen, these get `noalias` and `dereferenceable`; otherwise neither.
NewPermission::FreezeSensitive {
freeze_perm: Permission::SharedReadOnly,
freeze_access: Some(AccessKind::Read),
freeze_protector: protector,
nonfreeze_perm: Permission::SharedReadWrite,
// Inside UnsafeCell, this does *not* count as an access, as there
// might actually be mutable references further up the stack that
// we have to keep alive.
nonfreeze_access: None,
// We do not protect inside UnsafeCell.
// This fixes https://github.com/rust-lang/rust/issues/55005.
}
}
ty::RawPtr(_, Mutability::Not) => {
assert!(protector.is_none()); // RetagKind can't be both FnEntry and Raw.
// `*const T`, when freshly created, are read-only in the frozen part.
NewPermission::FreezeSensitive {
freeze_perm: Permission::SharedReadOnly,
freeze_access: Some(AccessKind::Read),
freeze_protector: None,
nonfreeze_perm: Permission::SharedReadWrite,
nonfreeze_access: None,
}
}
_ => unreachable!(),
}
}
fn from_box_ty<'tcx>(ty: Ty<'tcx>, kind: RetagKind, cx: &crate::MiriInterpCx<'tcx>) -> Self {
// `ty` is not the `Box` but the field of the Box with this pointer (due to allocator handling).
let pointee = ty.builtin_deref(true).unwrap();
if pointee.is_unpin(*cx.tcx, cx.param_env()) {
// A regular box. On `FnEntry` this is `noalias`, but not `dereferenceable` (hence only
// a weak protector).
NewPermission::Uniform {
perm: Permission::Unique,
access: Some(AccessKind::Write),
protector: (kind == RetagKind::FnEntry).then_some(ProtectorKind::WeakProtector),
}
} else {
// `!Unpin` boxes do not get `noalias` nor `dereferenceable`.
NewPermission::Uniform {
perm: Permission::SharedReadWrite,
access: None,
protector: None,
}
}
}
fn protector(&self) -> Option<ProtectorKind> {
match self {
NewPermission::Uniform { protector, .. } => *protector,
NewPermission::FreezeSensitive { freeze_protector, .. } => *freeze_protector,
}
}
}
// # Stacked Borrows Core Begin
/// We need to make at least the following things true:
///
/// U1: After creating a `Uniq`, it is at the top.
/// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it.
/// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
///
/// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
/// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
/// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
/// gets popped.
/// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
/// F3: If an access happens with an `&` outside `UnsafeCell`,
/// it requires the `SharedReadOnly` to still be in the stack.
/// Core relation on `Permission` to define which accesses are allowed
impl Permission {
/// This defines for a given permission, whether it permits the given kind of access.
fn grants(self, access: AccessKind) -> bool {
// Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
self != Permission::Disabled
&& (access == AccessKind::Read || self != Permission::SharedReadOnly)
}
}
/// Determines whether an item was invalidated by a conflicting access, or by deallocation.
#[derive(Copy, Clone, Debug)]
enum ItemInvalidationCause {
Conflict,
Dealloc,
}
/// Core per-location operations: access, dealloc, reborrow.
impl<'tcx> Stack {
/// Find the first write-incompatible item above the given one --
/// i.e, find the height to which the stack will be truncated when writing to `granting`.
fn find_first_write_incompatible(&self, granting: usize) -> usize {
let perm = self.get(granting).unwrap().perm();
match perm {
Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
Permission::Disabled => bug!("Cannot use Disabled for anything"),
Permission::Unique => {
// On a write, everything above us is incompatible.
granting + 1
}
Permission::SharedReadWrite => {
// The SharedReadWrite *just* above us are compatible, to skip those.
let mut idx = granting + 1;
while let Some(item) = self.get(idx) {
if item.perm() == Permission::SharedReadWrite {
// Go on.
idx += 1;
} else {
// Found first incompatible!
break;
}
}
idx
}
}
}
/// The given item was invalidated -- check its protectors for whether that will cause UB.
fn item_invalidated(
item: &Item,
global: &GlobalStateInner,
dcx: &DiagnosticCx<'_, '_, 'tcx>,
cause: ItemInvalidationCause,
) -> InterpResult<'tcx> {
if !global.tracked_pointer_tags.is_empty() {
dcx.check_tracked_tag_popped(item, global);
}
if !item.protected() {
return Ok(());
}
// We store tags twice, once in global.protected_tags and once in each call frame.
// We do this because consulting a single global set in this function is faster
// than attempting to search all call frames in the program for the `FrameExtra`
// (if any) which is protecting the popped tag.
//
// This duplication trades off making `end_call` slower to make this function faster. This
// trade-off is profitable in practice for a combination of two reasons.
// 1. A single protected tag can (and does in some programs) protect thousands of `Item`s.
// Therefore, adding overhead in function call/return is profitable even if it only
// saves a little work in this function.
// 2. Most frames protect only one or two tags. So this duplicative global turns a search
// which ends up about linear in the number of protected tags in the program into a
// constant time check (and a slow linear, because the tags in the frames aren't contiguous).
if let Some(&protector_kind) = global.protected_tags.get(&item.tag()) {
// The only way this is okay is if the protector is weak and we are deallocating with
// the right pointer.
let allowed = matches!(cause, ItemInvalidationCause::Dealloc)
&& matches!(protector_kind, ProtectorKind::WeakProtector);
if !allowed {
return Err(dcx.protector_error(item, protector_kind).into());
}
}
Ok(())
}
/// Test if a memory `access` using pointer tagged `tag` is granted.
/// If yes, return the index of the item that granted it.
/// `range` refers the entire operation, and `offset` refers to the specific offset into the
/// allocation that we are currently checking.
fn access(
&mut self,
access: AccessKind,
tag: ProvenanceExtra,
global: &GlobalStateInner,
dcx: &mut DiagnosticCx<'_, '_, 'tcx>,
exposed_tags: &FxHashSet<BorTag>,
) -> InterpResult<'tcx> {
// Two main steps: Find granting item, remove incompatible items above.
// Step 1: Find granting item.
let granting_idx =
self.find_granting(access, tag, exposed_tags).map_err(|()| dcx.access_error(self))?;
// Step 2: Remove incompatible items above them. Make sure we do not remove protected
// items. Behavior differs for reads and writes.
// In case of wildcards/unknown matches, we remove everything that is *definitely* gone.
if access == AccessKind::Write {
// Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
// pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
let first_incompatible_idx = if let Some(granting_idx) = granting_idx {
// The granting_idx *might* be approximate, but any lower idx would remove more
// things. Even if this is a Unique and the lower idx is an SRW (which removes
// less), there is an SRW group boundary here so strictly more would get removed.
self.find_first_write_incompatible(granting_idx)
} else {
// We are writing to something in the unknown part.
// There is a SRW group boundary between the unknown and the known, so everything is incompatible.
0
};
self.pop_items_after(first_incompatible_idx, |item| {
Stack::item_invalidated(&item, global, dcx, ItemInvalidationCause::Conflict)?;
dcx.log_invalidation(item.tag());
Ok(())
})?;
} else {
// On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
// The reason this is not following the stack discipline (by removing the first Unique and
// everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
// would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
// `SharedReadWrite` for `raw`.
// This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
// reference and use that.
// We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
let first_incompatible_idx = if let Some(granting_idx) = granting_idx {
// The granting_idx *might* be approximate, but any lower idx would disable more things.
granting_idx + 1
} else {
// We are reading from something in the unknown part. That means *all* `Unique` we know about are dead now.
0
};
self.disable_uniques_starting_at(first_incompatible_idx, |item| {
Stack::item_invalidated(&item, global, dcx, ItemInvalidationCause::Conflict)?;
dcx.log_invalidation(item.tag());
Ok(())
})?;
}
// If this was an approximate action, we now collapse everything into an unknown.
if granting_idx.is_none() || matches!(tag, ProvenanceExtra::Wildcard) {
// Compute the upper bound of the items that remain.
// (This is why we did all the work above: to reduce the items we have to consider here.)
let mut max = BorTag::one();
for i in 0..self.len() {
let item = self.get(i).unwrap();
// Skip disabled items, they cannot be matched anyway.
if !matches!(item.perm(), Permission::Disabled) {
// We are looking for a strict upper bound, so add 1 to this tag.
max = cmp::max(item.tag().succ().unwrap(), max);
}
}
if let Some(unk) = self.unknown_bottom() {
max = cmp::max(unk, max);
}
// Use `max` as new strict upper bound for everything.
trace!(
"access: forgetting stack to upper bound {max} due to wildcard or unknown access",
max = max.get(),
);
self.set_unknown_bottom(max);
}
// Done.
Ok(())
}
/// Deallocate a location: Like a write access, but also there must be no
/// active protectors at all because we will remove all items.
fn dealloc(
&mut self,
tag: ProvenanceExtra,
global: &GlobalStateInner,
dcx: &mut DiagnosticCx<'_, '_, 'tcx>,
exposed_tags: &FxHashSet<BorTag>,
) -> InterpResult<'tcx> {
// Step 1: Make a write access.
// As part of this we do regular protector checking, i.e. even weakly protected items cause UB when popped.
self.access(AccessKind::Write, tag, global, dcx, exposed_tags)?;
// Step 2: Pretend we remove the remaining items, checking if any are strongly protected.
for idx in (0..self.len()).rev() {
let item = self.get(idx).unwrap();
Stack::item_invalidated(&item, global, dcx, ItemInvalidationCause::Dealloc)?;
}
Ok(())
}
/// Derive a new pointer from one with the given tag.
///
/// `access` indicates which kind of memory access this retag itself should correspond to.
fn grant(
&mut self,
derived_from: ProvenanceExtra,
new: Item,
access: Option<AccessKind>,
global: &GlobalStateInner,
dcx: &mut DiagnosticCx<'_, '_, 'tcx>,
exposed_tags: &FxHashSet<BorTag>,
) -> InterpResult<'tcx> {
dcx.start_grant(new.perm());
// Compute where to put the new item.
// Either way, we ensure that we insert the new item in a way such that between
// `derived_from` and the new one, there are only items *compatible with* `derived_from`.
let new_idx = if let Some(access) = access {
// Simple case: We are just a regular memory access, and then push our thing on top,
// like a regular stack.
// This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
self.access(access, derived_from, global, dcx, exposed_tags)?;
// We insert "as far up as possible": We know only compatible items are remaining
// on top of `derived_from`, and we want the new item at the top so that we
// get the strongest possible guarantees.
// This ensures U1 and F1.
self.len()
} else {
// The tricky case: creating a new SRW permission without actually being an access.
assert!(new.perm() == Permission::SharedReadWrite);
// First we figure out which item grants our parent (`derived_from`) this kind of access.
// We use that to determine where to put the new item.
let granting_idx = self
.find_granting(AccessKind::Write, derived_from, exposed_tags)
.map_err(|()| dcx.grant_error(self))?;
let (Some(granting_idx), ProvenanceExtra::Concrete(_)) = (granting_idx, derived_from)
else {
// The parent is a wildcard pointer or matched the unknown bottom.
// This is approximate. Nobody knows what happened, so forget everything.
// The new thing is SRW anyway, so we cannot push it "on top of the unknown part"
// (for all we know, it might join an SRW group inside the unknown).
trace!(
"reborrow: forgetting stack entirely due to SharedReadWrite reborrow from wildcard or unknown"
);
self.set_unknown_bottom(global.next_ptr_tag);
return Ok(());
};
// SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
// access. Instead of popping the stack, we insert the item at the place the stack would
// be popped to (i.e., we insert it above all the write-compatible items).
// This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
self.find_first_write_incompatible(granting_idx)
};
// Put the new item there.
trace!("reborrow: adding item {:?}", new);
self.insert(new_idx, new);
Ok(())
}
}
// # Stacked Borrows Core End
/// Integration with the BorTag garbage collector
impl Stacks {
pub fn remove_unreachable_tags(&mut self, live_tags: &FxHashSet<BorTag>) {
for (_stack_range, stack) in self.stacks.iter_mut_all() {
stack.retain(live_tags);
}
self.history.retain(live_tags);
}
}
impl VisitProvenance for Stacks {
fn visit_provenance(&self, visit: &mut VisitWith<'_>) {
for tag in self.exposed_tags.iter().copied() {
visit(None, Some(tag));
}
}
}
/// Map per-stack operations to higher-level per-location-range operations.
impl<'tcx> Stacks {
/// Creates a new stack with an initial tag. For diagnostic purposes, we also need to know
/// the [`AllocId`] of the allocation this is associated with.
fn new(
size: Size,
perm: Permission,
tag: BorTag,
id: AllocId,
machine: &MiriMachine<'_>,
) -> Self {
let item = Item::new(tag, perm, false);
let stack = Stack::new(item);
Stacks {
stacks: RangeMap::new(size, stack),
history: AllocHistory::new(id, item, machine),
exposed_tags: FxHashSet::default(),
}
}
/// Call `f` on every stack in the range.
fn for_each(
&mut self,
range: AllocRange,
mut dcx_builder: DiagnosticCxBuilder<'_, 'tcx>,
mut f: impl FnMut(
&mut Stack,
&mut DiagnosticCx<'_, '_, 'tcx>,
&mut FxHashSet<BorTag>,
) -> InterpResult<'tcx>,
) -> InterpResult<'tcx> {
for (stack_range, stack) in self.stacks.iter_mut(range.start, range.size) {
let mut dcx = dcx_builder.build(&mut self.history, Size::from_bytes(stack_range.start));
f(stack, &mut dcx, &mut self.exposed_tags)?;
dcx_builder = dcx.unbuild();
}
Ok(())
}
}
/// Glue code to connect with Miri Machine Hooks
impl Stacks {
pub fn new_allocation(
id: AllocId,
size: Size,
state: &mut GlobalStateInner,
kind: MemoryKind,
machine: &MiriMachine<'_>,
) -> Self {
let (base_tag, perm) = match kind {
// New unique borrow. This tag is not accessible by the program,
// so it will only ever be used when using the local directly (i.e.,
// not through a pointer). That is, whenever we directly write to a local, this will pop
// everything else off the stack, invalidating all previous pointers,
// and in particular, *all* raw pointers.
MemoryKind::Stack => (state.root_ptr_tag(id, machine), Permission::Unique),
// Everything else is shared by default.
_ => (state.root_ptr_tag(id, machine), Permission::SharedReadWrite),
};
Stacks::new(size, perm, base_tag, id, machine)
}
#[inline(always)]
pub fn before_memory_read<'ecx, 'tcx>(
&mut self,
alloc_id: AllocId,
tag: ProvenanceExtra,
range: AllocRange,
machine: &'ecx MiriMachine<'tcx>,
) -> InterpResult<'tcx>
where
'tcx: 'ecx,
{
trace!(
"read access with tag {:?}: {:?}, size {}",
tag,
interpret::Pointer::new(alloc_id, range.start),
range.size.bytes()
);
let dcx = DiagnosticCxBuilder::read(machine, tag, range);
let state = machine.borrow_tracker.as_ref().unwrap().borrow();
self.for_each(range, dcx, |stack, dcx, exposed_tags| {
stack.access(AccessKind::Read, tag, &state, dcx, exposed_tags)
})
}
#[inline(always)]
pub fn before_memory_write<'tcx>(
&mut self,
alloc_id: AllocId,
tag: ProvenanceExtra,
range: AllocRange,
machine: &MiriMachine<'tcx>,
) -> InterpResult<'tcx> {
trace!(
"write access with tag {:?}: {:?}, size {}",
tag,
interpret::Pointer::new(alloc_id, range.start),
range.size.bytes()
);
let dcx = DiagnosticCxBuilder::write(machine, tag, range);
let state = machine.borrow_tracker.as_ref().unwrap().borrow();
self.for_each(range, dcx, |stack, dcx, exposed_tags| {
stack.access(AccessKind::Write, tag, &state, dcx, exposed_tags)
})
}
#[inline(always)]
pub fn before_memory_deallocation<'tcx>(
&mut self,
alloc_id: AllocId,
tag: ProvenanceExtra,
size: Size,
machine: &MiriMachine<'tcx>,
) -> InterpResult<'tcx> {
trace!("deallocation with tag {:?}: {:?}, size {}", tag, alloc_id, size.bytes());
let dcx = DiagnosticCxBuilder::dealloc(machine, tag);
let state = machine.borrow_tracker.as_ref().unwrap().borrow();
self.for_each(alloc_range(Size::ZERO, size), dcx, |stack, dcx, exposed_tags| {
stack.dealloc(tag, &state, dcx, exposed_tags)
})?;
Ok(())
}
}
/// Retagging/reborrowing. There is some policy in here, such as which permissions
/// to grant for which references, and when to add protectors.
impl<'tcx, 'ecx> EvalContextPrivExt<'tcx, 'ecx> for crate::MiriInterpCx<'tcx> {}
trait EvalContextPrivExt<'tcx, 'ecx>: crate::MiriInterpCxExt<'tcx> {
/// Returns the provenance that should be used henceforth.
fn sb_reborrow(
&mut self,
place: &MPlaceTy<'tcx>,
size: Size,
new_perm: NewPermission,
new_tag: BorTag,
retag_info: RetagInfo, // diagnostics info about this retag
) -> InterpResult<'tcx, Option<Provenance>> {
let this = self.eval_context_mut();
// Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
this.check_ptr_access(place.ptr(), size, CheckInAllocMsg::InboundsTest)?;
// It is crucial that this gets called on all code paths, to ensure we track tag creation.
let log_creation = |this: &MiriInterpCx<'tcx>,
loc: Option<(AllocId, Size, ProvenanceExtra)>| // alloc_id, base_offset, orig_tag
-> InterpResult<'tcx> {
let global = this.machine.borrow_tracker.as_ref().unwrap().borrow();
let ty = place.layout.ty;
if global.tracked_pointer_tags.contains(&new_tag) {
let mut kind_str = String::new();
match new_perm {
NewPermission::Uniform { perm, .. } =>
write!(kind_str, "{perm:?} permission").unwrap(),
NewPermission::FreezeSensitive { freeze_perm, .. } if ty.is_freeze(*this.tcx, this.param_env()) =>
write!(kind_str, "{freeze_perm:?} permission").unwrap(),
NewPermission::FreezeSensitive { freeze_perm, nonfreeze_perm, .. } =>
write!(kind_str, "{freeze_perm:?}/{nonfreeze_perm:?} permission for frozen/non-frozen parts").unwrap(),
}
write!(kind_str, " (pointee type {ty})").unwrap();
this.emit_diagnostic(NonHaltingDiagnostic::CreatedPointerTag(
new_tag.inner(),
Some(kind_str),
loc.map(|(alloc_id, base_offset, orig_tag)| (alloc_id, alloc_range(base_offset, size), orig_tag)),
));
}
drop(global); // don't hold that reference any longer than we have to
let Some((alloc_id, base_offset, orig_tag)) = loc else {
return Ok(())
};
let (_size, _align, alloc_kind) = this.get_alloc_info(alloc_id);
match alloc_kind {
AllocKind::LiveData => {
// This should have alloc_extra data, but `get_alloc_extra` can still fail
// if converting this alloc_id from a global to a local one
// uncovers a non-supported `extern static`.
let extra = this.get_alloc_extra(alloc_id)?;
let mut stacked_borrows = extra
.borrow_tracker_sb()
.borrow_mut();
// Note that we create a *second* `DiagnosticCxBuilder` below for the actual retag.
// FIXME: can this be done cleaner?
let dcx = DiagnosticCxBuilder::retag(
&this.machine,
retag_info,
new_tag,
orig_tag,
alloc_range(base_offset, size),
);
let mut dcx = dcx.build(&mut stacked_borrows.history, base_offset);
dcx.log_creation();
if new_perm.protector().is_some() {
dcx.log_protector();
}
},
AllocKind::Function | AllocKind::VTable | AllocKind::Dead => {
// No stacked borrows on these allocations.
}
}
Ok(())
};
if size == Size::ZERO {
trace!(
"reborrow of size 0: reference {:?} derived from {:?} (pointee {})",
new_tag,
place.ptr(),
place.layout.ty,
);
// Don't update any stacks for a zero-sized access; borrow stacks are per-byte and this
// touches no bytes so there is no stack to put this tag in.
// However, if the pointer for this operation points at a real allocation we still
// record where it was created so that we can issue a helpful diagnostic if there is an
// attempt to use it for a non-zero-sized access.
// Dangling slices are a common case here; it's valid to get their length but with raw
// pointer tagging for example all calls to get_unchecked on them are invalid.
if let Ok((alloc_id, base_offset, orig_tag)) = this.ptr_try_get_alloc_id(place.ptr(), 0)
{
log_creation(this, Some((alloc_id, base_offset, orig_tag)))?;
// Still give it the new provenance, it got retagged after all.
return Ok(Some(Provenance::Concrete { alloc_id, tag: new_tag }));
} else {
// This pointer doesn't come with an AllocId. :shrug:
log_creation(this, None)?;
// Provenance unchanged.
return Ok(place.ptr().provenance);
}
}
let (alloc_id, base_offset, orig_tag) = this.ptr_get_alloc_id(place.ptr(), 0)?;
log_creation(this, Some((alloc_id, base_offset, orig_tag)))?;
trace!(
"reborrow: reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
new_tag,
orig_tag,
place.layout.ty,
interpret::Pointer::new(alloc_id, base_offset),
size.bytes()
);
if let Some(protect) = new_perm.protector() {
// See comment in `Stack::item_invalidated` for why we store the tag twice.
this.frame_mut()
.extra
.borrow_tracker
.as_mut()
.unwrap()
.protected_tags
.push((alloc_id, new_tag));
this.machine
.borrow_tracker
.as_mut()
.unwrap()
.get_mut()
.protected_tags
.insert(new_tag, protect);
}
// Update the stacks, according to the new permission information we are given.
match new_perm {
NewPermission::Uniform { perm, access, protector } => {
assert!(perm != Permission::SharedReadOnly);
// Here we can avoid `borrow()` calls because we have mutable references.
// Note that this asserts that the allocation is mutable -- but since we are creating a
// mutable pointer, that seems reasonable.
let (alloc_extra, machine) = this.get_alloc_extra_mut(alloc_id)?;
let stacked_borrows = alloc_extra.borrow_tracker_sb_mut().get_mut();
let item = Item::new(new_tag, perm, protector.is_some());
let range = alloc_range(base_offset, size);
let global = machine.borrow_tracker.as_ref().unwrap().borrow();
let dcx = DiagnosticCxBuilder::retag(
machine,
retag_info,
new_tag,
orig_tag,
alloc_range(base_offset, size),
);
stacked_borrows.for_each(range, dcx, |stack, dcx, exposed_tags| {
stack.grant(orig_tag, item, access, &global, dcx, exposed_tags)
})?;
drop(global);
if let Some(access) = access {
assert_eq!(access, AccessKind::Write);
// Make sure the data race model also knows about this.
if let Some(data_race) = alloc_extra.data_race.as_mut() {
data_race.write(
alloc_id,
range,
NaWriteType::Retag,
Some(place.layout.ty),
machine,
)?;
}
}
}
NewPermission::FreezeSensitive {
freeze_perm,
freeze_access,
freeze_protector,
nonfreeze_perm,
nonfreeze_access,
} => {
// The permission is not uniform across the entire range!
// We need a frozen-sensitive reborrow.
// We have to use shared references to alloc/memory_extra here since
// `visit_freeze_sensitive` needs to access the global state.
let alloc_extra = this.get_alloc_extra(alloc_id)?;
let mut stacked_borrows = alloc_extra.borrow_tracker_sb().borrow_mut();
this.visit_freeze_sensitive(place, size, |mut range, frozen| {
// Adjust range.
range.start += base_offset;
// We are only ever `SharedReadOnly` inside the frozen bits.
let (perm, access, protector) = if frozen {
(freeze_perm, freeze_access, freeze_protector)
} else {
(nonfreeze_perm, nonfreeze_access, None)
};
let item = Item::new(new_tag, perm, protector.is_some());
let global = this.machine.borrow_tracker.as_ref().unwrap().borrow();
let dcx = DiagnosticCxBuilder::retag(
&this.machine,
retag_info,
new_tag,
orig_tag,
alloc_range(base_offset, size),
);
stacked_borrows.for_each(range, dcx, |stack, dcx, exposed_tags| {
stack.grant(orig_tag, item, access, &global, dcx, exposed_tags)
})?;
drop(global);
if let Some(access) = access {
assert_eq!(access, AccessKind::Read);
// Make sure the data race model also knows about this.
if let Some(data_race) = alloc_extra.data_race.as_ref() {
data_race.read(
alloc_id,
range,
NaReadType::Retag,
Some(place.layout.ty),
&this.machine,
)?;
}
}
Ok(())
})?;
}
}
Ok(Some(Provenance::Concrete { alloc_id, tag: new_tag }))
}
fn sb_retag_place(
&mut self,
place: &MPlaceTy<'tcx>,
new_perm: NewPermission,
info: RetagInfo, // diagnostics info about this retag
) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
let this = self.eval_context_mut();
let size = this.size_and_align_of_mplace(place)?.map(|(size, _)| size);
// FIXME: If we cannot determine the size (because the unsized tail is an `extern type`),
// bail out -- we cannot reasonably figure out which memory range to reborrow.
// See https://github.com/rust-lang/unsafe-code-guidelines/issues/276.
let size = match size {
Some(size) => size,
None => {
// The first time this happens, show a warning.
thread_local! { static WARNING_SHOWN: RefCell<bool> = const { RefCell::new(false) }; }
WARNING_SHOWN.with_borrow_mut(|shown| {
if *shown {
return;
}
// Not yet shown. Show it!
*shown = true;
this.emit_diagnostic(NonHaltingDiagnostic::ExternTypeReborrow);
});
return Ok(place.clone());
}
};
// Compute new borrow.
let new_tag = this.machine.borrow_tracker.as_mut().unwrap().get_mut().new_ptr();
// Reborrow.
let new_prov = this.sb_reborrow(place, size, new_perm, new_tag, info)?;
// Adjust place.
// (If the closure gets called, that means the old provenance was `Some`, and hence the new
// one must also be `Some`.)
Ok(place.clone().map_provenance(|_| new_prov.unwrap()))
}
/// Retags an individual pointer, returning the retagged version.
/// `kind` indicates what kind of reference is being created.
fn sb_retag_reference(
&mut self,
val: &ImmTy<'tcx>,
new_perm: NewPermission,
info: RetagInfo, // diagnostics info about this retag
) -> InterpResult<'tcx, ImmTy<'tcx>> {
let this = self.eval_context_mut();
let place = this.ref_to_mplace(val)?;
let new_place = this.sb_retag_place(&place, new_perm, info)?;
Ok(ImmTy::from_immediate(new_place.to_ref(this), val.layout))
}
}
impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
fn sb_retag_ptr_value(
&mut self,
kind: RetagKind,
val: &ImmTy<'tcx>,
) -> InterpResult<'tcx, ImmTy<'tcx>> {
let this = self.eval_context_mut();
let new_perm = NewPermission::from_ref_ty(val.layout.ty, kind, this);
let cause = match kind {
RetagKind::TwoPhase { .. } => RetagCause::TwoPhase,
RetagKind::FnEntry => unreachable!(),
RetagKind::Raw | RetagKind::Default => RetagCause::Normal,
};
this.sb_retag_reference(val, new_perm, RetagInfo { cause, in_field: false })
}
fn sb_retag_place_contents(
&mut self,
kind: RetagKind,
place: &PlaceTy<'tcx>,
) -> InterpResult<'tcx> {
let this = self.eval_context_mut();
let retag_fields = this.machine.borrow_tracker.as_mut().unwrap().get_mut().retag_fields;
let retag_cause = match kind {
RetagKind::TwoPhase { .. } => unreachable!(), // can only happen in `retag_ptr_value`
RetagKind::FnEntry => RetagCause::FnEntry,
RetagKind::Default | RetagKind::Raw => RetagCause::Normal,
};
let mut visitor =
RetagVisitor { ecx: this, kind, retag_cause, retag_fields, in_field: false };
return visitor.visit_value(place);
// The actual visitor.
struct RetagVisitor<'ecx, 'tcx> {
ecx: &'ecx mut MiriInterpCx<'tcx>,
kind: RetagKind,
retag_cause: RetagCause,
retag_fields: RetagFields,
in_field: bool,
}
impl<'ecx, 'tcx> RetagVisitor<'ecx, 'tcx> {
#[inline(always)] // yes this helps in our benchmarks
fn retag_ptr_inplace(
&mut self,
place: &PlaceTy<'tcx>,
new_perm: NewPermission,
) -> InterpResult<'tcx> {
let val = self.ecx.read_immediate(&self.ecx.place_to_op(place)?)?;
let val = self.ecx.sb_retag_reference(
&val,
new_perm,
RetagInfo { cause: self.retag_cause, in_field: self.in_field },
)?;
self.ecx.write_immediate(*val, place)?;
Ok(())
}
}
impl<'ecx, 'tcx> ValueVisitor<'tcx, MiriMachine<'tcx>> for RetagVisitor<'ecx, 'tcx> {
type V = PlaceTy<'tcx>;
#[inline(always)]
fn ecx(&self) -> &MiriInterpCx<'tcx> {
self.ecx
}
fn visit_box(&mut self, box_ty: Ty<'tcx>, place: &PlaceTy<'tcx>) -> InterpResult<'tcx> {
// Only boxes for the global allocator get any special treatment.
if box_ty.is_box_global(*self.ecx.tcx) {
// Boxes get a weak protectors, since they may be deallocated.
let new_perm = NewPermission::from_box_ty(place.layout.ty, self.kind, self.ecx);
self.retag_ptr_inplace(place, new_perm)?;
}
Ok(())
}
fn visit_value(&mut self, place: &PlaceTy<'tcx>) -> InterpResult<'tcx> {
// If this place is smaller than a pointer, we know that it can't contain any
// pointers we need to retag, so we can stop recursion early.
// This optimization is crucial for ZSTs, because they can contain way more fields
// than we can ever visit.
if place.layout.is_sized() && place.layout.size < self.ecx.pointer_size() {
return Ok(());
}
// Check the type of this value to see what to do with it (retag, or recurse).
match place.layout.ty.kind() {
ty::Ref(..) | ty::RawPtr(..) => {
if matches!(place.layout.ty.kind(), ty::Ref(..))
|| self.kind == RetagKind::Raw
{
let new_perm =
NewPermission::from_ref_ty(place.layout.ty, self.kind, self.ecx);
self.retag_ptr_inplace(place, new_perm)?;
}
}
ty::Adt(adt, _) if adt.is_box() => {
// Recurse for boxes, they require some tricky handling and will end up in `visit_box` above.
// (Yes this means we technically also recursively retag the allocator itself
// even if field retagging is not enabled. *shrug*)
self.walk_value(place)?;
}
_ => {
// Not a reference/pointer/box. Only recurse if configured appropriately.
let recurse = match self.retag_fields {
RetagFields::No => false,
RetagFields::Yes => true,
RetagFields::OnlyScalar => {
// Matching `ArgAbi::new` at the time of writing, only fields of
// `Scalar` and `ScalarPair` ABI are considered.
matches!(place.layout.abi, Abi::Scalar(..) | Abi::ScalarPair(..))
}
};
if recurse {
let in_field = mem::replace(&mut self.in_field, true); // remember and restore old value
self.walk_value(place)?;
self.in_field = in_field;
}
}
}
Ok(())
}
}
}
/// Protect a place so that it cannot be used any more for the duration of the current function
/// call.
///
/// This is used to ensure soundness of in-place function argument/return passing.
fn sb_protect_place(&mut self, place: &MPlaceTy<'tcx>) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
let this = self.eval_context_mut();
// Retag it. With protection! That is the entire point.
let new_perm = NewPermission::Uniform {
perm: Permission::Unique,
access: Some(AccessKind::Write),
protector: Some(ProtectorKind::StrongProtector),
};
this.sb_retag_place(
place,
new_perm,
RetagInfo { cause: RetagCause::InPlaceFnPassing, in_field: false },
)
}
/// Mark the given tag as exposed. It was found on a pointer with the given AllocId.
fn sb_expose_tag(&mut self, alloc_id: AllocId, tag: BorTag) -> InterpResult<'tcx> {
let this = self.eval_context_mut();
// Function pointers and dead objects don't have an alloc_extra so we ignore them.
// This is okay because accessing them is UB anyway, no need for any Stacked Borrows checks.
// NOT using `get_alloc_extra_mut` since this might be a read-only allocation!
let (_size, _align, kind) = this.get_alloc_info(alloc_id);
match kind {
AllocKind::LiveData => {
// This should have alloc_extra data, but `get_alloc_extra` can still fail
// if converting this alloc_id from a global to a local one
// uncovers a non-supported `extern static`.
let alloc_extra = this.get_alloc_extra(alloc_id)?;
trace!("Stacked Borrows tag {tag:?} exposed in {alloc_id:?}");
alloc_extra.borrow_tracker_sb().borrow_mut().exposed_tags.insert(tag);
}
AllocKind::Function | AllocKind::VTable | AllocKind::Dead => {
// No stacked borrows on these allocations.
}
}
Ok(())
}
fn print_stacks(&mut self, alloc_id: AllocId) -> InterpResult<'tcx> {
let this = self.eval_context_mut();
let alloc_extra = this.get_alloc_extra(alloc_id)?;
let stacks = alloc_extra.borrow_tracker_sb().borrow();
for (range, stack) in stacks.stacks.iter_all() {
print!("{range:?}: [");
if let Some(bottom) = stack.unknown_bottom() {
print!(" unknown-bottom(..{bottom:?})");
}
for i in 0..stack.len() {
let item = stack.get(i).unwrap();
print!(" {:?}{:?}", item.perm(), item.tag());
}
println!(" ]");
}
Ok(())
}
}