Optional semantics for Unique

README.md

@@ -435,6 +435,9 @@ to Miri failing to detect cases of undefined behavior in a program.
   so with this flag.
 * `-Zmiri-force-page-size=<num>` overrides the default page size for an architecture, in multiples of 1k.
   `4` is default for most targets. This value should always be a power of 2 and nonzero.
+* `-Zmiri-unique-is-unique` performs additional aliasing checks for `core::ptr::Unique` to ensure
+  that it could theoretically be considered `noalias`. This flag is experimental and has
+  an effect only when used with `-Zmiri-tree-borrows`.
 
 [function ABI]: https://doc.rust-lang.org/reference/items/functions.html#extern-function-qualifier
 

src/bin/miri.rs

@@ -340,6 +340,8 @@ fn main() {
             miri_config.borrow_tracker = None;
         } else if arg == "-Zmiri-tree-borrows" {
             miri_config.borrow_tracker = Some(BorrowTrackerMethod::TreeBorrows);
+        } else if arg == "-Zmiri-unique-is-unique" {
+            miri_config.unique_is_unique = true;
         } else if arg == "-Zmiri-disable-data-race-detector" {
             miri_config.data_race_detector = false;
             miri_config.weak_memory_emulation = false;
@@ -557,6 +559,14 @@ fn main() {
             rustc_args.push(arg);
         }
     }
+    // `-Zmiri-unique-is-unique` should only be used with `-Zmiri-tree-borrows`
+    if miri_config.unique_is_unique
+        && !matches!(miri_config.borrow_tracker, Some(BorrowTrackerMethod::TreeBorrows))
+    {
+        show_error!(
+            "-Zmiri-unique-is-unique only has an effect when -Zmiri-tree-borrows is also used"
+        );
+    }
 
     debug!("rustc arguments: {:?}", rustc_args);
     debug!("crate arguments: {:?}", miri_config.args);

src/borrow_tracker/mod.rs

@@ -103,6 +103,8 @@ pub struct GlobalStateInner {
     pub tracked_call_ids: FxHashSet<CallId>,
     /// Whether to recurse into datatypes when searching for pointers to retag.
     pub retag_fields: RetagFields,
+    /// Whether `core::ptr::Unique` gets special (`Box`-like) handling.
+    pub unique_is_unique: bool,
 }
 
 impl VisitTags for GlobalStateInner {
@@ -170,6 +172,7 @@ impl GlobalStateInner {
         tracked_pointer_tags: FxHashSet<BorTag>,
         tracked_call_ids: FxHashSet<CallId>,
         retag_fields: RetagFields,
+        unique_is_unique: bool,
     ) -> Self {
         GlobalStateInner {
             borrow_tracker_method,
@@ -180,6 +183,7 @@ impl GlobalStateInner {
             tracked_pointer_tags,
             tracked_call_ids,
             retag_fields,
+            unique_is_unique,
         }
     }
 
@@ -244,6 +248,7 @@ impl BorrowTrackerMethod {
             config.tracked_pointer_tags.clone(),
             config.tracked_call_ids.clone(),
             config.retag_fields,
+            config.unique_is_unique,
         ))
     }
 }

src/borrow_tracker/tree_borrows/mod.rs

@@ -11,6 +11,7 @@ use rustc_middle::{
         Ty,
     },
 };
+use rustc_span::def_id::DefId;
 
 use crate::*;
 
@@ -99,9 +100,9 @@ impl<'tcx> Tree {
 /// Policy for a new borrow.
 #[derive(Debug, Clone, Copy)]
 struct NewPermission {
-    /// Whether this borrow requires a read access on its parent.
-    /// `perform_read_access` is `true` for all pointers marked `dereferenceable`.
-    perform_read_access: bool,
+    /// Optionally ignore the actual size to do a zero-size reborrow.
+    /// If this is set then `dereferenceable` is not enforced.
+    zero_size: bool,
     /// Which permission should the pointer start with.
     initial_state: Permission,
     /// Whether this pointer is part of the arguments of a function call.
@@ -128,28 +129,27 @@ impl<'tcx> NewPermission {
             // `&`s, which are excluded above.
             _ => return None,
         };
-        // This field happens to be redundant since right now we always do a read,
-        // but it could be useful in the future.
-        let perform_read_access = true;
 
         let protector = (kind == RetagKind::FnEntry).then_some(ProtectorKind::StrongProtector);
-        Some(Self { perform_read_access, initial_state, protector })
+        Some(Self { zero_size: false, initial_state, protector })
     }
 
-    // Boxes are not handled by `from_ref_ty`, they need special behavior
-    // implemented here.
-    fn from_box_ty(
+    /// Compute permission for `Box`-like type (`Box` always, and also `Unique` if enabled).
+    /// These pointers allow deallocation so need a different kind of protector not handled
+    /// by `from_ref_ty`.
+    fn from_unique_ty(
         ty: Ty<'tcx>,
         kind: RetagKind,
         cx: &crate::MiriInterpCx<'_, 'tcx>,
+        zero_size: bool,
     ) -> Option<Self> {
         let pointee = ty.builtin_deref(true).unwrap().ty;
         pointee.is_unpin(*cx.tcx, cx.param_env()).then_some(()).map(|()| {
             // Regular `Unpin` box, give it `noalias` but only a weak protector
             // because it is valid to deallocate it within the function.
             let ty_is_freeze = ty.is_freeze(*cx.tcx, cx.param_env());
             Self {
-                perform_read_access: true,
+                zero_size,
                 initial_state: Permission::new_unique_2phase(ty_is_freeze),
                 protector: (kind == RetagKind::FnEntry).then_some(ProtectorKind::WeakProtector),
             }
@@ -201,6 +201,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
             Ok(())
         };
 
+        trace!("Reborrow of size {:?}", ptr_size);
         let (alloc_id, base_offset, parent_prov) = if ptr_size > Size::ZERO {
             this.ptr_get_alloc_id(place.ptr)?
         } else {
@@ -276,8 +277,8 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         let range = alloc_range(base_offset, ptr_size);
         let mut tree_borrows = alloc_extra.borrow_tracker_tb().borrow_mut();
 
-        if new_perm.perform_read_access {
-            // Count this reborrow as a read access
+        // All reborrows incur a (possibly zero-sized) read access to the parent
+        {
             let global = &this.machine.borrow_tracker.as_ref().unwrap();
             let span = this.machine.current_span();
             tree_borrows.perform_access(
@@ -308,12 +309,19 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         // We want a place for where the ptr *points to*, so we get one.
         let place = this.ref_to_mplace(val)?;
 
-        // Get a lower bound of the size of this place.
-        // (When `extern type` are involved, use the size of the known prefix.)
-        let size = this
-            .size_and_align_of_mplace(&place)?
-            .map(|(size, _)| size)
-            .unwrap_or(place.layout.size);
+        // Determine the size of the reborrow.
+        // For most types this is the entire size of the place, however
+        // - when `extern type` is involved we use the size of the known prefix,
+        // - if the pointer is not reborrowed (raw pointer) or if `zero_size` is set
+        // then we override the size to do a zero-length reborrow.
+        let reborrow_size = match new_perm {
+            Some(NewPermission { zero_size: false, .. }) =>
+                this.size_and_align_of_mplace(&place)?
+                    .map(|(size, _)| size)
+                    .unwrap_or(place.layout.size),
+            _ => Size::from_bytes(0),
+        };
+        trace!("Creating new permission: {:?} with size {:?}", new_perm, reborrow_size);
 
         // This new tag is not guaranteed to actually be used.
         //
@@ -324,7 +332,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         let new_tag = this.machine.borrow_tracker.as_mut().unwrap().get_mut().new_ptr();
 
         // Compute the actual reborrow.
-        let reborrowed = this.tb_reborrow(&place, size, new_perm, new_tag)?;
+        let reborrowed = this.tb_reborrow(&place, reborrow_size, new_perm, new_tag)?;
 
         // Adjust pointer.
         let new_place = place.map_provenance(|p| {
@@ -359,10 +367,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         val: &ImmTy<'tcx, Provenance>,
     ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
         let this = self.eval_context_mut();
-        let new_perm = if let &ty::Ref(_, pointee, mutability) = val.layout.ty.kind() {
-            NewPermission::from_ref_ty(pointee, mutability, kind, this)
-        } else {
-            None
+        let new_perm = match val.layout.ty.kind() {
+            &ty::Ref(_, pointee, mutability) =>
+                NewPermission::from_ref_ty(pointee, mutability, kind, this),
+            _ => None,
         };
         this.tb_retag_reference(val, new_perm)
     }
@@ -374,15 +382,19 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         place: &PlaceTy<'tcx, Provenance>,
     ) -> InterpResult<'tcx> {
         let this = self.eval_context_mut();
-        let retag_fields = this.machine.borrow_tracker.as_mut().unwrap().get_mut().retag_fields;
-        let mut visitor = RetagVisitor { ecx: this, kind, retag_fields };
+        let options = this.machine.borrow_tracker.as_mut().unwrap().get_mut();
+        let retag_fields = options.retag_fields;
+        let unique_did =
+            options.unique_is_unique.then(|| this.tcx.lang_items().ptr_unique()).flatten();
+        let mut visitor = RetagVisitor { ecx: this, kind, retag_fields, unique_did };
         return visitor.visit_value(place);
 
         // The actual visitor.
         struct RetagVisitor<'ecx, 'mir, 'tcx> {
             ecx: &'ecx mut MiriInterpCx<'mir, 'tcx>,
             kind: RetagKind,
             retag_fields: RetagFields,
+            unique_did: Option<DefId>,
         }
         impl<'ecx, 'mir, 'tcx> RetagVisitor<'ecx, 'mir, 'tcx> {
             #[inline(always)] // yes this helps in our benchmarks
@@ -407,8 +419,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 self.ecx
             }
 
+            /// Regardless of how `Unique` is handled, Boxes are always reborrowed.
+            /// When `Unique` is also reborrowed, then it behaves exactly like `Box`
+            /// except for the fact that `Box` has a non-zero-sized reborrow.
             fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
-                let new_perm = NewPermission::from_box_ty(place.layout.ty, self.kind, self.ecx);
+                let new_perm = NewPermission::from_unique_ty(
+                    place.layout.ty,
+                    self.kind,
+                    self.ecx,
+                    /* zero_size */ false,
+                );
                 self.retag_ptr_inplace(place, new_perm)
             }
 
@@ -440,6 +460,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                         // even if field retagging is not enabled. *shrug*)
                         self.walk_value(place)?;
                     }
+                    ty::Adt(adt, _) if self.unique_did == Some(adt.did()) => {
+                        let place = inner_ptr_of_unique(self.ecx, place)?;
+                        let new_perm = NewPermission::from_unique_ty(
+                            place.layout.ty,
+                            self.kind,
+                            self.ecx,
+                            /* zero_size */ true,
+                        );
+                        self.retag_ptr_inplace(&place, new_perm)?;
+                    }
                     _ => {
                         // Not a reference/pointer/box. Only recurse if configured appropriately.
                         let recurse = match self.retag_fields {
@@ -456,7 +486,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                         }
                     }
                 }
-
                 Ok(())
             }
         }
@@ -486,7 +515,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         // FIXME: do we truly want a 2phase borrow here?
         let new_perm = Some(NewPermission {
             initial_state: Permission::new_unique_2phase(/*freeze*/ false),
-            perform_read_access: true,
+            zero_size: false,
             protector: Some(ProtectorKind::StrongProtector),
         });
         let val = this.tb_retag_reference(&val, new_perm)?;
@@ -552,3 +581,27 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         tree_borrows.give_pointer_debug_name(tag, nth_parent, name)
     }
 }
+
+/// Takes a place for a `Unique` and turns it into a place with the inner raw pointer.
+/// I.e. input is what you get from the visitor upon encountering an `adt` that is `Unique`,
+/// and output can be used by `retag_ptr_inplace`.
+fn inner_ptr_of_unique<'tcx>(
+    ecx: &mut MiriInterpCx<'_, 'tcx>,
+    place: &PlaceTy<'tcx, Provenance>,
+) -> InterpResult<'tcx, PlaceTy<'tcx, Provenance>> {
+    // Follows the same layout as `interpret/visitor.rs:walk_value` for `Box` in
+    // `rustc_const_eval`, just with one fewer layer.
+    // Here we have a `Unique(NonNull(*mut), PhantomData)`
+    assert_eq!(place.layout.fields.count(), 2, "Unique must have exactly 2 fields");
+    let (nonnull, phantom) = (ecx.place_field(place, 0)?, ecx.place_field(place, 1)?);
+    assert!(
+        phantom.layout.ty.ty_adt_def().is_some_and(|adt| adt.is_phantom_data()),
+        "2nd field of `Unique` should be `PhantomData` but is `{:?}`",
+        phantom.layout.ty,
+    );
+    // Now down to `NonNull(*mut)`
+    assert_eq!(nonnull.layout.fields.count(), 1, "NonNull must have exactly 1 field");
+    let ptr = ecx.place_field(&nonnull, 0)?;
+    // Finally a plain `*mut`
+    Ok(ptr)
+}

src/eval.rs

@@ -90,6 +90,10 @@ pub struct MiriConfig {
     pub validate: bool,
     /// Determines if Stacked Borrows or Tree Borrows is enabled.
     pub borrow_tracker: Option<BorrowTrackerMethod>,
+    /// Whether `core::ptr::Unique` receives special treatment.
+    /// If `true` then `Unique` is reborrowed with its own new tag and permission,
+    /// otherwise `Unique` is just another raw pointer.
+    pub unique_is_unique: bool,
     /// Controls alignment checking.
     pub check_alignment: AlignmentCheck,
     /// Controls function [ABI](Abi) checking.
@@ -156,6 +160,7 @@ impl Default for MiriConfig {
             env: vec![],
             validate: true,
             borrow_tracker: Some(BorrowTrackerMethod::StackedBorrows),
+            unique_is_unique: false,
             check_alignment: AlignmentCheck::Int,
             check_abi: true,
             isolated_op: IsolatedOp::Reject(RejectOpWith::Abort),

tests/fail/tree_borrows/children-can-alias.default.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: entering unreachable code
+  --> $DIR/children-can-alias.rs:LL:CC
+   |
+LL |         std::hint::unreachable_unchecked();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/children-can-alias.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/fail/tree_borrows/children-can-alias.rs

@@ -0,0 +1,59 @@
+//@revisions: default uniq
+//@compile-flags: -Zmiri-tree-borrows
+//@[uniq]compile-flags: -Zmiri-unique-is-unique
+
+//! This is NOT intended behavior.
+//! We should eventually find a solution so that the version with `Unique` passes too,
+//! otherwise `Unique` is more strict than `&mut`!
+
+#![feature(ptr_internals)]
+
+use core::ptr::addr_of_mut;
+use core::ptr::Unique;
+
+fn main() {
+    let mut data = 0u8;
+    let raw = addr_of_mut!(data);
+    unsafe {
+        raw_children_of_refmut_can_alias(&mut *raw);
+        raw_children_of_unique_can_alias(Unique::new_unchecked(raw));
+
+        // Ultimately the intended behavior is that both above tests would
+        // succeed.
+        std::hint::unreachable_unchecked();
+        //~[default]^ ERROR: entering unreachable code
+    }
+}
+
+unsafe fn raw_children_of_refmut_can_alias(x: &mut u8) {
+    let child1 = addr_of_mut!(*x);
+    let child2 = addr_of_mut!(*x);
+    // We create two raw aliases of `x`: they have the exact same
+    // tag and can be used interchangeably.
+    child1.write(1);
+    child2.write(2);
+    child1.write(1);
+    child2.write(2);
+}
+
+unsafe fn raw_children_of_unique_can_alias(x: Unique<u8>) {
+    let child1 = x.as_ptr();
+    let child2 = x.as_ptr();
+    // Under `-Zmiri-unique-is-unique`, `Unique` accidentally offers more guarantees
+    // than `&mut`. Not because it responds differently to accesses but because
+    // there is no easy way to obtain a copy with the same tag.
+    //
+    // The closest (non-hack) attempt is two calls to `as_ptr`.
+    // - Without `-Zmiri-unique-is-unique`, independent `as_ptr` calls return pointers
+    //   with the same tag that can thus be used interchangeably.
+    // - With the current implementation of `-Zmiri-unique-is-unique`, they return cousin
+    //   tags with permissions that do not tolerate aliasing.
+    // Eventually we should make such aliasing allowed in some situations
+    // (e.g. when there is no protector), which will probably involve
+    // introducing a new kind of permission.
+    child1.write(1);
+    child2.write(2);
+    //~[uniq]^ ERROR: /write access through .* is forbidden/
+    child1.write(1);
+    child2.write(2);
+}