Auto merge of #2993 - Vanille-N:tb-protector, r=RalfJung

TB: Redefine trigger condition for protectors

The Coq formalization revealed that as currently implemented, read accesses did not always commute.
Indeed starting from a lazily initialized `Active` protected tag, applying a foreign read then a child read produces `Frozen`, but child read then foreign read triggers UB (because the child read initializes _before_ the `Active -> Frozen`).

This reformulation of when protectors trigger fixes that issue:
- instead of `Active + foreign read -> Frozen` and `Active -> Frozen` when protected is UB
- we do `Active + foreign read -> if protected { Disabled } else { Frozen }`

There is already precedent for transitions being dependent on the presence of a protector (`Reserved + foreign read -> if protected { Frozen } else { Reserved }`), and this has the nice side-effect of simplifying the protector trigger condition to just an equality check against `Disabled` since now there is protector UB iff a protected tag becomes `Disabled`.

In order not to introduce an extra `if`, it was decided that `Disabled -> Disabled` would be UB when protected, which was not the case previously. This is merely a theoretical for now because a protected `Disabled` is unreachable in the first place.

The extra test is not directly related to this modification, but also checks things related to protectors and lazy initialization.

src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs

@@ -227,10 +227,10 @@ pub(super) enum TransitionError {
     ChildAccessForbidden(Permission),
     /// A protector was triggered due to an invalid transition that loses
     /// too much permissions.
-    /// For example, if a protected tag goes from `Active` to `Frozen` due
+    /// For example, if a protected tag goes from `Active` to `Disabled` due
-    /// to a foreign write this will produce a `ProtectedTransition(PermTransition(Active, Frozen))`.
+    /// to a foreign write this will produce a `ProtectedDisabled(Active)`.
     /// This kind of error can only occur on foreign accesses.
-    ProtectedTransition(PermTransition),
+    ProtectedDisabled(Permission),
     /// Cannot deallocate because some tag in the allocation is strongly protected.
     /// This kind of error can only occur on deallocations.
     ProtectedDealloc,
@@ -302,7 +302,7 @@ impl TbError<'_> {
                 ));
                 (title, details, conflicting_tag_name)
             }
-            ProtectedTransition(transition) => {
+            ProtectedDisabled(before_disabled) => {
                 let conflicting_tag_name = "protected";
                 let access = cause.print_as_access(/* is_foreign */ true);
                 let details = vec![
@@ -310,12 +310,9 @@ impl TbError<'_> {
                         "the accessed tag {accessed} is foreign to the {conflicting_tag_name} tag {conflicting} (i.e., it is not a child)"
                     ),
                     format!(
-                        "this {access} would cause the {conflicting_tag_name} tag {conflicting} to transition {transition}"
+                        "this {access} would cause the {conflicting_tag_name} tag {conflicting} (currently {before_disabled}) to become Disabled"
-                    ),
-                    format!(
-                        "this transition would be {loss}, which is not allowed for protected tags",
-                        loss = transition.summary(),
                     ),
+                    format!("protected tags must never be Disabled"),
                 ];
                 (title, details, conflicting_tag_name)
             }

src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs

@@ -72,7 +72,12 @@ mod transition {
             // accesses, since the data is not being mutated. Hence the `{ .. }`
             res @ Reserved { .. } if !protected => res,
             Reserved { .. } => Frozen, // protected reserved
-            Active => Frozen,
+            Active =>
+                if protected {
+                    Disabled
+                } else {
+                    Frozen
+                },
             non_writeable @ (Frozen | Disabled) => non_writeable,
         })
     }
@@ -189,34 +194,9 @@ impl PermTransition {
         Permission { inner: self.from }
     }
 
-    /// Determines whether a transition that occured is compatible with the presence
+    /// Determines if this transition would disable the permission.
-    /// of a Protector. This is not included in the `transition` functions because
+    pub fn produces_disabled(self) -> bool {
-    /// it would distract from the few places where the transition is modified
+        self.to == Disabled
-    /// because of a protector, but not forbidden.
-    ///
-    /// Note: this is not in charge of checking that there *is* a protector,
-    /// it should be used as
-    /// ```
-    /// let no_protector_error = if is_protected(tag) {
-    ///     transition.is_allowed_by_protector()
-    /// };
-    /// ```
-    pub fn is_allowed_by_protector(&self) -> bool {
-        assert!(self.is_possible());
-        match (self.from, self.to) {
-            _ if self.from == self.to => true,
-            // It is always a protector violation to not be readable anymore
-            (_, Disabled) => false,
-            // In the case of a `Reserved` under a protector, both transitions
-            // `Reserved => Active` and `Reserved => Frozen` can legitimately occur.
-            // The first is standard (Child Write), the second is for Foreign Writes
-            // on protected Reserved where we must ensure that the pointer is not
-            // written to in the future.
-            (Reserved { .. }, Active) | (Reserved { .. }, Frozen) => true,
-            // This pointer should have stayed writeable for the whole function
-            (Active, Frozen) => false,
-            _ => unreachable!("Transition {} should never be possible", self),
-        }
     }
 }
 
@@ -298,14 +278,15 @@ pub mod diagnostics {
         ///
         /// This function assumes that its arguments apply to the same location
         /// and that they were obtained during a normal execution. It will panic otherwise.
-        /// - `err` cannot be a `ProtectedTransition(_)` of a noop transition, as those
-        /// never trigger protectors;
         /// - all transitions involved in `self` and `err` should be increasing
         /// (Reserved < Active < Frozen < Disabled);
         /// - between `self` and `err` the permission should also be increasing,
         /// so all permissions inside `err` should be greater than `self.1`;
         /// - `Active` and `Reserved` cannot cause an error due to insufficient permissions,
         /// so `err` cannot be a `ChildAccessForbidden(_)` of either of them;
+        /// - `err` should not be `ProtectedDisabled(Disabled)`, because the protected
+        /// tag should not have been `Disabled` in the first place (if this occurs it means
+        /// we have unprotected tags that become protected)
         pub(in super::super) fn is_relevant(&self, err: TransitionError) -> bool {
             // NOTE: `super::super` is the visibility of `TransitionError`
             assert!(self.is_possible());
@@ -342,17 +323,16 @@ pub mod diagnostics {
                             unreachable!("permissions between self and err must be increasing"),
                     }
                 }
-                TransitionError::ProtectedTransition(forbidden) => {
+                TransitionError::ProtectedDisabled(before_disabled) => {
-                    assert!(!forbidden.is_noop());
                     // Show how we got to the starting point of the forbidden transition,
                     // but ignore what came before.
                     // This eliminates transitions like `Reserved -> Active`
                     // when the error is a `Frozen -> Disabled`.
-                    match (self.to, forbidden.from, forbidden.to) {
+                    match (self.to, before_disabled.inner) {
                         // We absolutely want to know where it was activated.
-                        (Active, Active, Frozen | Disabled) => true,
+                        (Active, Active) => true,
                         // And knowing where it became Frozen is also important.
-                        (Frozen, Frozen, Disabled) => true,
+                        (Frozen, Frozen) => true,
                         // If the error is a transition `Frozen -> Disabled`, then we don't really
                         // care whether before that was `Reserved -> Active -> Frozen` or
                         // `Reserved -> Frozen` or even `Frozen` directly.
@@ -360,27 +340,22 @@ pub mod diagnostics {
                         // - created as Frozen, then Frozen -> Disabled is forbidden
                         // - created as Reserved, later became Frozen, then Frozen -> Disabled is forbidden
                         // In both cases the `Reserved -> Active` part is inexistant or irrelevant.
-                        (Active, Frozen, Disabled) => false,
+                        (Active, Frozen) => false,
 
-                        // `Reserved -> Frozen` does not trigger protectors.
+                        (_, Disabled) =>
-                        (_, Reserved { .. }, Frozen) =>
+                            unreachable!(
-                            unreachable!("this transition cannot cause an error"),
+                                "permission that results in Disabled should not itself be Disabled in the first place"
+                            ),
                         // No transition has `Reserved` as its `.to` unless it's a noop.
-                        (Reserved { .. }, _, _) => unreachable!("self is a noop transition"),
+                        (Reserved { .. }, _) => unreachable!("self is a noop transition"),
-                        (_, Disabled, Disabled) | (_, Frozen, Frozen) | (_, Active, Active) =>
-                            unreachable!("err contains a noop transition"),
 
                         // Permissions only evolve in the order `Reserved -> Active -> Frozen -> Disabled`,
                         // so permissions found must be increasing in the order
                         // `self.from < self.to <= forbidden.from < forbidden.to`.
-                        (Disabled, Reserved { .. } | Active | Frozen, _)
+                        (Disabled, Reserved { .. } | Active | Frozen)
-                        | (Frozen, Reserved { .. } | Active, _)
+                        | (Frozen, Reserved { .. } | Active)
-                        | (Active, Reserved { .. }, _) =>
+                        | (Active, Reserved { .. }) =>
                             unreachable!("permissions between self and err must be increasing"),
-                        (_, Disabled, Reserved { .. } | Active | Frozen)
-                        | (_, Frozen, Reserved { .. } | Active)
-                        | (_, Active, Reserved { .. }) =>
-                            unreachable!("permissions within err must be increasing"),
                     }
                 }
                 // We don't care because protectors evolve independently from
@@ -406,7 +381,7 @@ mod propagation_optimization_checks {
         pub use super::*;
         impl PermissionPriv {
             /// Enumerate all states
-            pub fn all() -> impl Iterator<Item = PermissionPriv> {
+            pub fn all() -> impl Iterator<Item = Self> {
                 vec![
                     Active,
                     Reserved { ty_is_freeze: true },
@@ -418,9 +393,15 @@ mod propagation_optimization_checks {
             }
         }
 
+        impl Permission {
+            pub fn all() -> impl Iterator<Item = Self> {
+                PermissionPriv::all().map(|inner| Self { inner })
+            }
+        }
+
         impl AccessKind {
             /// Enumerate all AccessKind.
-            pub fn all() -> impl Iterator<Item = AccessKind> {
+            pub fn all() -> impl Iterator<Item = Self> {
                 use AccessKind::*;
                 [Read, Write].into_iter()
             }
@@ -428,7 +409,7 @@ mod propagation_optimization_checks {
 
         impl AccessRelatedness {
             /// Enumerate all relative positions
-            pub fn all() -> impl Iterator<Item = AccessRelatedness> {
+            pub fn all() -> impl Iterator<Item = Self> {
                 use AccessRelatedness::*;
                 [This, StrictChildAccess, AncestorAccess, DistantAccess].into_iter()
             }

src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs

@@ -19,6 +19,7 @@ use rustc_target::abi::Size;
 
 use crate::borrow_tracker::tree_borrows::{
     diagnostics::{self, NodeDebugInfo, TbError, TransitionError},
+    perms::PermTransition,
     unimap::{UniEntry, UniIndex, UniKeyMap, UniValMap},
     Permission,
 };
@@ -28,17 +29,19 @@ use crate::*;
 /// Data for a single *location*.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(super) struct LocationState {
-    /// This pointer's current permission
+    /// A location is initialized when it is child-accessed for the first time (and the initial
-    permission: Permission,
+    /// retag initializes the location for the range covered by the type), and it then stays
-    /// A location is initialized when it is child accessed for the first time,
+    /// initialized forever.
-    /// and it then stays initialized forever.
+    /// For initialized locations, "permission" is the current permission. However, for
-    /// Before initialization we still apply some preemptive transitions on
+    /// uninitialized locations, we still need to track the "future initial permission": this will
-    /// `permission` to know what to do in case it ever gets initialized,
+    /// start out to be `default_initial_perm`, but foreign accesses need to be taken into account.
-    /// but these can never cause any immediate UB. There can however be UB
+    /// Crucially however, while transitions to `Disabled` would usually be UB if this location is
-    /// the moment we attempt to initialize (i.e. child-access) because some
+    /// protected, that is *not* the case for uninitialized locations. Instead we just have a latent
-    /// foreign access done between the creation and the initialization is
+    /// "future initial permission" of `Disabled`, causing UB only if an access is ever actually
-    /// incompatible with child accesses.
+    /// performed.
     initialized: bool,
+    /// This pointer's current permission / future initial permission.
+    permission: Permission,
     /// Strongest foreign access whose effects have already been applied to
     /// this node and all its children since the last child access.
     /// This is `None` if the most recent access is a child access,
@@ -69,6 +72,104 @@ impl LocationState {
     pub fn permission(&self) -> Permission {
         self.permission
     }
+
+    /// Apply the effect of an access to one location, including
+    /// - applying `Permission::perform_access` to the inner `Permission`,
+    /// - emitting protector UB if the location is initialized,
+    /// - updating the initialized status (child accesses produce initialized locations).
+    fn perform_access(
+        &mut self,
+        access_kind: AccessKind,
+        rel_pos: AccessRelatedness,
+        protected: bool,
+    ) -> Result<PermTransition, TransitionError> {
+        let old_perm = self.permission;
+        let transition = Permission::perform_access(access_kind, rel_pos, old_perm, protected)
+            .ok_or(TransitionError::ChildAccessForbidden(old_perm))?;
+        // Why do only initialized locations cause protector errors?
+        // Consider two mutable references `x`, `y` into disjoint parts of
+        // the same allocation. A priori, these may actually both be used to
+        // access the entire allocation, as long as only reads occur. However,
+        // a write to `y` needs to somehow record that `x` can no longer be used
+        // on that location at all. For these uninitialized locations (i.e., locations
+        // that haven't been accessed with `x` yet), we track the "future initial state":
+        // it defaults to whatever the initial state of the tag is,
+        // but the access to `y` moves that "future initial state" of `x` to `Disabled`.
+        // However, usually a `Reserved -> Disabled` transition would be UB due to the protector!
+        // So clearly protectors shouldn't fire for such "future initial state" transitions.
+        //
+        // See the test `two_mut_protected_same_alloc` in `tests/pass/tree_borrows/tree-borrows.rs`
+        // for an example of safe code that would be UB if we forgot to check `self.initialized`.
+        if protected && self.initialized && transition.produces_disabled() {
+            return Err(TransitionError::ProtectedDisabled(old_perm));
+        }
+        self.permission = transition.applied(old_perm).unwrap();
+        self.initialized |= !rel_pos.is_foreign();
+        Ok(transition)
+    }
+
+    // Helper to optimize the tree traversal.
+    // The optimization here consists of observing thanks to the tests
+    // `foreign_read_is_noop_after_write` and `all_transitions_idempotent`,
+    // that there are actually just three possible sequences of events that can occur
+    // in between two child accesses that produce different results.
+    //
+    // Indeed,
+    // - applying any number of foreign read accesses is the same as applying
+    //   exactly one foreign read,
+    // - applying any number of foreign read or write accesses is the same
+    //   as applying exactly one foreign write.
+    // therefore the three sequences of events that can produce different
+    // outcomes are
+    // - an empty sequence (`self.latest_foreign_access = None`)
+    // - a nonempty read-only sequence (`self.latest_foreign_access = Some(Read)`)
+    // - a nonempty sequence with at least one write (`self.latest_foreign_access = Some(Write)`)
+    //
+    // This function not only determines if skipping the propagation right now
+    // is possible, it also updates the internal state to keep track of whether
+    // the propagation can be skipped next time.
+    // It is a performance loss not to call this function when a foreign access occurs.
+    // It is unsound not to call this function when a child access occurs.
+    fn skip_if_known_noop(
+        &mut self,
+        access_kind: AccessKind,
+        rel_pos: AccessRelatedness,
+    ) -> ContinueTraversal {
+        if rel_pos.is_foreign() {
+            let new_access_noop = match (self.latest_foreign_access, access_kind) {
+                // Previously applied transition makes the new one a guaranteed
+                // noop in the two following cases:
+                // (1) justified by `foreign_read_is_noop_after_write`
+                (Some(AccessKind::Write), AccessKind::Read) => true,
+                // (2) justified by `all_transitions_idempotent`
+                (Some(old), new) if old == new => true,
+                // In all other cases there has been a recent enough
+                // child access that the effects of the new foreign access
+                // need to be applied to this subtree.
+                _ => false,
+            };
+            if new_access_noop {
+                // Abort traversal if the new transition is indeed guaranteed
+                // to be noop.
+                // No need to update `self.latest_foreign_access`,
+                // the type of the current streak among nonempty read-only
+                // or nonempty with at least one write has not changed.
+                ContinueTraversal::SkipChildren
+            } else {
+                // Otherwise propagate this time, and also record the
+                // access that just occurred so that we can skip the propagation
+                // next time.
+                self.latest_foreign_access = Some(access_kind);
+                ContinueTraversal::Recurse
+            }
+        } else {
+            // A child access occurred, this breaks the streak of foreign
+            // accesses in a row and the sequence since the previous child access
+            // is now empty.
+            self.latest_foreign_access = None;
+            ContinueTraversal::Recurse
+        }
+    }
 }
 
 /// Tree structure with both parents and children since we want to be
@@ -387,11 +488,15 @@ impl<'tcx> Tree {
         Ok(())
     }
 
-    /// Maps the following propagation procedure to each range:
+    /// Map the per-node and per-location `LocationState::perform_access`
-    /// - initialize if needed;
+    /// to each location of `access_range`, on every tag of the allocation.
-    /// - compute new state after transition;
+    ///
-    /// - check that there is no protector that would forbid this;
+    /// `LocationState::perform_access` will take care of raising transition
-    /// - record this specific location as accessed.
+    /// errors and updating the `initialized` status of each location,
+    /// this traversal adds to that:
+    /// - inserting into the map locations that do not exist yet,
+    /// - trimming the traversal,
+    /// - recording the history.
     pub fn perform_access(
         &mut self,
         access_kind: AccessKind,
@@ -411,55 +516,16 @@ impl<'tcx> Tree {
                         let old_state =
                             perm.or_insert_with(|| LocationState::new(node.default_initial_perm));
 
-                        // Optimize the tree traversal.
+                        match old_state.skip_if_known_noop(access_kind, rel_pos) {
-                        // The optimization here consists of observing thanks to the tests
+                            ContinueTraversal::SkipChildren =>
-                        // `foreign_read_is_noop_after_write` and `all_transitions_idempotent`
+                                return Ok(ContinueTraversal::SkipChildren),
-                        // that if we apply twice in a row the effects of a foreign access
+                            _ => {}
-                        // we can skip some branches.
-                        // "two foreign accesses in a row" occurs when `perm.latest_foreign_access` is `Some(_)`
-                        // AND the `rel_pos` of the current access corresponds to a foreign access.
-                        if rel_pos.is_foreign() {
-                            let new_access_noop =
-                                match (old_state.latest_foreign_access, access_kind) {
-                                    // Previously applied transition makes the new one a guaranteed
-                                    // noop in the two following cases:
-                                    // (1) justified by `foreign_read_is_noop_after_write`
-                                    (Some(AccessKind::Write), AccessKind::Read) => true,
-                                    // (2) justified by `all_transitions_idempotent`
-                                    (Some(old), new) if old == new => true,
-                                    // In all other cases there has been a recent enough
-                                    // child access that the effects of the new foreign access
-                                    // need to be applied to this subtree.
-                                    _ => false,
-                                };
-                            if new_access_noop {
-                                // Abort traversal if the new transition is indeed guaranteed
-                                // to be noop.
-                                return Ok(ContinueTraversal::SkipChildren);
-                            } else {
-                                // Otherwise propagate this time, and also record the
-                                // access that just occurred so that we can skip the propagation
-                                // next time.
-                                old_state.latest_foreign_access = Some(access_kind);
-                            }
-                        } else {
-                            // A child access occurred, this breaks the streak of "two foreign
-                            // accesses in a row" and we reset this field.
-                            old_state.latest_foreign_access = None;
                         }
 
-                        let old_perm = old_state.permission;
                         let protected = global.borrow().protected_tags.contains_key(&node.tag);
                         let transition =
-                            Permission::perform_access(access_kind, rel_pos, old_perm, protected)
+                            old_state.perform_access(access_kind, rel_pos, protected)?;
-                                .ok_or(TransitionError::ChildAccessForbidden(old_perm))?;
+
-                        if protected
-                            // Can't trigger Protector on uninitialized locations
-                            && old_state.initialized
-                            && !transition.is_allowed_by_protector()
-                        {
-                            return Err(TransitionError::ProtectedTransition(transition));
-                        }
                         // Record the event as part of the history
                         if !transition.is_noop() {
                             node.debug_info.history.push(diagnostics::Event {
@@ -470,10 +536,7 @@ impl<'tcx> Tree {
                                 transition_range: perms_range.clone(),
                                 span,
                             });
-                            old_state.permission =
-                                transition.applied(old_state.permission).unwrap();
                         }
-                        old_state.initialized |= !rel_pos.is_foreign();
                         Ok(ContinueTraversal::Recurse)
                     },
                     |args: ErrHandlerArgs<'_, TransitionError>| -> InterpError<'tcx> {
@@ -602,3 +665,57 @@ impl AccessRelatedness {
         }
     }
 }
+
+#[cfg(test)]
+mod commutation_tests {
+    use super::*;
+    impl LocationState {
+        pub fn all_without_access() -> impl Iterator<Item = Self> {
+            Permission::all().flat_map(|permission| {
+                [false, true].into_iter().map(move |initialized| {
+                    Self { permission, initialized, latest_foreign_access: None }
+                })
+            })
+        }
+    }
+
+    #[test]
+    #[rustfmt::skip]
+    // Exhaustive check that for any starting configuration loc,
+    // for any two read accesses r1 and r2, if `loc + r1 + r2` is not UB
+    // and results in `loc'`, then `loc + r2 + r1` is also not UB and results
+    // in the same final state `loc'`.
+    // This lets us justify arbitrary read-read reorderings.
+    fn all_read_accesses_commute() {
+        let kind = AccessKind::Read;
+        // Two of the four combinations of `AccessRelatedness` are trivial,
+        // but we might as well check them all.
+        for rel1 in AccessRelatedness::all() {
+            for rel2 in AccessRelatedness::all() {
+                // Any protector state works, but we can't move reads across function boundaries
+                // so the two read accesses occur under the same protector.
+                for &protected in &[true, false] {
+                    for loc in LocationState::all_without_access() {
+                        // Apply 1 then 2. Failure here means that there is UB in the source
+                        // and we skip the check in the target.
+                        let mut loc12 = loc;
+                        let Ok(_) = loc12.perform_access(kind, rel1, protected) else { continue; };
+                        let Ok(_) = loc12.perform_access(kind, rel2, protected) else { continue; };
+
+                        // If 1 followed by 2 succeeded, then 2 followed by 1 must also succeed...
+                        let mut loc21 = loc;
+                        loc21.perform_access(kind, rel2, protected).unwrap();
+                        loc21.perform_access(kind, rel1, protected).unwrap();
+
+                        // ... and produce the same final result.
+                        assert_eq!(
+                            loc12, loc21,
+                            "Read accesses {:?} followed by {:?} do not commute !",
+                            rel1, rel2
+                        );
+                    }
+                }
+            }
+        }
+    }
+}

src/tools/miri/tests/fail/both_borrows/aliasing_mut4.tree.stderr

@@ -6,8 +6,8 @@ LL |         ptr::write(dest, src);
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Frozen to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Frozen) to become Disabled
-   = help: this transition would be a loss of read permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/aliasing_mut4.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/box_noalias_violation.tree.stderr

@@ -6,8 +6,8 @@ LL |     *y
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign read access would cause the protected tag <TAG> to transition from Active to Frozen
+   = help: this foreign read access would cause the protected tag <TAG> (currently Active) to become Disabled
-   = help: this transition would be a loss of write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/box_noalias_violation.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/illegal_write6.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { *y = 2 };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Active to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Active) to become Disabled
-   = help: this transition would be a loss of read and write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/illegal_write6.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/invalidate_against_protector2.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { *x = 0 };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Frozen to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Frozen) to become Disabled
-   = help: this transition would be a loss of read permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/invalidate_against_protector2.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/invalidate_against_protector3.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { *x = 0 };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (root of the allocation) is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Frozen to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Frozen) to become Disabled
-   = help: this transition would be a loss of read permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/invalidate_against_protector3.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/newtype_pair_retagging.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this deallocation (acting as a foreign write access) would cause the protected tag <TAG> to transition from Frozen to Disabled
+   = help: this deallocation (acting as a foreign write access) would cause the protected tag <TAG> (currently Frozen) to become Disabled
-   = help: this transition would be a loss of read permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/newtype_pair_retagging.rs:LL:CC
    |

src/tools/miri/tests/fail/both_borrows/newtype_retagging.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this deallocation (acting as a foreign write access) would cause the protected tag <TAG> to transition from Frozen to Disabled
+   = help: this deallocation (acting as a foreign write access) would cause the protected tag <TAG> (currently Frozen) to become Disabled
-   = help: this transition would be a loss of read permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/newtype_retagging.rs:LL:CC
    |

src/tools/miri/tests/fail/function_calls/arg_inplace_mutate.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { ptr.write(S(0)) };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (root of the allocation) is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Active to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Active) to become Disabled
-   = help: this transition would be a loss of read and write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/arg_inplace_mutate.rs:LL:CC
    |

src/tools/miri/tests/fail/function_calls/arg_inplace_observe_during.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { ptr.read() };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (root of the allocation) is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign read access would cause the protected tag <TAG> to transition from Active to Frozen
+   = help: this foreign read access would cause the protected tag <TAG> (currently Active) to become Disabled
-   = help: this transition would be a loss of write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/arg_inplace_observe_during.rs:LL:CC
    |

src/tools/miri/tests/fail/function_calls/return_pointer_aliasing.tree.stderr

@@ -6,8 +6,8 @@ LL |     unsafe { ptr.read() };
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (root of the allocation) is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign read access would cause the protected tag <TAG> to transition from Active to Frozen
+   = help: this foreign read access would cause the protected tag <TAG> (currently Active) to become Disabled
-   = help: this transition would be a loss of write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/return_pointer_aliasing.rs:LL:CC
    |

src/tools/miri/tests/fail/tree_borrows/outside-range.stderr

@@ -6,8 +6,8 @@ LL |     *y.add(3) = 42;
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> to transition from Reserved to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (currently Reserved) to become Disabled
-   = help: this transition would be a loss of read and write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/outside-range.rs:LL:CC
    |

src/tools/miri/tests/fail/tree_borrows/reserved/cell-protected-write.stderr

@@ -16,8 +16,8 @@ LL |             *y = 1;
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (y, callee:y, caller:y) is foreign to the protected tag <TAG> (callee:x) (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> (callee:x) to transition from Reserved to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (callee:x) (currently Reserved) to become Disabled
-   = help: this transition would be a loss of read and write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/cell-protected-write.rs:LL:CC
    |

src/tools/miri/tests/fail/tree_borrows/reserved/int-protected-write.stderr

@@ -16,8 +16,8 @@ LL |             *y = 0;
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> (y, callee:y, caller:y) is foreign to the protected tag <TAG> (callee:x) (i.e., it is not a child)
-   = help: this foreign write access would cause the protected tag <TAG> (callee:x) to transition from Reserved to Disabled
+   = help: this foreign write access would cause the protected tag <TAG> (callee:x) (currently Reserved) to become Disabled
-   = help: this transition would be a loss of read and write permissions, which is not allowed for protected tags
+   = help: protected tags must never be Disabled
 help: the accessed tag <TAG> was created here
   --> $DIR/int-protected-write.rs:LL:CC
    |

src/tools/miri/tests/pass/tree_borrows/tree-borrows.rs

@@ -9,6 +9,7 @@ use std::ptr;
 fn main() {
     aliasing_read_only_mutable_refs();
     string_as_mut_ptr();
+    two_mut_protected_same_alloc();
 
     // Stacked Borrows tests
     read_does_not_invalidate1();
@@ -62,6 +63,23 @@ pub fn string_as_mut_ptr() {
     }
 }
 
+// This function checks that there is no issue with having two mutable references
+// from the same allocation both under a protector.
+// This is safe code, it must absolutely not be UB.
+// This test failing is a symptom of forgetting to check that only initialized
+// locations can cause protector UB.
+fn two_mut_protected_same_alloc() {
+    fn write_second(_x: &mut u8, y: &mut u8) {
+        // write through `y` will make some locations of `x` (protected)
+        // become Disabled. Those locations are outside of the range on which
+        // `x` is initialized, and the protector must not trigger.
+        *y = 1;
+    }
+
+    let mut data = (0u8, 1u8);
+    write_second(&mut data.0, &mut data.1);
+}
+
 // ----- The tests below were taken from Stacked Borrows ----
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,