Rollup merge of #130727 - compiler-errors:objects, r=RalfJung

Check vtable projections for validity in miri

Currently, miri does not catch when we transmute `dyn Trait<Assoc = A>` to `dyn Trait<Assoc = B>`. This PR implements such a check, and fixes rust-lang/miri#3905.

To do this, we modify `GlobalAlloc::VTable` to contain the *whole* list of `PolyExistentialPredicate`, and then modify `check_vtable_for_type` to validate the `PolyExistentialProjection`s of the vtable, along with the principal trait that was already being validated.

cc ``@RalfJung``
r? ``@lcnr`` or types

I also tweaked the diagnostics a bit.

---

**Open question:** We don't validate the auto traits. You can transmute `dyn Foo` into `dyn Foo + Send`. Should we check that? We currently have a test that *exercises* this as not being UB:

https://github.com/rust-lang/rust/blob/6c6d210089e4589afee37271862b9f88ba1d7755/src/tools/miri/tests/pass/dyn-upcast.rs#L14-L20

I'm not actually sure if we ever decided that's actually UB or not 🤔

We could perhaps still check that the underlying type of the object (i.e. the concrete type that was unsized) implements the auto traits, to catch UB like:

```rust
fn main() {
    let x: &dyn Trait = &std::ptr::null_mut::<()>();
    let _: &(dyn Trait + Send) = std::mem::transmute(x);
    //~^ this vtable is not allocated for a type that is `Send`!
}
```

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -161,13 +161,13 @@ pub(crate) fn codegen_const_value<'tcx>(
                             fx.module.declare_func_in_func(func_id, &mut fx.bcx.func);
                         fx.bcx.ins().func_addr(fx.pointer_type, local_func_id)
                     }
-                    GlobalAlloc::VTable(ty, trait_ref) => {
+                    GlobalAlloc::VTable(ty, dyn_ty) => {
                         let data_id = data_id_for_vtable(
                             fx.tcx,
                             &mut fx.constants_cx,
                             fx.module,
                             ty,
-                            trait_ref,
+                            dyn_ty.principal(),
                         );
                         let local_data_id =
                             fx.module.declare_data_in_func(data_id, &mut fx.bcx.func);
@@ -456,8 +456,8 @@ fn define_all_allocs(tcx: TyCtxt<'_>, module: &mut dyn Module, cx: &mut Constant
                 GlobalAlloc::Memory(target_alloc) => {
                     data_id_for_alloc_id(cx, module, alloc_id, target_alloc.inner().mutability)
                 }
-                GlobalAlloc::VTable(ty, trait_ref) => {
-                    data_id_for_vtable(tcx, cx, module, ty, trait_ref)
+                GlobalAlloc::VTable(ty, dyn_ty) => {
+                    data_id_for_vtable(tcx, cx, module, ty, dyn_ty.principal())
                 }
                 GlobalAlloc::Static(def_id) => {
                     if tcx.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::THREAD_LOCAL)

compiler/rustc_codegen_gcc/src/common.rs

@@ -224,10 +224,10 @@ impl<'gcc, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
                         value
                     }
                     GlobalAlloc::Function { instance, .. } => self.get_fn_addr(instance),
-                    GlobalAlloc::VTable(ty, trait_ref) => {
+                    GlobalAlloc::VTable(ty, dyn_ty) => {
                         let alloc = self
                             .tcx
-                            .global_alloc(self.tcx.vtable_allocation((ty, trait_ref)))
+                            .global_alloc(self.tcx.vtable_allocation((ty, dyn_ty.principal())))
                             .unwrap_memory();
                         let init = const_alloc_to_gcc(self, alloc);
                         self.static_addr_of(init, alloc.inner().align, None)

compiler/rustc_codegen_llvm/src/common.rs

@@ -290,10 +290,10 @@ impl<'ll, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                         self.get_fn_addr(instance.polymorphize(self.tcx)),
                         self.data_layout().instruction_address_space,
                     ),
-                    GlobalAlloc::VTable(ty, trait_ref) => {
+                    GlobalAlloc::VTable(ty, dyn_ty) => {
                         let alloc = self
                             .tcx
-                            .global_alloc(self.tcx.vtable_allocation((ty, trait_ref)))
+                            .global_alloc(self.tcx.vtable_allocation((ty, dyn_ty.principal())))
                             .unwrap_memory();
                         let init = const_alloc_to_llvm(self, alloc, /*static*/ false);
                         let value = self.static_addr_of(init, alloc.inner().align, None);

compiler/rustc_const_eval/messages.ftl

@@ -198,7 +198,7 @@ const_eval_invalid_vtable_pointer =
     using {$pointer} as vtable pointer but it does not point to a vtable
 
 const_eval_invalid_vtable_trait =
-    using vtable for trait `{$vtable_trait}` but trait `{$expected_trait}` was expected
+    using vtable for `{$vtable_dyn_type}` but `{$expected_dyn_type}` was expected
 
 const_eval_lazy_lock =
     consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
@@ -459,7 +459,7 @@ const_eval_validation_invalid_fn_ptr = {$front_matter}: encountered {$value}, bu
 const_eval_validation_invalid_ref_meta = {$front_matter}: encountered invalid reference metadata: total size is bigger than largest supported object
 const_eval_validation_invalid_ref_slice_meta = {$front_matter}: encountered invalid reference metadata: slice is bigger than largest supported object
 const_eval_validation_invalid_vtable_ptr = {$front_matter}: encountered {$value}, but expected a vtable pointer
-const_eval_validation_invalid_vtable_trait = {$front_matter}: wrong trait in wide pointer vtable: expected `{$ref_trait}`, but encountered `{$vtable_trait}`
+const_eval_validation_invalid_vtable_trait = {$front_matter}: wrong trait in wide pointer vtable: expected `{$expected_dyn_type}`, but encountered `{$vtable_dyn_type}`
 const_eval_validation_mutable_ref_to_immutable = {$front_matter}: encountered mutable reference or box pointing to read-only memory
 const_eval_validation_never_val = {$front_matter}: encountered a value of the never type `!`
 const_eval_validation_null_box = {$front_matter}: encountered a null box

compiler/rustc_const_eval/src/errors.rs

@@ -522,12 +522,9 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             UnterminatedCString(ptr) | InvalidFunctionPointer(ptr) | InvalidVTablePointer(ptr) => {
                 diag.arg("pointer", ptr);
             }
-            InvalidVTableTrait { expected_trait, vtable_trait } => {
-                diag.arg("expected_trait", expected_trait.to_string());
-                diag.arg(
-                    "vtable_trait",
-                    vtable_trait.map(|t| t.to_string()).unwrap_or_else(|| format!("<trivial>")),
-                );
+            InvalidVTableTrait { expected_dyn_type, vtable_dyn_type } => {
+                diag.arg("expected_dyn_type", expected_dyn_type.to_string());
+                diag.arg("vtable_dyn_type", vtable_dyn_type.to_string());
             }
             PointerUseAfterFree(alloc_id, msg) => {
                 diag.arg("alloc_id", alloc_id)
@@ -777,12 +774,9 @@ impl<'tcx> ReportErrorExt for ValidationErrorInfo<'tcx> {
             DanglingPtrNoProvenance { pointer, .. } => {
                 err.arg("pointer", pointer);
             }
-            InvalidMetaWrongTrait { expected_trait: ref_trait, vtable_trait } => {
-                err.arg("ref_trait", ref_trait.to_string());
-                err.arg(
-                    "vtable_trait",
-                    vtable_trait.map(|t| t.to_string()).unwrap_or_else(|| format!("<trivial>")),
-                );
+            InvalidMetaWrongTrait { vtable_dyn_type, expected_dyn_type } => {
+                err.arg("vtable_dyn_type", vtable_dyn_type.to_string());
+                err.arg("expected_dyn_type", expected_dyn_type.to_string());
             }
             NullPtr { .. }
             | ConstRefToMutable

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -128,7 +128,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             CastKind::DynStar => {
                 if let ty::Dynamic(data, _, ty::DynStar) = cast_ty.kind() {
                     // Initial cast from sized to dyn trait
-                    let vtable = self.get_vtable_ptr(src.layout.ty, data.principal())?;
+                    let vtable = self.get_vtable_ptr(src.layout.ty, data)?;
                     let vtable = Scalar::from_maybe_pointer(vtable, self);
                     let data = self.read_immediate(src)?.to_scalar();
                     let _assert_pointer_like = data.to_pointer(self)?;
@@ -446,12 +446,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 }
 
                 // Get the destination trait vtable and return that.
-                let new_vptr = self.get_vtable_ptr(ty, data_b.principal())?;
+                let new_vptr = self.get_vtable_ptr(ty, data_b)?;
                 self.write_immediate(Immediate::new_dyn_trait(old_data, new_vptr, self), dest)
             }
             (_, &ty::Dynamic(data, _, ty::Dyn)) => {
                 // Initial cast from sized to dyn trait
-                let vtable = self.get_vtable_ptr(src_pointee_ty, data.principal())?;
+                let vtable = self.get_vtable_ptr(src_pointee_ty, data)?;
                 let ptr = self.read_pointer(src)?;
                 let val = Immediate::new_dyn_trait(ptr, vtable, &*self.tcx);
                 self.write_immediate(val, dest)

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -943,12 +943,13 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         if offset.bytes() != 0 {
             throw_ub!(InvalidVTablePointer(Pointer::new(alloc_id, offset)))
         }
-        let Some(GlobalAlloc::VTable(ty, vtable_trait)) = self.tcx.try_get_global_alloc(alloc_id)
+        let Some(GlobalAlloc::VTable(ty, vtable_dyn_type)) =
+            self.tcx.try_get_global_alloc(alloc_id)
         else {
             throw_ub!(InvalidVTablePointer(Pointer::new(alloc_id, offset)))
         };
-        if let Some(expected_trait) = expected_trait {
-            self.check_vtable_for_type(vtable_trait, expected_trait)?;
+        if let Some(expected_dyn_type) = expected_trait {
+            self.check_vtable_for_type(vtable_dyn_type, expected_dyn_type)?;
         }
         Ok(ty)
     }
@@ -1113,11 +1114,8 @@ impl<'a, 'tcx, M: Machine<'tcx>> std::fmt::Debug for DumpAllocs<'a, 'tcx, M> {
                         Some(GlobalAlloc::Function { instance, .. }) => {
                             write!(fmt, " (fn: {instance})")?;
                         }
-                        Some(GlobalAlloc::VTable(ty, Some(trait_ref))) => {
-                            write!(fmt, " (vtable: impl {trait_ref} for {ty})")?;
-                        }
-                        Some(GlobalAlloc::VTable(ty, None)) => {
-                            write!(fmt, " (vtable: impl <auto trait> for {ty})")?;
+                        Some(GlobalAlloc::VTable(ty, dyn_ty)) => {
+                            write!(fmt, " (vtable: impl {dyn_ty} for {ty})")?;
                         }
                         Some(GlobalAlloc::Static(did)) => {
                             write!(fmt, " (static: {})", self.ecx.tcx.def_path_str(did))?;

compiler/rustc_const_eval/src/interpret/traits.rs

@@ -1,6 +1,6 @@
 use rustc_middle::mir::interpret::{InterpResult, Pointer};
 use rustc_middle::ty::layout::LayoutOf;
-use rustc_middle::ty::{self, Ty, TyCtxt, VtblEntry};
+use rustc_middle::ty::{self, ExistentialPredicateStableCmpExt, Ty, TyCtxt, VtblEntry};
 use rustc_target::abi::{Align, Size};
 use tracing::trace;
 
@@ -11,26 +11,25 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     /// Creates a dynamic vtable for the given type and vtable origin. This is used only for
     /// objects.
     ///
-    /// The `trait_ref` encodes the erased self type. Hence, if we are making an object `Foo<Trait>`
-    /// from a value of type `Foo<T>`, then `trait_ref` would map `T: Trait`. `None` here means that
-    /// this is an auto trait without any methods, so we only need the basic vtable (drop, size,
-    /// align).
+    /// The `dyn_ty` encodes the erased self type. Hence, if we are making an object
+    /// `Foo<dyn Trait<Assoc = A> + Send>` from a value of type `Foo<T>`, then `dyn_ty`
+    /// would be `Trait<Assoc = A> + Send`. If this list doesn't have a principal trait ref,
+    /// we only need the basic vtable prefix (drop, size, align).
     pub fn get_vtable_ptr(
         &self,
         ty: Ty<'tcx>,
-        poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        dyn_ty: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
     ) -> InterpResult<'tcx, Pointer<Option<M::Provenance>>> {
-        trace!("get_vtable(trait_ref={:?})", poly_trait_ref);
+        trace!("get_vtable(ty={ty:?}, dyn_ty={dyn_ty:?})");
 
-        let (ty, poly_trait_ref) = self.tcx.erase_regions((ty, poly_trait_ref));
+        let (ty, dyn_ty) = self.tcx.erase_regions((ty, dyn_ty));
 
         // All vtables must be monomorphic, bail out otherwise.
         ensure_monomorphic_enough(*self.tcx, ty)?;
-        ensure_monomorphic_enough(*self.tcx, poly_trait_ref)?;
+        ensure_monomorphic_enough(*self.tcx, dyn_ty)?;
 
         let salt = M::get_global_alloc_salt(self, None);
-        let vtable_symbolic_allocation =
-            self.tcx.reserve_and_set_vtable_alloc(ty, poly_trait_ref, salt);
+        let vtable_symbolic_allocation = self.tcx.reserve_and_set_vtable_alloc(ty, dyn_ty, salt);
         let vtable_ptr = self.global_root_pointer(Pointer::from(vtable_symbolic_allocation))?;
         Ok(vtable_ptr.into())
     }
@@ -64,17 +63,45 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     /// expected trait type.
     pub(super) fn check_vtable_for_type(
         &self,
-        vtable_trait: Option<ty::PolyExistentialTraitRef<'tcx>>,
-        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        vtable_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        expected_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
     ) -> InterpResult<'tcx> {
-        let eq = match (expected_trait.principal(), vtable_trait) {
-            (Some(a), Some(b)) => self.eq_in_param_env(a, b),
-            (None, None) => true,
-            _ => false,
-        };
-        if !eq {
-            throw_ub!(InvalidVTableTrait { expected_trait, vtable_trait });
+        // We check validity by comparing the lists of predicates for equality. We *could* instead
+        // check that the dynamic type to which the vtable belongs satisfies all the expected
+        // predicates, but that would likely be a lot slower and seems unnecessarily permissive.
+
+        // FIXME: we are skipping auto traits for now, but might revisit this in the future.
+        let mut sorted_vtable: Vec<_> = vtable_dyn_type.without_auto_traits().collect();
+        let mut sorted_expected: Vec<_> = expected_dyn_type.without_auto_traits().collect();
+        // `skip_binder` here is okay because `stable_cmp` doesn't look at binders
+        sorted_vtable.sort_by(|a, b| a.skip_binder().stable_cmp(*self.tcx, &b.skip_binder()));
+        sorted_vtable.dedup();
+        sorted_expected.sort_by(|a, b| a.skip_binder().stable_cmp(*self.tcx, &b.skip_binder()));
+        sorted_expected.dedup();
+
+        if sorted_vtable.len() != sorted_expected.len() {
+            throw_ub!(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type });
+        }
+
+        for (a_pred, b_pred) in std::iter::zip(sorted_vtable, sorted_expected) {
+            let is_eq = match (a_pred.skip_binder(), b_pred.skip_binder()) {
+                (
+                    ty::ExistentialPredicate::Trait(a_data),
+                    ty::ExistentialPredicate::Trait(b_data),
+                ) => self.eq_in_param_env(a_pred.rebind(a_data), b_pred.rebind(b_data)),
+
+                (
+                    ty::ExistentialPredicate::Projection(a_data),
+                    ty::ExistentialPredicate::Projection(b_data),
+                ) => self.eq_in_param_env(a_pred.rebind(a_data), b_pred.rebind(b_data)),
+
+                _ => false,
+            };
+            if !is_eq {
+                throw_ub!(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type });
+            }
         }
+
         Ok(())
     }
 

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -452,8 +452,8 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                     self.path,
                     Ub(DanglingIntPointer{ .. } | InvalidVTablePointer(..)) =>
                         InvalidVTablePtr { value: format!("{vtable}") },
-                    Ub(InvalidVTableTrait { expected_trait, vtable_trait }) => {
-                        InvalidMetaWrongTrait { expected_trait, vtable_trait: *vtable_trait }
+                    Ub(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type }) => {
+                        InvalidMetaWrongTrait { vtable_dyn_type, expected_dyn_type }
                     },
                 );
             }
@@ -1281,8 +1281,8 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
                     self.path,
                     // It's not great to catch errors here, since we can't give a very good path,
                     // but it's better than ICEing.
-                    Ub(InvalidVTableTrait { expected_trait, vtable_trait }) => {
-                        InvalidMetaWrongTrait { expected_trait, vtable_trait: *vtable_trait }
+                    Ub(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type }) => {
+                        InvalidMetaWrongTrait { vtable_dyn_type, expected_dyn_type: *expected_dyn_type }
                     },
                 );
             }

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -365,8 +365,10 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     InvalidVTablePointer(Pointer<AllocId>),
     /// Using a vtable for the wrong trait.
     InvalidVTableTrait {
-        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
-        vtable_trait: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        /// The vtable that was actually referenced by the wide pointer metadata.
+        vtable_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        /// The vtable that was expected at the point in MIR that it was accessed.
+        expected_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
     },
     /// Using a string that is not valid UTF-8,
     InvalidStr(std::str::Utf8Error),
@@ -479,8 +481,10 @@ pub enum ValidationErrorKind<'tcx> {
         value: String,
     },
     InvalidMetaWrongTrait {
-        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
-        vtable_trait: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        /// The vtable that was actually referenced by the wide pointer metadata.
+        vtable_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        /// The vtable that was expected at the point in MIR that it was accessed.
+        expected_dyn_type: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
     },
     InvalidMetaSliceTooLarge {
         ptr_kind: PointerKind,

compiler/rustc_middle/src/mir/interpret/mod.rs

@@ -232,9 +232,8 @@ impl<'s> AllocDecodingSession<'s> {
             }
             AllocDiscriminant::VTable => {
                 trace!("creating vtable alloc ID");
-                let ty = <Ty<'_> as Decodable<D>>::decode(decoder);
-                let poly_trait_ref =
-                    <Option<ty::PolyExistentialTraitRef<'_>> as Decodable<D>>::decode(decoder);
+                let ty = Decodable::decode(decoder);
+                let poly_trait_ref = Decodable::decode(decoder);
                 trace!("decoded vtable alloc instance: {ty:?}, {poly_trait_ref:?}");
                 decoder.interner().reserve_and_set_vtable_alloc(ty, poly_trait_ref, CTFE_ALLOC_SALT)
             }
@@ -259,7 +258,10 @@ pub enum GlobalAlloc<'tcx> {
     /// The alloc ID is used as a function pointer.
     Function { instance: Instance<'tcx> },
     /// This alloc ID points to a symbolic (not-reified) vtable.
-    VTable(Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>),
+    /// We remember the full dyn type, not just the principal trait, so that
+    /// const-eval and Miri can detect UB due to invalid transmutes of
+    /// `dyn Trait` types.
+    VTable(Ty<'tcx>, &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>),
     /// The alloc ID points to a "lazy" static variable that did not get computed (yet).
     /// This is also used to break the cycle in recursive statics.
     Static(DefId),
@@ -293,7 +295,7 @@ impl<'tcx> GlobalAlloc<'tcx> {
     #[inline]
     pub fn unwrap_vtable(&self) -> (Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>) {
         match *self {
-            GlobalAlloc::VTable(ty, poly_trait_ref) => (ty, poly_trait_ref),
+            GlobalAlloc::VTable(ty, dyn_ty) => (ty, dyn_ty.principal()),
             _ => bug!("expected vtable, got {:?}", self),
         }
     }
@@ -398,10 +400,10 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn reserve_and_set_vtable_alloc(
         self,
         ty: Ty<'tcx>,
-        poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        dyn_ty: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
         salt: usize,
     ) -> AllocId {
-        self.reserve_and_set_dedup(GlobalAlloc::VTable(ty, poly_trait_ref), salt)
+        self.reserve_and_set_dedup(GlobalAlloc::VTable(ty, dyn_ty), salt)
     }
 
     /// Interns the `Allocation` and return a new `AllocId`, even if there's already an identical