Check all dangling pointers in validation instead of in interning

Author: oli-obk

compiler/rustc_const_eval/messages.ftl

@@ -49,7 +49,6 @@ const_eval_dangling_int_pointer =
 const_eval_dangling_null_pointer =
     {$bad_pointer_message}: null pointer is a dangling pointer (it has no provenance)
 
-const_eval_dangling_ptr_in_final = encountered dangling pointer in final value of {const_eval_intern_kind}
 const_eval_dead_local =
     accessing a dead local variable
 const_eval_dealloc_immutable =

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -403,7 +403,7 @@ fn const_validate_mplace<'mir, 'tcx>(
                 CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
             }
         };
-        ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)
+        ecx.const_validate_operand(mplace, path, &mut ref_tracking, mode)
             // Instead of just reporting the `InterpError` via the usual machinery, we give a more targetted
             // error about the validation failure.
             .map_err(|error| report_validation_error(&ecx, error, alloc_id))?;

compiler/rustc_const_eval/src/errors.rs

@@ -17,14 +17,6 @@ use rustc_target::abi::{Size, WrappingRange};
 
 use crate::interpret::InternKind;
 
-#[derive(Diagnostic)]
-#[diag(const_eval_dangling_ptr_in_final)]
-pub(crate) struct DanglingPtrInFinal {
-    #[primary_span]
-    pub span: Span,
-    pub kind: InternKind,
-}
-
 #[derive(Diagnostic)]
 #[diag(const_eval_mutable_ptr_in_final)]
 pub(crate) struct MutablePtrInFinal {

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -26,7 +26,7 @@ use rustc_span::sym;
 
 use super::{AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy};
 use crate::const_eval;
-use crate::errors::{DanglingPtrInFinal, MutablePtrInFinal};
+use crate::errors::MutablePtrInFinal;
 
 pub trait CompileTimeMachine<'mir, 'tcx: 'mir, T> = Machine<
         'mir,
@@ -278,9 +278,7 @@ pub fn intern_const_alloc_recursive<
             continue;
         }
         just_interned.insert(alloc_id);
-        todo.extend(intern_shallow(ecx, alloc_id).map_err(|()| {
-            ecx.tcx.dcx().emit_err(DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind })
-        })?);
+        todo.extend(intern_shallow(ecx, alloc_id).unwrap());
     }
     if found_bad_mutable_pointer {
         return Err(ecx

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -606,17 +606,14 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 if place.layout.is_unsized() {
                     self.check_wide_ptr_meta(place.meta(), place.layout)?;
                 }
-                if let Some(prov) = place.ptr().provenance {
-                    if let Some(alloc_id) = prov.get_alloc_id() {
-                        if let AllocKind::Dead = self.ecx.get_alloc_info(alloc_id).2 {
-                            throw_validation_failure!(
-                                self.path,
-                                DanglingPtrUseAfterFree {
-                                    ptr_kind: PointerKind::Ref(Mutability::Not)
-                                }
-                            )
-                        }
-                    }
+                if self.ref_tracking.is_some()
+                    && let Some(alloc_id) = place.ptr().provenance.and_then(|p| p.get_alloc_id())
+                    && let AllocKind::Dead = self.ecx.get_alloc_info(alloc_id).2
+                {
+                    throw_validation_failure!(
+                        self.path,
+                        DanglingPtrUseAfterFree { ptr_kind: PointerKind::Ref(Mutability::Not) }
+                    )
                 }
                 Ok(true)
             }
@@ -988,15 +985,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         path: Vec<PathElem>,
         ref_tracking: Option<&mut RefTracking<MPlaceTy<'tcx, M::Provenance>, Vec<PathElem>>>,
         ctfe_mode: Option<CtfeValidationMode>,
-    ) -> InterpResult<'tcx> {
+    ) -> InterpResult<'tcx, Vec<PathElem>> {
         trace!("validate_operand_internal: {:?}, {:?}", *op, op.layout.ty);
 
         // Construct a visitor
         let mut visitor = ValidityVisitor { path, ref_tracking, ctfe_mode, ecx: self };
 
         // Run it.
         match self.run_for_validation(|| visitor.visit_value(op)) {
-            Ok(()) => Ok(()),
+            Ok(()) => Ok(visitor.path),
             // Pass through validation failures and "invalid program" issues.
             Err(err)
                 if matches!(
@@ -1016,7 +1013,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
         }
     }
+}
 
+impl<'mir, 'tcx> InterpCx<'mir, 'tcx, crate::const_eval::CompileTimeInterpreter<'mir, 'tcx>> {
     /// This function checks the data at `op` to be const-valid.
     /// `op` is assumed to cover valid memory if it is an indirect operand.
     /// It will error if the bits at the destination do not match the ones described by the layout.
@@ -1030,14 +1029,38 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     #[inline(always)]
     pub(crate) fn const_validate_operand(
         &self,
-        op: &OpTy<'tcx, M::Provenance>,
+        mplace: MPlaceTy<'tcx>,
         path: Vec<PathElem>,
-        ref_tracking: &mut RefTracking<MPlaceTy<'tcx, M::Provenance>, Vec<PathElem>>,
+        ref_tracking: &mut RefTracking<MPlaceTy<'tcx>, Vec<PathElem>>,
         ctfe_mode: CtfeValidationMode,
     ) -> InterpResult<'tcx> {
-        self.validate_operand_internal(op, path, Some(ref_tracking), Some(ctfe_mode))
+        let prov = mplace.ptr().provenance;
+        let path = self.validate_operand_internal(
+            &mplace.into(),
+            path,
+            Some(ref_tracking),
+            Some(ctfe_mode),
+        )?;
+
+        // There was no error, so let's check the rest of the relocations in the pointed-to allocation for
+        // dangling pointers.
+        if let Some(prov) = prov
+            && let Some((_, alloc)) = self.memory.alloc_map().get(&prov.alloc_id())
+        {
+            for (_, prov) in alloc.provenance().ptrs().iter() {
+                if let AllocKind::Dead = self.get_alloc_info(prov.alloc_id()).2 {
+                    throw_validation_failure!(
+                        path,
+                        DanglingPtrUseAfterFree { ptr_kind: PointerKind::Ref(Mutability::Not) }
+                    )
+                }
+            }
+        }
+        Ok(())
     }
+}
 
+impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// This function checks the data at `op` to be runtime-valid.
     /// `op` is assumed to cover valid memory if it is an indirect operand.
     /// It will error if the bits at the destination do not match the ones described by the layout.
@@ -1047,6 +1070,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // still correct to not use `ctfe_mode`: that mode is for validation of the final constant
         // value, it rules out things like `UnsafeCell` in awkward places. It also can make checking
         // recurse through references which, for now, we don't want here, either.
-        self.validate_operand_internal(op, vec![], None, None)
+        self.validate_operand_internal(op, vec![], None, None).map(drop)
     }
 }

tests/ui/consts/dangling_raw_ptr.rs

@@ -7,7 +7,7 @@ union Union {
     ptr: *const u32
 }
 
-const BAR: Union = { //~ ERROR encountered dangling pointer in final value
+const BAR: Union = { //~ ERROR it is undefined behavior
     let x = 42;
     Union { ptr: &x }
 };

tests/ui/consts/dangling_raw_ptr.stderr

@@ -9,11 +9,16 @@ LL | const FOO: *const u32 = {
                ╾ALLOC0<imm>╼                         │ ╾──────╼
            }
 
-error: encountered dangling pointer in final value of constant
+error[E0080]: it is undefined behavior to use this value
   --> $DIR/dangling_raw_ptr.rs:10:1
    |
 LL | const BAR: Union = {
-   | ^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling reference (use-after-free)
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: 8, align: 8) {
+               ╾ALLOC1<imm>╼                         │ ╾──────╼
+           }
 
 error: aborting due to 2 previous errors