properly track why we checked whether a pointer is in-bounds

also simplify the in-bounds checking in Miri's borrow trackers

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -282,7 +282,7 @@ const_eval_pointer_out_of_bounds =
         *[many] bytes
     } starting at offset {$ptr_offset} is out-of-bounds
 const_eval_pointer_use_after_free =
-    pointer to {$allocation} was dereferenced after this allocation got freed
+    {$bad_pointer_message}: {$alloc_id} has been freed, so this pointer is dangling
 const_eval_ptr_as_bytes_1 =
     this code performed an operation that depends on the underlying bytes representing a pointer
 const_eval_ptr_as_bytes_2 =

compiler/rustc_const_eval/src/errors.rs

@@ -492,7 +492,7 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             InvalidMeta(InvalidMetaKind::SliceTooBig) => const_eval_invalid_meta_slice,
             InvalidMeta(InvalidMetaKind::TooBig) => const_eval_invalid_meta,
             UnterminatedCString(_) => const_eval_unterminated_c_string,
-            PointerUseAfterFree(_) => const_eval_pointer_use_after_free,
+            PointerUseAfterFree(_, _) => const_eval_pointer_use_after_free,
             PointerOutOfBounds { ptr_size: Size::ZERO, .. } => const_eval_zst_pointer_out_of_bounds,
             PointerOutOfBounds { .. } => const_eval_pointer_out_of_bounds,
             DanglingIntPointer(0, _) => const_eval_dangling_null_pointer,
@@ -545,8 +545,10 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             UnterminatedCString(ptr) | InvalidFunctionPointer(ptr) | InvalidVTablePointer(ptr) => {
                 builder.set_arg("pointer", ptr);
             }
-            PointerUseAfterFree(allocation) => {
-                builder.set_arg("allocation", allocation);
+            PointerUseAfterFree(alloc_id, msg) => {
+                builder
+                    .set_arg("alloc_id", alloc_id)
+                    .set_arg("bad_pointer_message", bad_pointer_message(msg, handler));
             }
             PointerOutOfBounds { alloc_id, alloc_size, ptr_offset, ptr_size, msg } => {
                 builder

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -317,7 +317,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         kind = "static_mem"
                     )
                 }
-                None => err_ub!(PointerUseAfterFree(alloc_id)),
+                None => err_ub!(PointerUseAfterFree(alloc_id, CheckInAllocMsg::MemoryAccessTest)),
             }
             .into());
         };
@@ -380,7 +380,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             M::enforce_alignment(self),
             CheckInAllocMsg::MemoryAccessTest,
             |alloc_id, offset, prov| {
-                let (size, align) = self.get_live_alloc_size_and_align(alloc_id)?;
+                let (size, align) = self
+                    .get_live_alloc_size_and_align(alloc_id, CheckInAllocMsg::MemoryAccessTest)?;
                 Ok((size, align, (alloc_id, offset, prov)))
             },
         )
@@ -404,7 +405,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             CheckAlignment::Error,
             msg,
             |alloc_id, _, _| {
-                let (size, align) = self.get_live_alloc_size_and_align(alloc_id)?;
+                let (size, align) = self.get_live_alloc_size_and_align(alloc_id, msg)?;
                 Ok((size, align, ()))
             },
         )?;
@@ -414,7 +415,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Low-level helper function to check if a ptr is in-bounds and potentially return a reference
     /// to the allocation it points to. Supports both shared and mutable references, as the actual
     /// checking is offloaded to a helper closure. `align` defines whether and which alignment check
-    /// is done. Returns `None` for size 0, and otherwise `Some` of what `alloc_size` returned.
+    /// is done.
+    ///
+    /// If this returns `None`, the size is 0; it can however return `Some` even for size 0.
     fn check_and_deref_ptr<T>(
         &self,
         ptr: Pointer<Option<M::Provenance>>,
@@ -515,7 +518,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
             Some(GlobalAlloc::Function(..)) => throw_ub!(DerefFunctionPointer(id)),
             Some(GlobalAlloc::VTable(..)) => throw_ub!(DerefVTablePointer(id)),
-            None => throw_ub!(PointerUseAfterFree(id)),
+            None => throw_ub!(PointerUseAfterFree(id, CheckInAllocMsg::MemoryAccessTest)),
             Some(GlobalAlloc::Static(def_id)) => {
                 assert!(self.tcx.is_static(def_id));
                 assert!(!self.tcx.is_thread_local_static(def_id));
@@ -761,11 +764,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    /// Obtain the size and alignment of a live allocation.
-    pub fn get_live_alloc_size_and_align(&self, id: AllocId) -> InterpResult<'tcx, (Size, Align)> {
+    /// Obtain the size and alignment of a *live* allocation.
+    fn get_live_alloc_size_and_align(
+        &self,
+        id: AllocId,
+        msg: CheckInAllocMsg,
+    ) -> InterpResult<'tcx, (Size, Align)> {
         let (size, align, kind) = self.get_alloc_info(id);
         if matches!(kind, AllocKind::Dead) {
-            throw_ub!(PointerUseAfterFree(id))
+            throw_ub!(PointerUseAfterFree(id, msg))
         }
         Ok((size, align))
     }

compiler/rustc_const_eval/src/interpret/step.rs

@@ -224,8 +224,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Len(place) => {
                 let src = self.eval_place(place)?;
-                let op = self.place_to_op(&src)?;
-                let len = op.len(self)?;
+                let len = src.len(self)?;
                 self.write_scalar(Scalar::from_target_usize(len, self), &dest)?;
             }
 

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

@@ -282,8 +282,8 @@ pub enum UndefinedBehaviorInfo<'a> {
     InvalidMeta(InvalidMetaKind),
     /// Reading a C string that does not end within its allocation.
     UnterminatedCString(Pointer),
-    /// Dereferencing a dangling pointer after it got freed.
-    PointerUseAfterFree(AllocId),
+    /// Using a pointer after it got freed.
+    PointerUseAfterFree(AllocId, CheckInAllocMsg),
     /// Used a pointer outside the bounds it is valid for.
     /// (If `ptr_size > 0`, determines the size of the memory range that was expected to be in-bounds.)
     PointerOutOfBounds {

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -18,7 +18,7 @@ use rustc_middle::ty::{
     layout::{HasParamEnv, LayoutOf},
     Ty,
 };
-use rustc_target::abi::{Abi, Size};
+use rustc_target::abi::{Abi, Align, Size};
 
 use crate::borrow_tracker::{
     stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder},
@@ -619,6 +619,8 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         retag_info: RetagInfo, // diagnostics info about this retag
     ) -> InterpResult<'tcx, Option<AllocId>> {
         let this = self.eval_context_mut();
+        // Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
+        this.check_ptr_access_align(place.ptr, size, Align::ONE, CheckInAllocMsg::InboundsTest)?;
 
         // It is crucial that this gets called on all code paths, to ensure we track tag creation.
         let log_creation = |this: &MiriInterpCx<'mir, 'tcx>,
@@ -707,18 +709,6 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         let (alloc_id, base_offset, orig_tag) = this.ptr_get_alloc_id(place.ptr)?;
         log_creation(this, Some((alloc_id, base_offset, orig_tag)))?;
 
-        // Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
-        let (alloc_size, _) = this.get_live_alloc_size_and_align(alloc_id)?;
-        if base_offset + size > alloc_size {
-            throw_ub!(PointerOutOfBounds {
-                alloc_id,
-                alloc_size,
-                ptr_offset: this.target_usize_to_isize(base_offset.bytes()),
-                ptr_size: size,
-                msg: CheckInAllocMsg::InboundsTest
-            });
-        }
-
         trace!(
             "reborrow: reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
             new_tag,

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

@@ -1,6 +1,6 @@
 use log::trace;
 
-use rustc_target::abi::{Abi, Size};
+use rustc_target::abi::{Abi, Align, Size};
 
 use crate::borrow_tracker::{AccessKind, GlobalStateInner, ProtectorKind, RetagFields};
 use rustc_middle::{
@@ -182,6 +182,8 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         new_tag: BorTag,
     ) -> InterpResult<'tcx, Option<(AllocId, BorTag)>> {
         let this = self.eval_context_mut();
+        // Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
+        this.check_ptr_access_align(place.ptr, ptr_size, Align::ONE, CheckInAllocMsg::InboundsTest)?;
 
         // It is crucial that this gets called on all code paths, to ensure we track tag creation.
         let log_creation = |this: &MiriInterpCx<'mir, 'tcx>,
@@ -202,51 +204,33 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         };
 
         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 {
-            match this.ptr_try_get_alloc_id(place.ptr) {
-                Ok(data) => data,
-                Err(_) => {
-                    // This pointer doesn't come with an AllocId, so there's no
-                    // memory to do retagging in.
-                    trace!(
-                        "reborrow of size 0: reference {:?} derived from {:?} (pointee {})",
-                        new_tag,
-                        place.ptr,
-                        place.layout.ty,
-                    );
-                    log_creation(this, None)?;
-                    return Ok(None);
-                }
+        let (alloc_id, base_offset, parent_prov) = match this.ptr_try_get_alloc_id(place.ptr) {
+            Ok(data) => {
+                // Unlike SB, we *do* a proper retag for size 0 if can identify the allocation.
+                // After all, the pointer may be lazily initialized outside this initial range.
+                data
+            },
+            Err(_) => {
+                assert_eq!(ptr_size, Size::ZERO); // we did the deref check above, size has to be 0 here
+                // This pointer doesn't come with an AllocId, so there's no
+                // memory to do retagging in.
+                trace!(
+                    "reborrow of size 0: reference {:?} derived from {:?} (pointee {})",
+                    new_tag,
+                    place.ptr,
+                    place.layout.ty,
+                );
+                log_creation(this, None)?;
+                return Ok(None);
             }
         };
+        log_creation(this, Some((alloc_id, base_offset, parent_prov)))?;
+
         let orig_tag = match parent_prov {
             ProvenanceExtra::Wildcard => return Ok(None), // TODO: handle wildcard pointers
             ProvenanceExtra::Concrete(tag) => tag,
         };
 
-        // Protection against trying to get a reference to a vtable:
-        // vtables do not have an alloc_extra so the call to
-        // `get_alloc_extra` that follows fails.
-        let (alloc_size, _align, alloc_kind) = this.get_alloc_info(alloc_id);
-        if ptr_size == Size::ZERO && !matches!(alloc_kind, AllocKind::LiveData) {
-            return Ok(Some((alloc_id, orig_tag)));
-        }
-
-        log_creation(this, Some((alloc_id, base_offset, parent_prov)))?;
-
-        // Ensure we bail out if the pointer goes out-of-bounds (see miri#1050).
-        if base_offset + ptr_size > alloc_size {
-            throw_ub!(PointerOutOfBounds {
-                alloc_id,
-                alloc_size,
-                ptr_offset: this.target_usize_to_isize(base_offset.bytes()),
-                ptr_size,
-                msg: CheckInAllocMsg::InboundsTest
-            });
-        }
-
         trace!(
             "reborrow: reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
             new_tag,

src/tools/miri/tests/fail/alloc/deallocate-twice.rs

@@ -1,6 +1,6 @@
 use std::alloc::{alloc, dealloc, Layout};
 
-//@error-in-other-file: dereferenced after this allocation got freed
+//@error-in-other-file: has been freed
 
 fn main() {
     unsafe {

src/tools/miri/tests/fail/alloc/deallocate-twice.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: memory access failed: ALLOC has been freed, so this pointer is dangling
   --> RUSTLIB/alloc/src/alloc.rs:LL:CC
    |
 LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ memory access failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/alloc/reallocate-change-alloc.rs

@@ -4,6 +4,6 @@ fn main() {
     unsafe {
         let x = alloc(Layout::from_size_align_unchecked(1, 1));
         let _y = realloc(x, Layout::from_size_align_unchecked(1, 1), 1);
-        let _z = *x; //~ ERROR: dereferenced after this allocation got freed
+        let _z = *x; //~ ERROR: has been freed
     }
 }

src/tools/miri/tests/fail/alloc/reallocate-change-alloc.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
   --> $DIR/reallocate-change-alloc.rs:LL:CC
    |
 LL |         let _z = *x;
-   |                  ^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |                  ^^ dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/alloc/reallocate-dangling.rs

@@ -1,6 +1,6 @@
 use std::alloc::{alloc, dealloc, realloc, Layout};
 
-//@error-in-other-file: dereferenced after this allocation got freed
+//@error-in-other-file: has been freed
 
 fn main() {
     unsafe {

src/tools/miri/tests/fail/alloc/reallocate-dangling.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: memory access failed: ALLOC has been freed, so this pointer is dangling
   --> RUSTLIB/alloc/src/alloc.rs:LL:CC
    |
 LL |     unsafe { __rust_realloc(ptr, layout.size(), layout.align(), new_size) }
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ memory access failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/concurrency/thread_local_static_dealloc.rs

@@ -11,6 +11,6 @@ unsafe impl Send for SendRaw {}
 fn main() {
     unsafe {
         let dangling_ptr = std::thread::spawn(|| SendRaw(&TLS as *const u8)).join().unwrap();
-        let _val = *dangling_ptr.0; //~ ERROR: dereferenced after this allocation got freed
+        let _val = *dangling_ptr.0; //~ ERROR: has been freed
     }
 }

src/tools/miri/tests/fail/concurrency/thread_local_static_dealloc.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
   --> $DIR/thread_local_static_dealloc.rs:LL:CC
    |
 LL |         let _val = *dangling_ptr.0;
-   |                    ^^^^^^^^^^^^^^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |                    ^^^^^^^^^^^^^^^ dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_addr_of.rs

@@ -7,6 +7,6 @@ fn main() {
         let b = Box::new(42);
         &*b as *const i32
     };
-    let x = unsafe { ptr::addr_of!(*p) }; //~ ERROR: dereferenced after this allocation got freed
+    let x = unsafe { ptr::addr_of!(*p) }; //~ ERROR: has been freed
     panic!("this should never print: {:?}", x);
 }

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_addr_of.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
   --> $DIR/dangling_pointer_addr_of.rs:LL:CC
    |
 LL |     let x = unsafe { ptr::addr_of!(*p) };
-   |                      ^^^^^^^^^^^^^^^^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |                      ^^^^^^^^^^^^^^^^^ dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref.rs

@@ -6,6 +6,6 @@ fn main() {
         let b = Box::new(42);
         &*b as *const i32
     };
-    let x = unsafe { *p }; //~ ERROR: dereferenced after this allocation got freed
+    let x = unsafe { *p }; //~ ERROR: has been freed
     panic!("this should never print: {}", x);
 }

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+error: Undefined Behavior: dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
   --> $DIR/dangling_pointer_deref.rs:LL:CC
    |
 LL |     let x = unsafe { *p };
-   |                      ^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |                      ^^ dereferencing pointer failed: ALLOC has been freed, so this pointer is dangling
    |
    = 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

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_offset.rs

@@ -0,0 +1,11 @@
+// Make sure we find these even with many checks disabled.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32
+    };
+    let x = unsafe { p.offset(42) }; //~ ERROR: /out-of-bounds pointer arithmetic: .* has been freed/
+    panic!("this should never print: {:?}", x);
+}

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_offset.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: out-of-bounds pointer arithmetic: ALLOC has been freed, so this pointer is dangling
+  --> $DIR/dangling_pointer_offset.rs:LL:CC
+   |
+LL |     let x = unsafe { p.offset(42) };
+   |                      ^^^^^^^^^^^^ out-of-bounds pointer arithmetic: ALLOC has been freed, so this pointer is dangling
+   |
+   = 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/dangling_pointer_offset.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.rs renamed to src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_project_underscore.rs

@@ -7,7 +7,7 @@ fn main() {
         &*b as *const i32
     };
     unsafe {
-        let _ = *p; //~ ERROR: dereferenced after this allocation got freed
+        let _ = *p; //~ ERROR: has been freed
     }
     panic!("this should never print");
 }