CTFE: throw unsupported error when partially overwriting a pointer

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

@@ -12,7 +12,7 @@ use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Align, HasDataLayout, Size};
 
 use super::{
-    read_target_uint, write_target_uint, AllocId, InterpError, InterpResult, Pointer,
+    read_target_uint, write_target_uint, AllocId, InterpError, InterpResult, Pointer, Provenance,
     ResourceExhaustionInfo, Scalar, ScalarMaybeUninit, UndefinedBehaviorInfo, UninitBytesAccess,
     UnsupportedOpInfo,
 };
@@ -53,18 +53,22 @@ pub struct Allocation<Tag = AllocId, Extra = ()> {
 pub enum AllocError {
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
+    /// Partially overwriting a pointer.
+    PartialPointerOverwrite(Size),
     /// Using uninitialized data where it is not allowed.
     InvalidUninitBytes(Option<UninitBytesAccess>),
 }
 pub type AllocResult<T = ()> = Result<T, AllocError>;
 
 impl AllocError {
     pub fn to_interp_error<'tcx>(self, alloc_id: AllocId) -> InterpError<'tcx> {
+        use AllocError::*;
         match self {
-            AllocError::ReadPointerAsBytes => {
-                InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes)
-            }
-            AllocError::InvalidUninitBytes(info) => InterpError::UndefinedBehavior(
+            ReadPointerAsBytes => InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes),
+            PartialPointerOverwrite(offset) => InterpError::Unsupported(
+                UnsupportedOpInfo::PartialPointerOverwrite(Pointer::new(alloc_id, offset)),
+            ),
+            InvalidUninitBytes(info) => InterpError::UndefinedBehavior(
                 UndefinedBehaviorInfo::InvalidUninitBytes(info.map(|b| (alloc_id, b))),
             ),
         }
@@ -218,7 +222,7 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
 }
 
 /// Byte accessors.
-impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
+impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
     /// The last argument controls whether we error out when there are uninitialized
     /// or pointer bytes. You should never call this, call `get_bytes` or
     /// `get_bytes_with_uninit_and_ptr` instead,
@@ -275,30 +279,35 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
     /// It is the caller's responsibility to check bounds and alignment beforehand.
     /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
     /// on `InterpCx` instead.
-    pub fn get_bytes_mut(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> &mut [u8] {
+    pub fn get_bytes_mut(
+        &mut self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+    ) -> AllocResult<&mut [u8]> {
         self.mark_init(range, true);
-        self.clear_relocations(cx, range);
+        self.clear_relocations(cx, range)?;
 
-        &mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
+        Ok(&mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
     }
 
     /// A raw pointer variant of `get_bytes_mut` that avoids invalidating existing aliases into this memory.
-    pub fn get_bytes_mut_ptr(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> *mut [u8] {
+    pub fn get_bytes_mut_ptr(
+        &mut self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+    ) -> AllocResult<*mut [u8]> {
         self.mark_init(range, true);
-        // This also clears relocations that just overlap with the written range. So writing to some
-        // byte can de-initialize its neighbors! See
-        // <https://github.com/rust-lang/rust/issues/87184> for details.
-        self.clear_relocations(cx, range);
+        self.clear_relocations(cx, range)?;
 
         assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
         let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
         let len = range.end().bytes_usize() - range.start.bytes_usize();
-        ptr::slice_from_raw_parts_mut(begin_ptr, len)
+        Ok(ptr::slice_from_raw_parts_mut(begin_ptr, len))
     }
 }
 
 /// Reading and writing.
-impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
+impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
     /// Validates that `ptr.offset` and `ptr.offset + size` do not point to the middle of a
     /// relocation. If `allow_uninit_and_ptr` is `false`, also enforces that the memory in the
     /// given range contains neither relocations nor uninitialized bytes.
@@ -395,7 +404,7 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         };
 
         let endian = cx.data_layout().endian;
-        let dst = self.get_bytes_mut(cx, range);
+        let dst = self.get_bytes_mut(cx, range)?;
         write_target_uint(endian, dst, bytes).unwrap();
 
         // See if we have to also write a relocation.
@@ -433,13 +442,16 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
     /// uninitialized. This is a somewhat odd "spooky action at a distance",
     /// but it allows strictly more code to run than if we would just error
     /// immediately in that case.
-    fn clear_relocations(&mut self, cx: &impl HasDataLayout, range: AllocRange) {
+    fn clear_relocations(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult
+    where
+        Tag: Provenance,
+    {
         // Find the start and end of the given range and its outermost relocations.
         let (first, last) = {
             // Find all relocations overlapping the given range.
             let relocations = self.get_relocations(cx, range);
             if relocations.is_empty() {
-                return;
+                return Ok(());
             }
 
             (
@@ -450,17 +462,27 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         let start = range.start;
         let end = range.end();
 
-        // Mark parts of the outermost relocations as uninitialized if they partially fall outside the
-        // given range.
+        // We need to handle clearing the relocations from parts of a pointer. See
+        // <https://github.com/rust-lang/rust/issues/87184> for details.
         if first < start {
+            if Tag::ERR_ON_PARTIAL_PTR_OVERWRITE {
+                return Err(AllocError::PartialPointerOverwrite(first));
+            }
             self.init_mask.set_range(first, start, false);
         }
         if last > end {
+            if Tag::ERR_ON_PARTIAL_PTR_OVERWRITE {
+                return Err(AllocError::PartialPointerOverwrite(
+                    last - cx.data_layout().pointer_size,
+                ));
+            }
             self.init_mask.set_range(end, last, false);
         }
 
         // Forget all the relocations.
         self.relocations.0.remove_range(first..last);
+
+        Ok(())
     }
 
     /// Errors if there are relocations overlapping with the edges of the

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

@@ -404,6 +404,9 @@ pub enum UnsupportedOpInfo {
     Unsupported(String),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
+    /// Overwriting parts of a pointer; the resulting state cannot be represented in our
+    /// `Allocation` data structure.
+    PartialPointerOverwrite(Pointer<AllocId>),
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     //
@@ -418,9 +421,12 @@ impl fmt::Display for UnsupportedOpInfo {
         use UnsupportedOpInfo::*;
         match self {
             Unsupported(ref msg) => write!(f, "{}", msg),
-            ReadExternStatic(did) => write!(f, "cannot read from extern static ({:?})", did),
-            ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes",),
+            ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes"),
+            PartialPointerOverwrite(ptr) => {
+                write!(f, "unable to overwrite parts of a pointer in memory at {:?}", ptr)
+            }
             ThreadLocalStatic(did) => write!(f, "cannot access thread local static ({:?})", did),
+            ReadExternStatic(did) => write!(f, "cannot read from extern static ({:?})", did),
         }
     }
 }

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

@@ -108,6 +108,10 @@ pub trait Provenance: Copy + fmt::Debug {
     /// If `false`, ptr-to-int casts are not supported. The offset *must* be relative in that case.
     const OFFSET_IS_ADDR: bool;
 
+    /// We also use this trait to control whether to abort execution when a pointer is being partially overwritten
+    /// (this avoids a separate trait in `allocation.rs` just for this purpose).
+    const ERR_ON_PARTIAL_PTR_OVERWRITE: bool;
+
     /// Determines how a pointer should be printed.
     fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result
     where
@@ -123,6 +127,9 @@ impl Provenance for AllocId {
     // so ptr-to-int casts are not possible (since we do not know the global physical offset).
     const OFFSET_IS_ADDR: bool = false;
 
+    // For now, do not allow this, so that we keep our options open.
+    const ERR_ON_PARTIAL_PTR_OVERWRITE: bool = true;
+
     fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // Forward `alternate` flag to `alloc_id` printing.
         if f.alternate() {

compiler/rustc_mir/src/interpret/memory.rs

@@ -907,7 +907,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> std::fmt::Debug for DumpAllocs<'a,
 }
 
 /// Reading and writing.
-impl<'tcx, 'a, Tag: Copy, Extra> AllocRefMut<'a, 'tcx, Tag, Extra> {
+impl<'tcx, 'a, Tag: Provenance, Extra> AllocRefMut<'a, 'tcx, Tag, Extra> {
     pub fn write_scalar(
         &mut self,
         range: AllocRange,
@@ -928,7 +928,7 @@ impl<'tcx, 'a, Tag: Copy, Extra> AllocRefMut<'a, 'tcx, Tag, Extra> {
     }
 }
 
-impl<'tcx, 'a, Tag: Copy, Extra> AllocRef<'a, 'tcx, Tag, Extra> {
+impl<'tcx, 'a, Tag: Provenance, Extra> AllocRef<'a, 'tcx, Tag, Extra> {
     pub fn read_scalar(&self, range: AllocRange) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
         Ok(self
             .alloc
@@ -998,7 +998,11 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
 
         // Side-step AllocRef and directly access the underlying bytes more efficiently.
         // (We are staying inside the bounds here so all is good.)
-        let bytes = alloc_ref.alloc.get_bytes_mut(&alloc_ref.tcx, alloc_ref.range);
+        let alloc_id = alloc_ref.alloc_id;
+        let bytes = alloc_ref
+            .alloc
+            .get_bytes_mut(&alloc_ref.tcx, alloc_ref.range)
+            .map_err(move |e| e.to_interp_error(alloc_id))?;
         // `zip` would stop when the first iterator ends; we want to definitely
         // cover all of `bytes`.
         for dest in bytes {
@@ -1072,7 +1076,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         let (dest_alloc, extra) = self.get_raw_mut(dest_alloc_id)?;
         let dest_range = alloc_range(dest_offset, size * num_copies);
         M::memory_written(extra, &mut dest_alloc.extra, dest.provenance, dest_range)?;
-        let dest_bytes = dest_alloc.get_bytes_mut_ptr(&tcx, dest_range).as_mut_ptr();
+        let dest_bytes = dest_alloc
+            .get_bytes_mut_ptr(&tcx, dest_range)
+            .map_err(|e| e.to_interp_error(dest_alloc_id))?
+            .as_mut_ptr();
 
         if compressed.no_bytes_init() {
             // Fast path: If all bytes are `uninit` then there is nothing to copy. The target range

src/test/ui/consts/const-eval/partial_ptr_overwrite.rs

@@ -0,0 +1,15 @@
+// Test for the behavior described in <https://github.com/rust-lang/rust/issues/87184>.
+#![feature(const_mut_refs, const_raw_ptr_deref)]
+
+const PARTIAL_OVERWRITE: () = {
+    let mut p = &42;
+    unsafe {
+        let ptr: *mut _ = &mut p;
+        *(ptr as *mut u8) = 123; //~ ERROR any use of this value
+        //~| unable to overwrite parts of a pointer
+        //~| WARN previously accepted
+    }
+    let x = *p;
+};
+
+fn main() {}

src/test/ui/consts/const-eval/partial_ptr_overwrite.stderr

@@ -0,0 +1,20 @@
+error: any use of this value will cause an error
+  --> $DIR/partial_ptr_overwrite.rs:8:9
+   |
+LL | / const PARTIAL_OVERWRITE: () = {
+LL | |     let mut p = &42;
+LL | |     unsafe {
+LL | |         let ptr: *mut _ = &mut p;
+LL | |         *(ptr as *mut u8) = 123;
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^ unable to overwrite parts of a pointer in memory at alloc4
+...  |
+LL | |     let x = *p;
+LL | | };
+   | |__-
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to previous error
+