Auto merge of #97684 - RalfJung:better-provenance-control, r=oli-obk

interpret: better control over whether we read data with provenance

The resolution in rust-lang/unsafe-code-guidelines#286 seems to be that when we load data at integer type, we implicitly strip provenance. So let's implement that in Miri at least for scalar loads. This makes use of the fact that `Scalar` layouts distinguish pointer-sized integers and pointers -- so I was expecting some wild bugs where layouts set this incorrectly, but so far that does not seem to happen.

This does not entirely implement the solution to rust-lang/unsafe-code-guidelines#286; we still do the wrong thing for integers in larger types: we will `copy_op` them and then do validation, and validation will complain about the provenance. To fix that we need mutating validation; validation needs to strip the provenance rather than complaining about it. This is a larger undertaking (but will also help resolve rust-lang/miri#845 since we can reset padding to `Uninit`).

The reason this is useful is that we can now implement `addr` as a `transmute` from a pointer to an integer, and actually get the desired behavior of stripping provenance without exposing it!

Author: bors

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -908,11 +908,15 @@ impl<'tcx, 'a, Tag: Provenance, Extra> AllocRefMut<'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>> {
+    pub fn read_scalar(
+        &self,
+        range: AllocRange,
+        read_provenance: bool,
+    ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
         let range = self.range.subrange(range);
         let res = self
             .alloc
-            .read_scalar(&self.tcx, range)
+            .read_scalar(&self.tcx, range, read_provenance)
             .map_err(|e| e.to_interp_error(self.alloc_id))?;
         debug!(
             "read_scalar in {} at {:#x}, size {}: {:?}",
@@ -924,8 +928,19 @@ impl<'tcx, 'a, Tag: Provenance, Extra> AllocRef<'a, 'tcx, Tag, Extra> {
         Ok(res)
     }
 
-    pub fn read_ptr_sized(&self, offset: Size) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
-        self.read_scalar(alloc_range(offset, self.tcx.data_layout().pointer_size))
+    pub fn read_integer(
+        &self,
+        offset: Size,
+        size: Size,
+    ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
+        self.read_scalar(alloc_range(offset, size), /*read_provenance*/ false)
+    }
+
+    pub fn read_pointer(&self, offset: Size) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
+        self.read_scalar(
+            alloc_range(offset, self.tcx.data_layout().pointer_size),
+            /*read_provenance*/ true,
+        )
     }
 
     pub fn check_bytes(

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -15,8 +15,8 @@ use rustc_target::abi::{VariantIdx, Variants};
 
 use super::{
     alloc_range, from_known_layout, mir_assign_valid_types, AllocId, ConstValue, GlobalId,
-    InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer, Provenance,
-    Scalar, ScalarMaybeUninit,
+    InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer,
+    PointerArithmetic, Provenance, Scalar, ScalarMaybeUninit,
 };
 
 /// An `Immediate` represents a single immediate self-contained Rust value.
@@ -284,11 +284,18 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Abi::Scalar(s) if force => Some(s.primitive()),
             _ => None,
         };
-        if let Some(_s) = scalar_layout {
+        let read_provenance = |s: abi::Primitive, size| {
+            // Should be just `s.is_ptr()`, but we support a Miri flag that accepts more
+            // questionable ptr-int transmutes.
+            let number_may_have_provenance = !M::enforce_number_no_provenance(self);
+            s.is_ptr() || (number_may_have_provenance && size == self.pointer_size())
+        };
+        if let Some(s) = scalar_layout {
             //FIXME(#96185): let size = s.size(self);
             //FIXME(#96185): assert_eq!(size, mplace.layout.size, "abi::Scalar size does not match layout size");
             let size = mplace.layout.size; //FIXME(#96185): remove this line
-            let scalar = alloc.read_scalar(alloc_range(Size::ZERO, size))?;
+            let scalar =
+                alloc.read_scalar(alloc_range(Size::ZERO, size), read_provenance(s, size))?;
             return Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }));
         }
         let scalar_pair_layout = match mplace.layout.abi {
@@ -306,8 +313,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             let (a_size, b_size) = (a.size(self), b.size(self));
             let b_offset = a_size.align_to(b.align(self).abi);
             assert!(b_offset.bytes() > 0); // in `operand_field` we use the offset to tell apart the fields
-            let a_val = alloc.read_scalar(alloc_range(Size::ZERO, a_size))?;
-            let b_val = alloc.read_scalar(alloc_range(b_offset, b_size))?;
+            let a_val =
+                alloc.read_scalar(alloc_range(Size::ZERO, a_size), read_provenance(a, a_size))?;
+            let b_val =
+                alloc.read_scalar(alloc_range(b_offset, b_size), read_provenance(b, b_size))?;
             return Ok(Some(ImmTy {
                 imm: Immediate::ScalarPair(a_val, b_val),
                 layout: mplace.layout,

compiler/rustc_const_eval/src/interpret/traits.rs

@@ -50,7 +50,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let vtable_slot = self
             .get_ptr_alloc(vtable_slot, ptr_size, self.tcx.data_layout.pointer_align.abi)?
             .expect("cannot be a ZST");
-        let fn_ptr = self.scalar_to_ptr(vtable_slot.read_ptr_sized(Size::ZERO)?.check_init()?)?;
+        let fn_ptr = self.scalar_to_ptr(vtable_slot.read_pointer(Size::ZERO)?.check_init()?)?;
         self.get_ptr_fn(fn_ptr)
     }
 
@@ -69,9 +69,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             )?
             .expect("cannot be a ZST");
         let drop_fn = vtable
-            .read_ptr_sized(
-                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_DROPINPLACE).unwrap(),
-            )?
+            .read_pointer(pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_DROPINPLACE).unwrap())?
             .check_init()?;
         // We *need* an instance here, no other kind of function value, to be able
         // to determine the type.
@@ -104,12 +102,18 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             )?
             .expect("cannot be a ZST");
         let size = vtable
-            .read_ptr_sized(pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_SIZE).unwrap())?
+            .read_integer(
+                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_SIZE).unwrap(),
+                pointer_size,
+            )?
             .check_init()?;
         let size = size.to_machine_usize(self)?;
         let size = Size::from_bytes(size);
         let align = vtable
-            .read_ptr_sized(pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_ALIGN).unwrap())?
+            .read_integer(
+                pointer_size * u64::try_from(COMMON_VTABLE_ENTRIES_ALIGN).unwrap(),
+                pointer_size,
+            )?
             .check_init()?;
         let align = align.to_machine_usize(self)?;
         let align = Align::from_bytes(align).map_err(|e| err_ub!(InvalidVtableAlignment(e)))?;
@@ -132,8 +136,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             .get_ptr_alloc(vtable_slot, pointer_size, self.tcx.data_layout.pointer_align.abi)?
             .expect("cannot be a ZST");
 
-        let new_vtable =
-            self.scalar_to_ptr(new_vtable.read_ptr_sized(Size::ZERO)?.check_init()?)?;
+        let new_vtable = self.scalar_to_ptr(new_vtable.read_pointer(Size::ZERO)?.check_init()?)?;
 
         Ok(new_vtable)
     }

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

@@ -264,9 +264,18 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
 
 /// Byte accessors.
 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,
+    /// This is the entirely abstraction-violating way to just grab the raw bytes without
+    /// caring about relocations. It just deduplicates some code between `read_scalar`
+    /// and `get_bytes_internal`.
+    fn get_bytes_even_more_internal(&self, range: AllocRange) -> &[u8] {
+        &self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
+    }
+
+    /// The last argument controls whether we error out when there are uninitialized or pointer
+    /// bytes. However, we *always* error when there are relocations overlapping the edges of the
+    /// range.
+    ///
+    /// You should never call this, call `get_bytes` or `get_bytes_with_uninit_and_ptr` instead,
     ///
     /// This function also guarantees that the resulting pointer will remain stable
     /// even when new allocations are pushed to the `HashMap`. `mem_copy_repeatedly` relies
@@ -287,7 +296,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
             self.check_relocation_edges(cx, range)?;
         }
 
-        Ok(&self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
+        Ok(self.get_bytes_even_more_internal(range))
     }
 
     /// Checks that these bytes are initialized and not pointer bytes, and then return them
@@ -373,6 +382,9 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
 
     /// Reads a *non-ZST* scalar.
     ///
+    /// If `read_provenance` is `true`, this will also read provenance; otherwise (if the machine
+    /// supports that) provenance is entirely ignored.
+    ///
     /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
     /// for ZSTness anyway due to integer pointers being valid for ZSTs.
     ///
@@ -382,35 +394,47 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
         &self,
         cx: &impl HasDataLayout,
         range: AllocRange,
+        read_provenance: bool,
     ) -> AllocResult<ScalarMaybeUninit<Tag>> {
-        // `get_bytes_with_uninit_and_ptr` tests relocation edges.
-        // We deliberately error when loading data that partially has provenance, or partially
-        // initialized data (that's the check below), into a scalar. The LLVM semantics of this are
-        // unclear so we are conservative. See <https://github.com/rust-lang/rust/issues/69488> for
-        // further discussion.
-        let bytes = self.get_bytes_with_uninit_and_ptr(cx, range)?;
-        // Uninit check happens *after* we established that the alignment is correct.
-        // We must not return `Ok()` for unaligned pointers!
+        if read_provenance {
+            assert_eq!(range.size, cx.data_layout().pointer_size);
+        }
+
+        // First and foremost, if anything is uninit, bail.
         if self.is_init(range).is_err() {
             // This inflates uninitialized bytes to the entire scalar, even if only a few
             // bytes are uninitialized.
             return Ok(ScalarMaybeUninit::Uninit);
         }
-        // Now we do the actual reading.
-        let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
-        // See if we got a pointer.
-        if range.size != cx.data_layout().pointer_size {
-            // Not a pointer.
-            // *Now*, we better make sure that the inside is free of relocations too.
-            self.check_relocations(cx, range)?;
-        } else {
-            // Maybe a pointer.
-            if let Some(&prov) = self.relocations.get(&range.start) {
-                let ptr = Pointer::new(prov, Size::from_bytes(bits));
-                return Ok(ScalarMaybeUninit::from_pointer(ptr, cx));
-            }
+
+        // If we are doing a pointer read, and there is a relocation exactly where we
+        // are reading, then we can put data and relocation back together and return that.
+        if read_provenance && let Some(&prov) = self.relocations.get(&range.start) {
+            // We already checked init and relocations, so we can use this function.
+            let bytes = self.get_bytes_even_more_internal(range);
+            let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
+            let ptr = Pointer::new(prov, Size::from_bytes(bits));
+            return Ok(ScalarMaybeUninit::from_pointer(ptr, cx));
         }
-        // We don't. Just return the bits.
+
+        // If we are *not* reading a pointer, and we can just ignore relocations,
+        // then do exactly that.
+        if !read_provenance && Tag::OFFSET_IS_ADDR {
+            // We just strip provenance.
+            let bytes = self.get_bytes_even_more_internal(range);
+            let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
+            return Ok(ScalarMaybeUninit::Scalar(Scalar::from_uint(bits, range.size)));
+        }
+
+        // It's complicated. Better make sure there is no provenance anywhere.
+        // FIXME: If !OFFSET_IS_ADDR, this is the best we can do. But if OFFSET_IS_ADDR, then
+        // `read_pointer` is true and we ideally would distinguish the following two cases:
+        // - The entire `range` is covered by 2 relocations for the same provenance.
+        //   Then we should return a pointer with that provenance.
+        // - The range has inhomogeneous provenance. Then we should return just the
+        //   underlying bits.
+        let bytes = self.get_bytes(cx, range)?;
+        let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
         Ok(ScalarMaybeUninit::Scalar(Scalar::from_uint(bits, range.size)))
     }
 
@@ -513,8 +537,9 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         let start = range.start;
         let end = range.end();
 
-        // We need to handle clearing the relocations from parts of a pointer. See
-        // <https://github.com/rust-lang/rust/issues/87184> for details.
+        // We need to handle clearing the relocations from parts of a pointer.
+        // FIXME: Miri should preserve partial relocations; see
+        // https://github.com/rust-lang/miri/issues/2181.
         if first < start {
             if Tag::ERR_ON_PARTIAL_PTR_OVERWRITE {
                 return Err(AllocError::PartialPointerOverwrite(first));

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

@@ -428,7 +428,7 @@ pub enum UnsupportedOpInfo {
     /// 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.
+    /// `Allocation` data structure. See <https://github.com/rust-lang/miri/issues/2181>.
     PartialPointerOverwrite(Pointer<AllocId>),
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.

compiler/rustc_target/src/abi/mod.rs

@@ -747,6 +747,11 @@ impl Primitive {
     pub fn is_int(self) -> bool {
         matches!(self, Int(..))
     }
+
+    #[inline]
+    pub fn is_ptr(self) -> bool {
+        matches!(self, Pointer)
+    }
 }
 
 /// Inclusive wrap-around range of valid values, that is, if