Rollup merge of #96162 - RalfJung:mark-uninit, r=oli-obk

interpret: Fix writing uninit to an allocation

When calling `mark_init`, we need to also be mindful of what happens with the relocations! Specifically, when we de-init memory, we need to clear relocations in that range as well or else strange things will happen (and printing will not show the de-init, since relocations take precedence there).

Fixes rust-lang/miri#2068.

Here's the Miri testcase that this fixes (requires `-Zmiri-disable-validation`):
```rust
use std::mem::MaybeUninit;

fn main() { unsafe {
    let mut x = MaybeUninit::<i64>::uninit();
    // Put in a ptr.
    x.as_mut_ptr().cast::<&i32>().write_unaligned(&0);
    // Overwrite parts of that pointer with 'uninit' through a Scalar.
    let ptr = x.as_mut_ptr().cast::<i32>();
    *ptr = MaybeUninit::uninit().assume_init();
    // Reading this back should hence work fine.
    let _c = *ptr;
} }
```
Previously this failed with
```
error: unsupported operation: unable to turn pointer into raw bytes
  --> ../miri/uninit.rs:11:14
   |
11 |     let _c = *ptr;
   |              ^^^^ unable to turn pointer into raw bytes
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support

   = note: inside `main` at ../miri/uninit.rs:11:14
```

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -892,8 +892,11 @@ impl<'tcx, 'a, Tag: Provenance, Extra> AllocRefMut<'a, 'tcx, Tag, Extra> {
     }
 
     /// Mark the entire referenced range as uninitalized
-    pub fn write_uninit(&mut self) {
-        self.alloc.mark_init(self.range, false);
+    pub fn write_uninit(&mut self) -> InterpResult<'tcx> {
+        Ok(self
+            .alloc
+            .write_uninit(&self.tcx, self.range)
+            .map_err(|e| e.to_interp_error(self.alloc_id))?)
     }
 }
 
@@ -1053,8 +1056,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // This also avoids writing to the target bytes so that the backing allocation is never
             // touched if the bytes stay uninitialized for the whole interpreter execution. On contemporary
             // operating system this can avoid physically allocating the page.
-            dest_alloc.mark_init(dest_range, false); // `Size` multiplication
-            dest_alloc.mark_relocation_range(relocations);
+            dest_alloc
+                .write_uninit(&tcx, dest_range)
+                .map_err(|e| e.to_interp_error(dest_alloc_id))?;
+            // We can forget about the relocations, this is all not initialized anyway.
             return Ok(());
         }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -823,7 +823,7 @@ where
             // Zero-sized access
             return Ok(());
         };
-        alloc.write_uninit();
+        alloc.write_uninit()?;
         Ok(())
     }
 

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

@@ -269,7 +269,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
     /// `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`. `copy_repeatedly` relies
+    /// even when new allocations are pushed to the `HashMap`. `mem_copy_repeatedly` relies
     /// on that.
     ///
     /// It is the caller's responsibility to check bounds and alignment beforehand.
@@ -429,8 +429,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
         let val = match val {
             ScalarMaybeUninit::Scalar(scalar) => scalar,
             ScalarMaybeUninit::Uninit => {
-                self.mark_init(range, false);
-                return Ok(());
+                return self.write_uninit(cx, range);
             }
         };
 
@@ -455,6 +454,13 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
 
         Ok(())
     }
+
+    /// Write "uninit" to the given memory range.
+    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        self.mark_init(range, false);
+        self.clear_relocations(cx, range)?;
+        return Ok(());
+    }
 }
 
 /// Relocations.
@@ -561,8 +567,10 @@ impl<Tag> Deref for Relocations<Tag> {
 }
 
 /// A partial, owned list of relocations to transfer into another allocation.
+///
+/// Offsets are already adjusted to the destination allocation.
 pub struct AllocationRelocations<Tag> {
-    relative_relocations: Vec<(Size, Tag)>,
+    dest_relocations: Vec<(Size, Tag)>,
 }
 
 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
@@ -575,12 +583,17 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
     ) -> AllocationRelocations<Tag> {
         let relocations = self.get_relocations(cx, src);
         if relocations.is_empty() {
-            return AllocationRelocations { relative_relocations: Vec::new() };
+            return AllocationRelocations { dest_relocations: Vec::new() };
         }
 
         let size = src.size;
         let mut new_relocations = Vec::with_capacity(relocations.len() * (count as usize));
 
+        // If `count` is large, this is rather wasteful -- we are allocating a big array here, which
+        // is mostly filled with redundant information since it's just N copies of the same `Tag`s
+        // at slightly adjusted offsets. The reason we do this is so that in `mark_relocation_range`
+        // we can use `insert_presorted`. That wouldn't work with an `Iterator` that just produces
+        // the right sequence of relocations for all N copies.
         for i in 0..count {
             new_relocations.extend(relocations.iter().map(|&(offset, reloc)| {
                 // compute offset for current repetition
@@ -593,14 +606,17 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
             }));
         }
 
-        AllocationRelocations { relative_relocations: new_relocations }
+        AllocationRelocations { dest_relocations: new_relocations }
     }
 
     /// Applies a relocation copy.
     /// The affected range, as defined in the parameters to `prepare_relocation_copy` is expected
     /// to be clear of relocations.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
     pub fn mark_relocation_range(&mut self, relocations: AllocationRelocations<Tag>) {
-        self.relocations.0.insert_presorted(relocations.relative_relocations);
+        self.relocations.0.insert_presorted(relocations.dest_relocations);
     }
 }
 
@@ -1056,7 +1072,7 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         })
     }
 
-    pub fn mark_init(&mut self, range: AllocRange, is_init: bool) {
+    fn mark_init(&mut self, range: AllocRange, is_init: bool) {
         if range.size.bytes() == 0 {
             return;
         }
@@ -1118,6 +1134,9 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
     }
 
     /// Applies multiple instances of the run-length encoding to the initialization mask.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
     pub fn mark_compressed_init_range(
         &mut self,
         defined: &InitMaskCompressed,

compiler/rustc_middle/src/mir/pretty.rs

@@ -851,6 +851,7 @@ fn write_allocation_bytes<'tcx, Tag: Provenance, Extra>(
         }
         if let Some(&tag) = alloc.relocations().get(&i) {
             // Memory with a relocation must be defined
+            assert!(alloc.init_mask().is_range_initialized(i, i + ptr_size).is_ok());
             let j = i.bytes_usize();
             let offset = alloc
                 .inspect_with_uninit_and_ptr_outside_interpreter(j..j + ptr_size.bytes_usize());