Rollup merge of #122233 - RalfJung:custom-alloc-box, r=oli-obk

miri: do not apply aliasing restrictions to Box with custom allocator

This is the Miri side of https://github.com/rust-lang/rust/pull/122018. The "intrinsics with body" made this much more pleasant. :)

Fixes https://github.com/rust-lang/miri/issues/3341.
r? `@oli-obk`

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -17,8 +17,8 @@ use rustc_middle::mir::interpret::{
     ExpectedKind, InterpError, InvalidMetaKind, Misalignment, PointerKind, Provenance,
     ValidationErrorInfo, ValidationErrorKind, ValidationErrorKind::*,
 };
-use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
+use rustc_middle::ty::{self, Ty};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_target::abi::{
     Abi, FieldIdx, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange,
@@ -783,7 +783,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
     }
 
     #[inline]
-    fn visit_box(&mut self, op: &OpTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
+    fn visit_box(
+        &mut self,
+        _box_ty: Ty<'tcx>,
+        op: &OpTy<'tcx, M::Provenance>,
+    ) -> InterpResult<'tcx> {
         self.check_safe_pointer(op, PointerKind::Box)?;
         Ok(())
     }

compiler/rustc_const_eval/src/interpret/visitor.rs

@@ -3,7 +3,7 @@
 
 use rustc_index::IndexVec;
 use rustc_middle::mir::interpret::InterpResult;
-use rustc_middle::ty;
+use rustc_middle::ty::{self, Ty};
 use rustc_target::abi::FieldIdx;
 use rustc_target::abi::{FieldsShape, VariantIdx, Variants};
 
@@ -47,10 +47,10 @@ pub trait ValueVisitor<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>>: Sized {
         Ok(())
     }
     /// Visits the given value as the pointer of a `Box`. There is nothing to recurse into.
-    /// The type of `v` will be a raw pointer, but this is a field of `Box<T>` and the
+    /// The type of `v` will be a raw pointer to `T`, but this is a field of `Box<T>` and the
-    /// pointee type is the actual `T`.
+    /// pointee type is the actual `T`. `box_ty` provides the full type of the `Box` itself.
     #[inline(always)]
-    fn visit_box(&mut self, _v: &Self::V) -> InterpResult<'tcx> {
+    fn visit_box(&mut self, _box_ty: Ty<'tcx>, _v: &Self::V) -> InterpResult<'tcx> {
         Ok(())
     }
 
@@ -144,7 +144,7 @@ pub trait ValueVisitor<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>>: Sized {
                 assert_eq!(nonnull_ptr.layout().fields.count(), 1);
                 let raw_ptr = self.ecx().project_field(&nonnull_ptr, 0)?; // the actual raw ptr
                 // ... whose only field finally is a raw ptr we can dereference.
-                self.visit_box(&raw_ptr)?;
+                self.visit_box(ty, &raw_ptr)?;
 
                 // The second `Box` field is the allocator, which we recursively check for validity
                 // like in regular structs.

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -657,6 +657,10 @@ pub fn check_intrinsic_type(
             sym::simd_shuffle => (3, 0, vec![param(0), param(0), param(1)], param(2)),
             sym::simd_shuffle_generic => (2, 1, vec![param(0), param(0)], param(1)),
 
+            sym::retag_box_to_raw => {
+                (2, 0, vec![Ty::new_mut_ptr(tcx, param(0))], Ty::new_mut_ptr(tcx, param(0)))
+            }
+
             other => {
                 tcx.dcx().emit_err(UnrecognizedIntrinsicFunction { span, name: other });
                 return;

compiler/rustc_middle/src/ty/sty.rs

@@ -2008,13 +2008,10 @@ impl<'tcx> Ty<'tcx> {
                     // Single-argument Box is always global. (for "minicore" tests)
                     return true;
                 };
-                if let Some(alloc_adt) = alloc.expect_ty().ty_adt_def() {
+                alloc.expect_ty().ty_adt_def().is_some_and(|alloc_adt| {
                     let global_alloc = tcx.require_lang_item(LangItem::GlobalAlloc, None);
                     alloc_adt.did() == global_alloc
-                } else {
+                })
-                    // Allocator is not an ADT...
-                    false
-                }
             }
             _ => false,
         }

compiler/rustc_span/src/symbol.rs

@@ -1450,6 +1450,7 @@ symbols! {
         residual,
         result,
         resume,
+        retag_box_to_raw,
         return_position_impl_trait_in_trait,
         return_type_notation,
         rhs,

library/alloc/src/boxed.rs

@@ -155,6 +155,7 @@ use core::error::Error;
 use core::fmt;
 use core::future::Future;
 use core::hash::{Hash, Hasher};
+use core::intrinsics::retag_box_to_raw;
 use core::iter::FusedIterator;
 use core::marker::Tuple;
 use core::marker::Unsize;
@@ -1110,8 +1111,16 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
     pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
-        let (leaked, alloc) = Box::into_unique(b);
+        // This is the transition point from `Box` to raw pointers. For Stacked Borrows, these casts
-        (leaked.as_ptr(), alloc)
+        // are relevant -- if this is a global allocator Box and we just get the pointer from `b.0`,
+        // it will have `Unique` permission, which is not what we want from a raw pointer. We could
+        // fix that by going through `&mut`, but then if this is *not* a global allocator Box, we'd
+        // be adding uniqueness assertions that we do not want. So for Miri's sake we pass this
+        // pointer through an intrinsic for box-to-raw casts, which can do the right thing wrt the
+        // aliasing model.
+        let b = mem::ManuallyDrop::new(b);
+        let alloc = unsafe { ptr::read(&b.1) };
+        (unsafe { retag_box_to_raw::<T, A>(b.0.as_ptr()) }, alloc)
     }
 
     #[unstable(
@@ -1122,13 +1131,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     #[inline]
     #[doc(hidden)]
     pub fn into_unique(b: Self) -> (Unique<T>, A) {
-        // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
+        let (ptr, alloc) = Box::into_raw_with_allocator(b);
-        // raw pointer for the type system. Turning it directly into a raw pointer would not be
+        unsafe { (Unique::from(&mut *ptr), alloc) }
-        // recognized as "releasing" the unique pointer to permit aliased raw accesses,
-        // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
-        // behaves correctly.
-        let alloc = unsafe { ptr::read(&b.1) };
-        (Unique::from(Box::leak(b)), alloc)
     }
 
     /// Returns a reference to the underlying allocator.
@@ -1184,7 +1188,7 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     where
         A: 'a,
     {
-        unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
+        unsafe { &mut *Box::into_raw(b) }
     }
 
     /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then

library/core/src/intrinsics.rs

@@ -2695,6 +2695,19 @@ pub unsafe fn vtable_size(_ptr: *const ()) -> usize {
     unreachable!()
 }
 
+/// Retag a box pointer as part of casting it to a raw pointer. This is the `Box` equivalent of
+/// `(x: &mut T) as *mut T`. The input pointer must be the pointer of a `Box` (passed as raw pointer
+/// to avoid all questions around move semantics and custom allocators), and `A` must be the `Box`'s
+/// allocator.
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_nounwind]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(bootstrap, inline)]
+pub unsafe fn retag_box_to_raw<T: ?Sized, A>(ptr: *mut T) -> *mut T {
+    // Miri needs to adjust the provenance, but for regular codegen this is not needed
+    ptr
+}
+
 // Some functions are defined here because they accidentally got made
 // available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
 // (`transmute` also falls into this category, but it cannot be wrapped due to the

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

@@ -5,7 +5,7 @@ use std::num::NonZero;
 use smallvec::SmallVec;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_middle::mir::RetagKind;
+use rustc_middle::{mir::RetagKind, ty::Ty};
 use rustc_target::abi::Size;
 
 use crate::*;
@@ -291,6 +291,19 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         }
     }
 
+    fn retag_box_to_raw(
+        &mut self,
+        val: &ImmTy<'tcx, Provenance>,
+        alloc_ty: Ty<'tcx>,
+    ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
+        let this = self.eval_context_mut();
+        let method = this.machine.borrow_tracker.as_ref().unwrap().borrow().borrow_tracker_method;
+        match method {
+            BorrowTrackerMethod::StackedBorrows => this.sb_retag_box_to_raw(val, alloc_ty),
+            BorrowTrackerMethod::TreeBorrows => this.tb_retag_box_to_raw(val, alloc_ty),
+        }
+    }
+
     fn retag_place_contents(
         &mut self,
         kind: RetagKind,

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

@@ -865,6 +865,24 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         this.sb_retag_reference(val, new_perm, RetagInfo { cause, in_field: false })
     }
 
+    fn sb_retag_box_to_raw(
+        &mut self,
+        val: &ImmTy<'tcx, Provenance>,
+        alloc_ty: Ty<'tcx>,
+    ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
+        let this = self.eval_context_mut();
+        let is_global_alloc = alloc_ty.ty_adt_def().is_some_and(|adt| {
+            let global_alloc = this.tcx.require_lang_item(rustc_hir::LangItem::GlobalAlloc, None);
+            adt.did() == global_alloc
+        });
+        if is_global_alloc {
+            // Retag this as-if it was a mutable reference.
+            this.sb_retag_ptr_value(RetagKind::Raw, val)
+        } else {
+            Ok(val.clone())
+        }
+    }
+
     fn sb_retag_place_contents(
         &mut self,
         kind: RetagKind,
@@ -916,10 +934,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 self.ecx
             }
 
-            fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
+            fn visit_box(
-                // Boxes get a weak protectors, since they may be deallocated.
+                &mut self,
-                let new_perm = NewPermission::from_box_ty(place.layout.ty, self.kind, self.ecx);
+                box_ty: Ty<'tcx>,
-                self.retag_ptr_inplace(place, new_perm)
+                place: &PlaceTy<'tcx, Provenance>,
+            ) -> InterpResult<'tcx> {
+                // Only boxes for the global allocator get any special treatment.
+                if box_ty.is_box_global(*self.ecx.tcx) {
+                    // Boxes get a weak protectors, since they may be deallocated.
+                    let new_perm = NewPermission::from_box_ty(place.layout.ty, self.kind, self.ecx);
+                    self.retag_ptr_inplace(place, new_perm)?;
+                }
+                Ok(())
             }
 
             fn visit_value(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {

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

@@ -392,6 +392,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         }
     }
 
+    fn tb_retag_box_to_raw(
+        &mut self,
+        val: &ImmTy<'tcx, Provenance>,
+        _alloc_ty: Ty<'tcx>,
+    ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
+        // Casts to raw pointers are NOPs in Tree Borrows.
+        Ok(val.clone())
+    }
+
     /// Retag all pointers that are stored in this place.
     fn tb_retag_place_contents(
         &mut self,
@@ -441,14 +450,22 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             /// Regardless of how `Unique` is handled, Boxes are always reborrowed.
             /// When `Unique` is also reborrowed, then it behaves exactly like `Box`
             /// except for the fact that `Box` has a non-zero-sized reborrow.
-            fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
+            fn visit_box(
-                let new_perm = NewPermission::from_unique_ty(
+                &mut self,
-                    place.layout.ty,
+                box_ty: Ty<'tcx>,
-                    self.kind,
+                place: &PlaceTy<'tcx, Provenance>,
-                    self.ecx,
+            ) -> InterpResult<'tcx> {
-                    /* zero_size */ false,
+                // Only boxes for the global allocator get any special treatment.
-                );
+                if box_ty.is_box_global(*self.ecx.tcx) {
-                self.retag_ptr_inplace(place, new_perm)
+                    let new_perm = NewPermission::from_unique_ty(
+                        place.layout.ty,
+                        self.kind,
+                        self.ecx,
+                        /* zero_size */ false,
+                    );
+                    self.retag_ptr_inplace(place, new_perm)?;
+                }
+                Ok(())
             }
 
             fn visit_value(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {

src/tools/miri/src/shims/intrinsics/mod.rs

@@ -129,6 +129,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 this.write_bytes_ptr(ptr, iter::repeat(val_byte).take(byte_count.bytes_usize()))?;
             }
 
+            // Memory model / provenance manipulation
             "ptr_mask" => {
                 let [ptr, mask] = check_arg_count(args)?;
 
@@ -139,6 +140,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
                 this.write_pointer(Pointer::new(ptr.provenance, masked_addr), dest)?;
             }
+            "retag_box_to_raw" => {
+                let [ptr] = check_arg_count(args)?;
+                let alloc_ty = generic_args[1].expect_ty();
+
+                let val = this.read_immediate(ptr)?;
+                let new_val = if this.machine.borrow_tracker.is_some() {
+                    this.retag_box_to_raw(&val, alloc_ty)?
+                } else {
+                    val
+                };
+                this.write_immediate(*new_val, dest)?;
+            }
 
             // We want to return either `true` or `false` at random, or else something like
             // ```

src/tools/miri/tests/pass/box-custom-alloc-aliasing.rs

@@ -0,0 +1,123 @@
+//! Regression test for <https://github.com/rust-lang/miri/issues/3341>:
+//! If `Box` has a local allocator, then it can't be `noalias` as the allocator
+//! may want to access allocator state based on the data pointer.
+
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
+#![feature(allocator_api)]
+#![feature(strict_provenance)]
+
+use std::{
+    alloc::{AllocError, Allocator, Layout},
+    cell::{Cell, UnsafeCell},
+    ptr::{self, addr_of, NonNull},
+    thread::{self, ThreadId},
+    mem,
+};
+
+const BIN_SIZE: usize = 8;
+
+// A bin represents a collection of blocks of a specific layout.
+#[repr(align(128))]
+struct MyBin {
+    top: Cell<usize>,
+    thread_id: ThreadId,
+    memory: UnsafeCell<[usize; BIN_SIZE]>,
+}
+
+impl MyBin {
+    fn pop(&self) -> Option<NonNull<u8>> {
+        let top = self.top.get();
+        if top == BIN_SIZE {
+            return None;
+        }
+        // Cast the *entire* thing to a raw pointer to not restrict its provenance.
+        let bin = self as *const MyBin;
+        let base_ptr = UnsafeCell::raw_get(unsafe{ addr_of!((*bin).memory )}).cast::<usize>();
+        let ptr = unsafe { NonNull::new_unchecked(base_ptr.add(top)) };
+        self.top.set(top + 1);
+        Some(ptr.cast())
+    }
+
+    // Pretends to not be a throwaway allocation method like this. A more realistic
+    // substitute is using intrusive linked lists, which requires access to the
+    // metadata of this bin as well.
+    unsafe fn push(&self, ptr: NonNull<u8>) {
+        // For now just check that this really is in this bin.
+        let start = self.memory.get().addr();
+        let end = start + BIN_SIZE * mem::size_of::<usize>();
+        let addr = ptr.addr().get();
+        assert!((start..end).contains(&addr));
+    }
+}
+
+// A collection of bins.
+struct MyAllocator {
+    thread_id: ThreadId,
+    // Pretends to be some complex collection of bins, such as an array of linked lists.
+    bins: Box<[MyBin; 1]>,
+}
+
+impl MyAllocator {
+    fn new() -> Self {
+        let thread_id = thread::current().id();
+        MyAllocator {
+            thread_id,
+            bins: Box::new(
+                [MyBin {
+                    top: Cell::new(0),
+                    thread_id,
+                    memory: UnsafeCell::default(),
+                }; 1],
+            ),
+        }
+    }
+
+    // Pretends to be expensive finding a suitable bin for the layout.
+    fn find_bin(&self, layout: Layout) -> Option<&MyBin> {
+        if layout == Layout::new::<usize>() {
+            Some(&self.bins[0])
+        } else {
+            None
+        }
+    }
+}
+
+unsafe impl Allocator for MyAllocator {
+    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        // Expensive bin search.
+        let bin = self.find_bin(layout).ok_or(AllocError)?;
+        let ptr = bin.pop().ok_or(AllocError)?;
+        Ok(NonNull::slice_from_raw_parts(ptr, layout.size()))
+    }
+
+    unsafe fn deallocate(&self, ptr: NonNull<u8>, _layout: Layout) {
+        // Since manually finding the corresponding bin of `ptr` is very expensive,
+        // doing pointer arithmetics is preferred.
+        // But this means we access `top` via `ptr` rather than `self`!
+        // That is fundamentally the source of the aliasing trouble in this example.
+        let their_bin = ptr.as_ptr().map_addr(|addr| addr & !127).cast::<MyBin>();
+        let thread_id = ptr::read(ptr::addr_of!((*their_bin).thread_id));
+        if self.thread_id == thread_id {
+            unsafe { (*their_bin).push(ptr) };
+        } else {
+            todo!("Deallocating from another thread")
+        }
+    }
+}
+
+// Make sure to involve `Box` in allocating these,
+// as that's where `noalias` may come from.
+fn v<T, A: Allocator>(t: T, a: A) -> Vec<T, A> {
+    (Box::new_in([t], a) as Box<[T], A>).into_vec()
+}
+
+fn main() {
+    assert!(mem::size_of::<MyBin>() <= 128); // if it grows bigger, the trick to access the "header" no longer works
+    let my_alloc = MyAllocator::new();
+    let a = v(1usize, &my_alloc);
+    let b = v(2usize, &my_alloc);
+    assert_eq!(a[0] + 1, b[0]);
+    assert_eq!(addr_of!(a[0]).wrapping_add(1), addr_of!(b[0]));
+    drop((a, b));
+}