Rollup merge of #128277 - RalfJung:offset_from_wildcard, r=oli-obk

miri: fix offset_from behavior on wildcard pointers

offset_from wouldn't behave correctly when the "end" pointer was a wildcard pointer (result of an int2ptr cast) just at the end of the allocation. Fix that by expressing the "same allocation" check in terms of two `check_ptr_access_signed` instead of something specific to offset_from, which is both more canonical and works better with wildcard pointers.

The second commit just improves diagnostics: I wanted the "pointer is dangling (has no provenance)" message to say how many bytes of memory it expected to see (since if it were 0 bytes, this would actually be legal, so it's good to tell the user that it's not 0 bytes). And then I was annoying that the error looks so different for when you deref a dangling pointer vs an out-of-bounds pointer so I made them more similar.

Fixes https://github.com/rust-lang/miri/issues/3767

3 files changed, 68 insertions, 54 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index c305a57e85a..1e3de224380 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -238,36 +238,22 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let isize_layout = self.layout_of(self.tcx.types.isize)?;
 
                 // Get offsets for both that are at least relative to the same base.
-                let (a_offset, b_offset) =
+                // With `OFFSET_IS_ADDR` this is trivial; without it we need either
+                // two integers or two pointers into the same allocation.
+                let (a_offset, b_offset, is_addr) = if M::Provenance::OFFSET_IS_ADDR {
+                    (a.addr().bytes(), b.addr().bytes(), /*is_addr*/ true)
+                } else {
                     match (self.ptr_try_get_alloc_id(a), self.ptr_try_get_alloc_id(b)) {
                         (Err(a), Err(b)) => {
-                            // Neither pointer points to an allocation.
-                            // This is okay only if they are the same.
-                            if a != b {
-                                // We'd catch this below in the "dereferenceable" check, but
-                                // show a nicer error for this particular case.
-                                throw_ub_custom!(
-                                    fluent::const_eval_offset_from_different_integers,
-                                    name = intrinsic_name,
-                                );
-                            }
-                            // This will always return 0.
-                            (a, b)
-                        }
-                        _ if M::Provenance::OFFSET_IS_ADDR && a.addr() == b.addr() => {
-                            // At least one of the pointers has provenance, but they also point to
-                            // the same address so it doesn't matter; this is fine. `(0, 0)` means
-                            // we pass all the checks below and return 0.
-                            (0, 0)
+                            // Neither pointer points to an allocation, so they are both absolute.
+                            (a, b, /*is_addr*/ true)
                         }
-                        // From here onwards, the pointers are definitely for different addresses
-                        // (or we can't determine their absolute address).
                         (Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _)))
                             if a_alloc_id == b_alloc_id =>
                         {
                             // Found allocation for both, and it's the same.
                             // Use these offsets for distance calculation.
-                            (a_offset.bytes(), b_offset.bytes())
+                            (a_offset.bytes(), b_offset.bytes(), /*is_addr*/ false)
                         }
                         _ => {
                             // Not into the same allocation -- this is UB.
@@ -276,9 +262,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                                 name = intrinsic_name,
                             );
                         }
-                    };
+                    }
+                };
 
-                // Compute distance.
+                // Compute distance: a - b.
                 let dist = {
                     // Addresses are unsigned, so this is a `usize` computation. We have to do the
                     // overflow check separately anyway.
@@ -295,6 +282,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                                 fluent::const_eval_offset_from_unsigned_overflow,
                                 a_offset = a_offset,
                                 b_offset = b_offset,
+                                is_addr = is_addr,
                             );
                         }
                         // The signed form of the intrinsic allows this. If we interpret the
@@ -323,14 +311,23 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                     }
                 };
 
-                // Check that the range between them is dereferenceable ("in-bounds or one past the
-                // end of the same allocation"). This is like the check in ptr_offset_inbounds.
-                let min_ptr = if dist >= 0 { b } else { a };
-                self.check_ptr_access(
-                    min_ptr,
-                    Size::from_bytes(dist.unsigned_abs()),
+                // Check that the memory between them is dereferenceable at all, starting from the
+                // base pointer: `dist` is `a - b`, so it is based on `b`.
+                self.check_ptr_access_signed(b, dist, CheckInAllocMsg::OffsetFromTest)?;
+                // Then check that this is also dereferenceable from `a`. This ensures that they are
+                // derived from the same allocation.
+                self.check_ptr_access_signed(
+                    a,
+                    dist.checked_neg().unwrap(), // i64::MIN is impossible as no allocation can be that large
                     CheckInAllocMsg::OffsetFromTest,
-                )?;
+                )
+                .map_err(|_| {
+                    // Make the error more specific.
+                    err_ub_custom!(
+                        fluent::const_eval_offset_from_different_allocations,
+                        name = intrinsic_name,
+                    )
+                })?;
 
                 // Perform division by size to compute return value.
                 let ret_layout = if intrinsic_name == sym::ptr_offset_from_unsigned {
@@ -577,27 +574,19 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     }
 
     /// Offsets a pointer by some multiple of its type, returning an error if the pointer leaves its
-    /// allocation. For integer pointers, we consider each of them their own tiny allocation of size
-    /// 0, so offset-by-0 (and only 0) is okay -- except that null cannot be offset by _any_ value.
+    /// allocation.
     pub fn ptr_offset_inbounds(
         &self,
         ptr: Pointer<Option<M::Provenance>>,
         offset_bytes: i64,
     ) -> InterpResult<'tcx, Pointer<Option<M::Provenance>>> {
-        // The offset being in bounds cannot rely on "wrapping around" the address space.
-        // So, first rule out overflows in the pointer arithmetic.
-        let offset_ptr = ptr.signed_offset(offset_bytes, self)?;
-        // ptr and offset_ptr must be in bounds of the same allocated object. This means all of the
-        // memory between these pointers must be accessible. Note that we do not require the
-        // pointers to be properly aligned (unlike a read/write operation).
-        let min_ptr = if offset_bytes >= 0 { ptr } else { offset_ptr };
-        // This call handles checking for integer/null pointers.
-        self.check_ptr_access(
-            min_ptr,
-            Size::from_bytes(offset_bytes.unsigned_abs()),
-            CheckInAllocMsg::PointerArithmeticTest,
-        )?;
-        Ok(offset_ptr)
+        // We first compute the pointer with overflow checks, to get a specific error for when it
+        // overflows (though technically this is redundant with the following inbounds check).
+        let result = ptr.signed_offset(offset_bytes, self)?;
+        // The offset must be in bounds starting from `ptr`.
+        self.check_ptr_access_signed(ptr, offset_bytes, CheckInAllocMsg::PointerArithmeticTest)?;
+        // Done.
+        Ok(result)
     }
 
     /// Copy `count*size_of::<T>()` many bytes from `*src` to `*dst`.
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 092174a3079..859f30137dc 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -411,6 +411,25 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         Ok(())
     }
 
+    /// Check whether the given pointer points to live memory for a signed amount of bytes.
+    /// A negative amounts means that the given range of memory to the left of the pointer
+    /// needs to be dereferenceable.
+    pub fn check_ptr_access_signed(
+        &self,
+        ptr: Pointer<Option<M::Provenance>>,
+        size: i64,
+        msg: CheckInAllocMsg,
+    ) -> InterpResult<'tcx> {
+        if let Ok(size) = u64::try_from(size) {
+            self.check_ptr_access(ptr, Size::from_bytes(size), msg)
+        } else {
+            // Compute the pointer at the beginning of the range, and do the standard
+            // dereferenceability check from there.
+            let begin_ptr = ptr.wrapping_signed_offset(size, self);
+            self.check_ptr_access(begin_ptr, Size::from_bytes(size.unsigned_abs()), msg)
+        }
+    }
+
     /// 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.
@@ -437,7 +456,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         Ok(match self.ptr_try_get_alloc_id(ptr) {
             Err(addr) => {
                 // We couldn't get a proper allocation.
-                throw_ub!(DanglingIntPointer(addr, msg));
+                throw_ub!(DanglingIntPointer { addr, inbounds_size: size, msg });
             }
             Ok((alloc_id, offset, prov)) => {
                 let (alloc_size, _alloc_align, ret_val) = alloc_size(alloc_id, offset, prov)?;
@@ -448,7 +467,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                         alloc_id,
                         alloc_size,
                         ptr_offset: self.target_usize_to_isize(offset.bytes()),
-                        ptr_size: size,
+                        inbounds_size: size,
                         msg,
                     })
                 }
@@ -1421,7 +1440,13 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         ptr: Pointer<Option<M::Provenance>>,
     ) -> InterpResult<'tcx, (AllocId, Size, M::ProvenanceExtra)> {
         self.ptr_try_get_alloc_id(ptr).map_err(|offset| {
-            err_ub!(DanglingIntPointer(offset, CheckInAllocMsg::InboundsTest)).into()
+            err_ub!(DanglingIntPointer {
+                addr: offset,
+                // We don't know the actually required size.
+                inbounds_size: Size::ZERO,
+                msg: CheckInAllocMsg::InboundsTest
+            })
+            .into()
         })
     }
 }
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 3171a888151..adb6ebabd73 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -348,7 +348,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                 try_validation!(
                     self.ecx.get_ptr_vtable_ty(vtable, Some(data)),
                     self.path,
-                    Ub(DanglingIntPointer(..) | InvalidVTablePointer(..)) =>
+                    Ub(DanglingIntPointer{ .. } | InvalidVTablePointer(..)) =>
                         InvalidVTablePtr { value: format!("{vtable}") },
                     Ub(InvalidVTableTrait { expected_trait, vtable_trait }) => {
                         InvalidMetaWrongTrait { expected_trait, vtable_trait: *vtable_trait }
@@ -405,8 +405,8 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                 CheckInAllocMsg::InboundsTest, // will anyway be replaced by validity message
             ),
             self.path,
-            Ub(DanglingIntPointer(0, _)) => NullPtr { ptr_kind },
-            Ub(DanglingIntPointer(i, _)) => DanglingPtrNoProvenance {
+            Ub(DanglingIntPointer { addr: 0, .. }) => NullPtr { ptr_kind },
+            Ub(DanglingIntPointer { addr: i, .. }) => DanglingPtrNoProvenance {
                 ptr_kind,
                 // FIXME this says "null pointer" when null but we need translate
                 pointer: format!("{}", Pointer::<Option<AllocId>>::from_addr_invalid(*i))
@@ -605,7 +605,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                     let _fn = try_validation!(
                         self.ecx.get_ptr_fn(ptr),
                         self.path,
-                        Ub(DanglingIntPointer(..) | InvalidFunctionPointer(..)) =>
+                        Ub(DanglingIntPointer{ .. } | InvalidFunctionPointer(..)) =>
                             InvalidFnPtr { value: format!("{ptr}") },
                     );
                     // FIXME: Check if the signature matches