miri: better ptr-out-of-bounds errors

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

@@ -240,12 +240,13 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     /// Dereferencing a dangling pointer after it got freed.
     PointerUseAfterFree(AllocId),
     /// Used a pointer outside the bounds it is valid for.
+    /// (If `ptr_size > 0`, determines the size of the memory range that was expected to be in-bounds.)
     PointerOutOfBounds {
         alloc_id: AllocId,
-        offset: Size,
+        alloc_size: Size,
-        size: Size,
+        ptr_offset: i64,
+        ptr_size: Size,
         msg: CheckInAllocMsg,
-        allocation_size: Size,
     },
     /// Using an integer as a pointer in the wrong way.
     DanglingIntPointer(u64, CheckInAllocMsg),
@@ -318,24 +319,25 @@ impl fmt::Display for UndefinedBehaviorInfo<'_> {
             PointerUseAfterFree(a) => {
                 write!(f, "pointer to {} was dereferenced after this allocation got freed", a)
             }
-            PointerOutOfBounds { alloc_id, offset, size: Size::ZERO, msg, allocation_size } => {
+            PointerOutOfBounds { alloc_id, alloc_size, ptr_offset, ptr_size: Size::ZERO, msg } => {
                 write!(
                     f,
-                    "{}{} has size {}, so pointer at offset {} is out-of-bounds",
+                    "{}{alloc_id} has size {alloc_size}, so pointer at offset {ptr_offset} is out-of-bounds",
                     msg,
-                    alloc_id,
+                    alloc_id = alloc_id,
-                    allocation_size.bytes(),
+                    alloc_size = alloc_size.bytes(),
-                    offset.bytes(),
+                    ptr_offset = ptr_offset,
                 )
             }
-            PointerOutOfBounds { alloc_id, offset, size, msg, allocation_size } => write!(
+            PointerOutOfBounds { alloc_id, alloc_size, ptr_offset, ptr_size, msg } => write!(
                 f,
-                "{}{} has size {}, so pointer to {} bytes starting at offset {} is out-of-bounds",
+                "{}{alloc_id} has size {alloc_size}, so pointer to {ptr_size} byte{ptr_size_p} starting at offset {ptr_offset} is out-of-bounds",
                 msg,
-                alloc_id,
+                alloc_id = alloc_id,
-                allocation_size.bytes(),
+                alloc_size = alloc_size.bytes(),
-                size.bytes(),
+                ptr_size = ptr_size.bytes(),
-                offset.bytes(),
+                ptr_size_p = pluralize!(ptr_size.bytes()),
+                ptr_offset = ptr_offset,
             ),
             DanglingIntPointer(0, CheckInAllocMsg::InboundsTest) => {
                 write!(f, "null pointer is not a valid pointer for this operation")

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

@@ -36,6 +36,20 @@ pub trait PointerArithmetic: HasDataLayout {
         i64::try_from(max_isize_plus_1 - 1).unwrap()
     }
 
+    #[inline]
+    fn machine_usize_to_isize(&self, val: u64) -> i64 {
+        let val = val as i64;
+        // Now clamp into the machine_isize range.
+        if val > self.machine_isize_max() {
+            // This can only happen the the ptr size is < 64, so we know max_usize_plus_1 fits into
+            // i64.
+            let max_usize_plus_1 = 1u128 << self.pointer_size().bits();
+            val - i64::try_from(max_usize_plus_1).unwrap()
+        } else {
+            val
+        }
+    }
+
     /// Helper function: truncate given value-"overflowed flag" pair to pointer size and
     /// update "overflowed flag" if there was an overflow.
     /// This should be called by all the other methods before returning!

compiler/rustc_mir/src/interpret/memory.rs

@@ -451,11 +451,17 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 None
             }
             Ok((alloc_id, offset, ptr)) => {
-                let (allocation_size, alloc_align, ret_val) = alloc_size(alloc_id, offset, ptr)?;
+                let (alloc_size, alloc_align, ret_val) = alloc_size(alloc_id, offset, ptr)?;
                 // Test bounds. This also ensures non-null.
                 // It is sufficient to check this for the end pointer. Also check for overflow!
-                if offset.checked_add(size, &self.tcx).map_or(true, |end| end > allocation_size) {
+                if offset.checked_add(size, &self.tcx).map_or(true, |end| end > alloc_size) {
-                    throw_ub!(PointerOutOfBounds { alloc_id, offset, size, allocation_size, msg })
+                    throw_ub!(PointerOutOfBounds {
+                        alloc_id,
+                        alloc_size,
+                        ptr_offset: self.machine_usize_to_isize(offset.bytes()),
+                        ptr_size: size,
+                        msg,
+                    })
                 }
                 // Test align. Check this last; if both bounds and alignment are violated
                 // we want the error to be about the bounds.

src/test/ui/consts/offset_ub.rs

@@ -13,6 +13,7 @@ pub const OVERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MAX)
 pub const UNDERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MIN) }; //~NOTE
 pub const OVERFLOW_ADDRESS_SPACE: *const u8 = unsafe { (usize::MAX as *const u8).offset(2) }; //~NOTE
 pub const UNDERFLOW_ADDRESS_SPACE: *const u8 = unsafe { (1 as *const u8).offset(-2) }; //~NOTE
+pub const NEGATIVE_OFFSET: *const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) }; //~NOTE
 
 pub const ZERO_SIZED_ALLOC: *const u8 = unsafe { [0u8; 0].as_ptr().offset(1) }; //~NOTE
 pub const DANGLING: *const u8 = unsafe { ptr::NonNull::<u8>::dangling().as_ptr().offset(4) }; //~NOTE

src/test/ui/consts/offset_ub.stderr

@@ -102,13 +102,27 @@ error[E0080]: evaluation of constant value failed
 LL |         unsafe { intrinsics::offset(self, count) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                  |
-   |                  pointer arithmetic failed: allocN has size 0, so pointer to 1 bytes starting at offset 0 is out-of-bounds
+   |                  pointer arithmetic failed: allocN has size 1, so pointer to 2 bytes starting at offset -4 is out-of-bounds
    |                  inside `ptr::const_ptr::<impl *const u8>::offset` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
    | 
-  ::: $DIR/offset_ub.rs:17:50
+  ::: $DIR/offset_ub.rs:16:49
+   |
+LL | pub const NEGATIVE_OFFSET: *const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) };
+   |                                                 ------------------------------------------------ inside `NEGATIVE_OFFSET` at $DIR/offset_ub.rs:16:49
+
+error[E0080]: evaluation of constant value failed
+  --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+LL |         unsafe { intrinsics::offset(self, count) }
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                  |
+   |                  pointer arithmetic failed: allocN has size 0, so pointer to 1 byte starting at offset 0 is out-of-bounds
+   |                  inside `ptr::const_ptr::<impl *const u8>::offset` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   | 
+  ::: $DIR/offset_ub.rs:18:50
    |
 LL | pub const ZERO_SIZED_ALLOC: *const u8 = unsafe { [0u8; 0].as_ptr().offset(1) };
-   |                                                  --------------------------- inside `ZERO_SIZED_ALLOC` at $DIR/offset_ub.rs:17:50
+   |                                                  --------------------------- inside `ZERO_SIZED_ALLOC` at $DIR/offset_ub.rs:18:50
 
 error[E0080]: evaluation of constant value failed
   --> $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
@@ -119,10 +133,10 @@ LL |         unsafe { intrinsics::offset(self, count) as *mut T }
    |                  0x1 is not a valid pointer
    |                  inside `ptr::mut_ptr::<impl *mut u8>::offset` at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
    | 
-  ::: $DIR/offset_ub.rs:18:42
+  ::: $DIR/offset_ub.rs:19:42
    |
 LL | pub const DANGLING: *const u8 = unsafe { ptr::NonNull::<u8>::dangling().as_ptr().offset(4) };
-   |                                          ------------------------------------------------- inside `DANGLING` at $DIR/offset_ub.rs:18:42
+   |                                          ------------------------------------------------- inside `DANGLING` at $DIR/offset_ub.rs:19:42
 
 error[E0080]: evaluation of constant value failed
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
@@ -133,10 +147,10 @@ LL |         unsafe { intrinsics::offset(self, count) }
    |                  pointer arithmetic failed: 0x0 is not a valid pointer
    |                  inside `ptr::const_ptr::<impl *const u8>::offset` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
    | 
-  ::: $DIR/offset_ub.rs:21:50
+  ::: $DIR/offset_ub.rs:22:50
    |
 LL | pub const NULL_OFFSET_ZERO: *const u8 = unsafe { ptr::null::<u8>().offset(0) };
-   |                                                  --------------------------- inside `NULL_OFFSET_ZERO` at $DIR/offset_ub.rs:21:50
+   |                                                  --------------------------- inside `NULL_OFFSET_ZERO` at $DIR/offset_ub.rs:22:50
 
 error[E0080]: evaluation of constant value failed
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
@@ -147,11 +161,11 @@ LL |         unsafe { intrinsics::offset(self, count) }
    |                  0x7f..f is not a valid pointer
    |                  inside `ptr::const_ptr::<impl *const u8>::offset` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
    | 
-  ::: $DIR/offset_ub.rs:24:47
+  ::: $DIR/offset_ub.rs:25:47
    |
 LL | pub const UNDERFLOW_ABS: *const u8 = unsafe { (usize::MAX as *const u8).offset(isize::MIN) };
-   |                                               -------------------------------------------- inside `UNDERFLOW_ABS` at $DIR/offset_ub.rs:24:47
+   |                                               -------------------------------------------- inside `UNDERFLOW_ABS` at $DIR/offset_ub.rs:25:47
 
-error: aborting due to 11 previous errors
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0080`.