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RalfJungRalfJung
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offset_from intrinsic: always allow pointers to point to the same address
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7 files changed

+67
-61
lines changed

7 files changed

+67
-61
lines changed

‎compiler/rustc_const_eval/src/interpret/intrinsics.rs

+18-15
Original file line numberDiff line numberDiff line change
@@ -20,7 +20,7 @@ use super::{
2020
err_inval, err_ub_custom, err_unsup_format, memory::MemoryKind, throw_inval, throw_ub_custom,
2121
throw_ub_format, util::ensure_monomorphic_enough, Allocation, CheckInAllocMsg, ConstAllocation,
2222
GlobalId, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, Pointer, PointerArithmetic,
23-
Scalar,
23+
Provenance, Scalar,
2424
};
2525

2626
use crate::fluent_generated as fluent;
@@ -259,25 +259,28 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
259259
// This will always return 0.
260260
(a, b)
261261
}
262-
(Err(_), _) | (_, Err(_)) => {
263-
// We managed to find a valid allocation for one pointer, but not the other.
264-
// That means they are definitely not pointing to the same allocation.
262+
_ if M::Provenance::OFFSET_IS_ADDR && a.addr() == b.addr() => {
263+
// At least one of the pointers has provenance, but they also point to
264+
// the same address so it doesn't matter; this is fine. `(0, 0)` means
265+
// we pass all the checks below and return 0.
266+
(0, 0)
267+
}
268+
// From here onwards, the pointers are definitely for different addresses
269+
// (or we can't determine their absolute address).
270+
(Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _)))
271+
if a_alloc_id == b_alloc_id =>
272+
{
273+
// Found allocation for both, and it's the same.
274+
// Use these offsets for distance calculation.
275+
(a_offset.bytes(), b_offset.bytes())
276+
}
277+
_ => {
278+
// Not into the same allocation -- this is UB.
265279
throw_ub_custom!(
266280
fluent::const_eval_offset_from_different_allocations,
267281
name = intrinsic_name,
268282
);
269283
}
270-
(Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _))) => {
271-
// Found allocation for both. They must be into the same allocation.
272-
if a_alloc_id != b_alloc_id {
273-
throw_ub_custom!(
274-
fluent::const_eval_offset_from_different_allocations,
275-
name = intrinsic_name,
276-
);
277-
}
278-
// Use these offsets for distance calculation.
279-
(a_offset.bytes(), b_offset.bytes())
280-
}
281284
};
282285

283286
// Compute distance.

‎library/core/src/ptr/const_ptr.rs

+6-6
Original file line numberDiff line numberDiff line change
@@ -604,9 +604,9 @@ impl<T: ?Sized> *const T {
604604
///
605605
/// * `self` and `origin` must either
606606
///
607+
/// * point to the same address, or
607608
/// * both be *derived from* a pointer to the same [allocated object], and the memory range between
608-
/// the two pointers must be either empty or in bounds of that object. (See below for an example.)
609-
/// * or both be derived from an integer literal/constant, and point to the same address.
609+
/// the two pointers must be in bounds of that object. (See below for an example.)
610610
///
611611
/// * The distance between the pointers, in bytes, must be an exact multiple
612612
/// of the size of `T`.
@@ -653,14 +653,14 @@ impl<T: ?Sized> *const T {
653653
/// let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
654654
/// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
655655
/// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
656-
/// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
657-
/// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
656+
/// // Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
657+
/// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff).wrapping_offset(1);
658658
/// assert_eq!(ptr2 as usize, ptr2_other as usize);
659659
/// // Since ptr2_other and ptr2 are derived from pointers to different objects,
660660
/// // computing their offset is undefined behavior, even though
661-
/// // they point to the same address!
661+
/// // they point to addresses that are in-bounds of the same object!
662662
/// unsafe {
663-
/// let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
663+
/// let one = ptr2_other.offset_from(ptr2); // Undefined Behavior! ⚠️
664664
/// }
665665
/// ```
666666
#[stable(feature = "ptr_offset_from", since = "1.47.0")]

‎library/core/src/ptr/mut_ptr.rs

+6-6
Original file line numberDiff line numberDiff line change
@@ -829,9 +829,9 @@ impl<T: ?Sized> *mut T {
829829
///
830830
/// * `self` and `origin` must either
831831
///
832+
/// * point to the same address, or
832833
/// * both be *derived from* a pointer to the same [allocated object], and the memory range between
833-
/// the two pointers must be either empty or in bounds of that object. (See below for an example.)
834-
/// * or both be derived from an integer literal/constant, and point to the same address.
834+
/// the two pointers must be in bounds of that object. (See below for an example.)
835835
///
836836
/// * The distance between the pointers, in bytes, must be an exact multiple
837837
/// of the size of `T`.
@@ -878,14 +878,14 @@ impl<T: ?Sized> *mut T {
878878
/// let ptr1 = Box::into_raw(Box::new(0u8));
879879
/// let ptr2 = Box::into_raw(Box::new(1u8));
880880
/// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
881-
/// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
882-
/// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
881+
/// // Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
882+
/// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff).wrapping_offset(1);
883883
/// assert_eq!(ptr2 as usize, ptr2_other as usize);
884884
/// // Since ptr2_other and ptr2 are derived from pointers to different objects,
885885
/// // computing their offset is undefined behavior, even though
886-
/// // they point to the same address!
886+
/// // they point to addresses that are in-bounds of the same object!
887887
/// unsafe {
888-
/// let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
888+
/// let one = ptr2_other.offset_from(ptr2); // Undefined Behavior! ⚠️
889889
/// }
890890
/// ```
891891
#[stable(feature = "ptr_offset_from", since = "1.47.0")]

‎library/core/src/ptr/non_null.rs

+7-6
Original file line numberDiff line numberDiff line change
@@ -735,9 +735,9 @@ impl<T: ?Sized> NonNull<T> {
735735
///
736736
/// * `self` and `origin` must either
737737
///
738+
/// * point to the same address, or
738739
/// * both be *derived from* a pointer to the same [allocated object], and the memory range between
739-
/// the two pointers must be either empty or in bounds of that object. (See below for an example.)
740-
/// * or both be derived from an integer literal/constant, and point to the same address.
740+
/// the two pointers must be in bounds of that object. (See below for an example.)
741741
///
742742
/// * The distance between the pointers, in bytes, must be an exact multiple
743743
/// of the size of `T`.
@@ -789,14 +789,15 @@ impl<T: ?Sized> NonNull<T> {
789789
/// let ptr1 = NonNull::new(Box::into_raw(Box::new(0u8))).unwrap();
790790
/// let ptr2 = NonNull::new(Box::into_raw(Box::new(1u8))).unwrap();
791791
/// let diff = (ptr2.addr().get() as isize).wrapping_sub(ptr1.addr().get() as isize);
792-
/// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
793-
/// let ptr2_other = NonNull::new(ptr1.as_ptr().wrapping_byte_offset(diff)).unwrap();
792+
/// // Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
793+
/// let diff_plus_1 = diff.wrapping_add(1);
794+
/// let ptr2_other = NonNull::new(ptr1.as_ptr().wrapping_byte_offset(diff_plus_1)).unwrap();
794795
/// assert_eq!(ptr2.addr(), ptr2_other.addr());
795796
/// // Since ptr2_other and ptr2 are derived from pointers to different objects,
796797
/// // computing their offset is undefined behavior, even though
797-
/// // they point to the same address!
798+
/// // they point to addresses that are in-bounds of the same object!
798799
///
799-
/// let zero = unsafe { ptr2_other.offset_from(ptr2) }; // Undefined Behavior
800+
/// let one = unsafe { ptr2_other.offset_from(ptr2) }; // Undefined Behavior! ⚠️
800801
/// ```
801802
#[inline]
802803
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces

‎src/tools/miri/tests/pass/zero-sized-accesses-and-offsets.rs

-3
Original file line numberDiff line numberDiff line change
@@ -39,8 +39,6 @@ fn test_ptr(ptr: *mut ()) {
3939
// Distance.
4040
let ptr = ptr.cast::<i32>();
4141
ptr.offset_from(ptr);
42-
/*
43-
FIXME: this is disabled for now as these cases are not yet allowed.
4442
// Distance from other "bad" pointers that have the same address, but different provenance. Some
4543
// of this is library UB, but we don't want it to be language UB since that would violate
4644
// provenance monotonicity: if we allow computing the distance between two ptrs with no
@@ -54,6 +52,5 @@ fn test_ptr(ptr: *mut ()) {
5452
// - Distance from use-after-free pointer
5553
drop(b);
5654
ptr.offset_from(other_ptr.with_addr(ptr.addr()));
57-
*/
5855
}
5956
}

‎tests/ui/consts/offset_from_ub.rs

+19-14
Original file line numberDiff line numberDiff line change
@@ -32,12 +32,6 @@ pub const NOT_MULTIPLE_OF_SIZE: isize = {
3232
//~| 1_isize cannot be divided by 2_isize without remainder
3333
};
3434

35-
pub const OFFSET_FROM_NULL: isize = {
36-
let ptr = 0 as *const u8;
37-
// Null isn't special for zero-sized "accesses" (i.e., the range between the two pointers)
38-
unsafe { ptr_offset_from(ptr, ptr) }
39-
};
40-
4135
pub const DIFFERENT_INT: isize = { // offset_from with two different integers: like DIFFERENT_ALLOC
4236
let ptr1 = 8 as *const u8;
4337
let ptr2 = 16 as *const u8;
@@ -63,14 +57,6 @@ const OUT_OF_BOUNDS_2: isize = {
6357
//~| pointer to 10 bytes starting at offset 0 is out-of-bounds
6458
};
6559

66-
const OUT_OF_BOUNDS_SAME: isize = {
67-
let start_ptr = &4 as *const _ as *const u8;
68-
let length = 10;
69-
let end_ptr = (start_ptr).wrapping_add(length);
70-
// Out-of-bounds is fine as long as the range between the pointers is empty.
71-
unsafe { ptr_offset_from(end_ptr, end_ptr) }
72-
};
73-
7460
pub const DIFFERENT_ALLOC_UNSIGNED: usize = {
7561
let uninit = std::mem::MaybeUninit::<Struct>::uninit();
7662
let base_ptr: *const Struct = &uninit as *const _ as *const Struct;
@@ -130,4 +116,23 @@ pub const OFFSET_VERY_FAR2: isize = {
130116
//~^ inside
131117
};
132118

119+
// If the pointers are the same, OOB/null/UAF is fine.
120+
pub const OFFSET_FROM_NULL_SAME: isize = {
121+
let ptr = 0 as *const u8;
122+
unsafe { ptr_offset_from(ptr, ptr) }
123+
};
124+
const OUT_OF_BOUNDS_SAME: isize = {
125+
let start_ptr = &4 as *const _ as *const u8;
126+
let length = 10;
127+
let end_ptr = (start_ptr).wrapping_add(length);
128+
unsafe { ptr_offset_from(end_ptr, end_ptr) }
129+
};
130+
const UAF_SAME: isize = {
131+
let uaf_ptr = {
132+
let x = 0;
133+
&x as *const i32
134+
};
135+
unsafe { ptr_offset_from(uaf_ptr, uaf_ptr) }
136+
};
137+
133138
fn main() {}

‎tests/ui/consts/offset_from_ub.stderr

+11-11
Original file line numberDiff line numberDiff line change
@@ -24,55 +24,55 @@ LL | unsafe { ptr_offset_from(field_ptr, base_ptr as *const u16) }
2424
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ exact_div: 1_isize cannot be divided by 2_isize without remainder
2525

2626
error[E0080]: evaluation of constant value failed
27-
--> $DIR/offset_from_ub.rs:44:14
27+
--> $DIR/offset_from_ub.rs:38:14
2828
|
2929
LL | unsafe { ptr_offset_from(ptr2, ptr1) }
3030
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from` called on different pointers without provenance (i.e., without an associated allocation)
3131

3232
error[E0080]: evaluation of constant value failed
33-
--> $DIR/offset_from_ub.rs:53:14
33+
--> $DIR/offset_from_ub.rs:47:14
3434
|
3535
LL | unsafe { ptr_offset_from(end_ptr, start_ptr) }
3636
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds `offset_from`: ALLOC0 has size 4, so pointer to 10 bytes starting at offset 0 is out-of-bounds
3737

3838
error[E0080]: evaluation of constant value failed
39-
--> $DIR/offset_from_ub.rs:62:14
39+
--> $DIR/offset_from_ub.rs:56:14
4040
|
4141
LL | unsafe { ptr_offset_from(start_ptr, end_ptr) }
4242
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds `offset_from`: ALLOC1 has size 4, so pointer to 10 bytes starting at offset 0 is out-of-bounds
4343

4444
error[E0080]: evaluation of constant value failed
45-
--> $DIR/offset_from_ub.rs:79:14
45+
--> $DIR/offset_from_ub.rs:65:14
4646
|
4747
LL | unsafe { ptr_offset_from_unsigned(field_ptr, base_ptr) }
4848
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from_unsigned` called on pointers into different allocations
4949

5050
error[E0080]: evaluation of constant value failed
51-
--> $DIR/offset_from_ub.rs:86:14
51+
--> $DIR/offset_from_ub.rs:72:14
5252
|
5353
LL | unsafe { ptr_offset_from(ptr2, ptr1) }
5454
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from` called when first pointer is too far ahead of second
5555

5656
error[E0080]: evaluation of constant value failed
57-
--> $DIR/offset_from_ub.rs:92:14
57+
--> $DIR/offset_from_ub.rs:78:14
5858
|
5959
LL | unsafe { ptr_offset_from(ptr1, ptr2) }
6060
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from` called when first pointer is too far before second
6161

6262
error[E0080]: evaluation of constant value failed
63-
--> $DIR/offset_from_ub.rs:100:14
63+
--> $DIR/offset_from_ub.rs:86:14
6464
|
6565
LL | unsafe { ptr_offset_from(ptr1, ptr2) }
6666
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from` called when first pointer is too far before second
6767

6868
error[E0080]: evaluation of constant value failed
69-
--> $DIR/offset_from_ub.rs:107:14
69+
--> $DIR/offset_from_ub.rs:93:14
7070
|
7171
LL | unsafe { ptr_offset_from_unsigned(p, p.add(2) ) }
7272
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from_unsigned` called when first pointer has smaller offset than second: 0 < 8
7373

7474
error[E0080]: evaluation of constant value failed
75-
--> $DIR/offset_from_ub.rs:114:14
75+
--> $DIR/offset_from_ub.rs:100:14
7676
|
7777
LL | unsafe { ptr_offset_from_unsigned(ptr2, ptr1) }
7878
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from_unsigned` called when first pointer is too far ahead of second
@@ -85,7 +85,7 @@ error[E0080]: evaluation of constant value failed
8585
note: inside `std::ptr::const_ptr::<impl *const u8>::offset_from`
8686
--> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
8787
note: inside `OFFSET_VERY_FAR1`
88-
--> $DIR/offset_from_ub.rs:123:14
88+
--> $DIR/offset_from_ub.rs:109:14
8989
|
9090
LL | unsafe { ptr2.offset_from(ptr1) }
9191
| ^^^^^^^^^^^^^^^^^^^^^^
@@ -98,7 +98,7 @@ error[E0080]: evaluation of constant value failed
9898
note: inside `std::ptr::const_ptr::<impl *const u8>::offset_from`
9999
--> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
100100
note: inside `OFFSET_VERY_FAR2`
101-
--> $DIR/offset_from_ub.rs:129:14
101+
--> $DIR/offset_from_ub.rs:115:14
102102
|
103103
LL | unsafe { ptr1.offset_from(ptr2.wrapping_offset(1)) }
104104
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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