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offset_from intrinsic: always allow pointers to point to the same add…
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@RalfJung
RalfJung committed May 10, 2024
1 parent 353c636 commit 8f8766a
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Showing 6 changed files with 59 additions and 54 deletions.
33 changes: 18 additions & 15 deletions compiler/rustc_const_eval/src/interpret/intrinsics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@ use rustc_target::abi::Size;
use super::{
memory::MemoryKind, util::ensure_monomorphic_enough, Allocation, CheckInAllocMsg,
ConstAllocation, GlobalId, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, Pointer,
PointerArithmetic, Scalar,
PointerArithmetic, Provenance, Scalar,
};

use crate::fluent_generated as fluent;
Expand Down Expand Up @@ -256,25 +256,28 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
// This will always return 0.
(a, b)
}
(Err(_), _) | (_, Err(_)) => {
// We managed to find a valid allocation for one pointer, but not the other.
// That means they are definitely not pointing to the same allocation.
_ 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)
}
// 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())
}
_ => {
// Not into the same allocation -- this is UB.
throw_ub_custom!(
fluent::const_eval_offset_from_different_allocations,
name = intrinsic_name,
);
}
(Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _))) => {
// Found allocation for both. They must be into the same allocation.
if a_alloc_id != b_alloc_id {
throw_ub_custom!(
fluent::const_eval_offset_from_different_allocations,
name = intrinsic_name,
);
}
// Use these offsets for distance calculation.
(a_offset.bytes(), b_offset.bytes())
}
};

// Compute distance.
Expand Down
12 changes: 6 additions & 6 deletions library/core/src/ptr/const_ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -679,9 +679,9 @@ impl<T: ?Sized> *const T {
///
/// * `self` and `origin` must either
///
/// * point to the same address, or
/// * both be *derived from* a pointer to the same [allocated object], and the memory range between
/// the two pointers must be either empty or in bounds of that object. (See below for an example.)
/// * or both be derived from an integer literal/constant, and point to the same address.
/// the two pointers must be in bounds of that object. (See below for an example.)
///
/// * The distance between the pointers, in bytes, must be an exact multiple
/// of the size of `T`.
Expand Down Expand Up @@ -744,14 +744,14 @@ impl<T: ?Sized> *const T {
/// let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
/// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
/// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
/// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
/// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
/// // Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
/// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff).wrapping_offset(1);
/// assert_eq!(ptr2 as usize, ptr2_other as usize);
/// // Since ptr2_other and ptr2 are derived from pointers to different objects,
/// // computing their offset is undefined behavior, even though
/// // they point to the same address!
/// // they point to addresses that are in-bounds of the same object!
/// unsafe {
/// let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
/// let one = ptr2_other.offset_from(ptr2); // Undefined Behavior
/// }
/// ```
#[stable(feature = "ptr_offset_from", since = "1.47.0")]
Expand Down
12 changes: 6 additions & 6 deletions library/core/src/ptr/mut_ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -907,9 +907,9 @@ impl<T: ?Sized> *mut T {
///
/// * `self` and `origin` must either
///
/// * point to the same address, or
/// * both be *derived from* a pointer to the same [allocated object], and the memory range between
/// the two pointers must be either empty or in bounds of that object. (See below for an example.)
/// * or both be derived from an integer literal/constant, and point to the same address.
/// the two pointers must be in bounds of that object. (See below for an example.)
///
/// * The distance between the pointers, in bytes, must be an exact multiple
/// of the size of `T`.
Expand Down Expand Up @@ -972,14 +972,14 @@ impl<T: ?Sized> *mut T {
/// let ptr1 = Box::into_raw(Box::new(0u8));
/// let ptr2 = Box::into_raw(Box::new(1u8));
/// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
/// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
/// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
/// // Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
/// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff).wrapping_offset(1);
/// assert_eq!(ptr2 as usize, ptr2_other as usize);
/// // Since ptr2_other and ptr2 are derived from pointers to different objects,
/// // computing their offset is undefined behavior, even though
/// // they point to the same address!
/// // they point to addresses that are in-bounds of the same object!
/// unsafe {
/// let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
/// let one = ptr2_other.offset_from(ptr2); // Undefined Behavior
/// }
/// ```
#[stable(feature = "ptr_offset_from", since = "1.47.0")]
Expand Down
3 changes: 0 additions & 3 deletions src/tools/miri/tests/pass/zero-sized-accesses-and-offsets.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,6 @@ fn test_ptr(ptr: *mut ()) {
// Distance.
let ptr = ptr.cast::<i32>();
ptr.offset_from(ptr);
/*
FIXME: this is disabled for now as these cases are not yet allowed.
// Distance from other "bad" pointers that have the same address, but different provenance. Some
// of this is library UB, but we don't want it to be language UB since that would violate
// provenance monotonicity: if we allow computing the distance between two ptrs with no
Expand All @@ -54,6 +52,5 @@ fn test_ptr(ptr: *mut ()) {
// - Distance from use-after-free pointer
drop(b);
ptr.offset_from(other_ptr.with_addr(ptr.addr()));
*/
}
}
33 changes: 19 additions & 14 deletions tests/ui/consts/offset_from_ub.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -32,12 +32,6 @@ pub const NOT_MULTIPLE_OF_SIZE: isize = {
//~| 1_isize cannot be divided by 2_isize without remainder
};

pub const OFFSET_FROM_NULL: isize = {
let ptr = 0 as *const u8;
// Null isn't special for zero-sized "accesses" (i.e., the range between the two pointers)
unsafe { ptr_offset_from(ptr, ptr) }
};

pub const DIFFERENT_INT: isize = { // offset_from with two different integers: like DIFFERENT_ALLOC
let ptr1 = 8 as *const u8;
let ptr2 = 16 as *const u8;
Expand All @@ -63,14 +57,6 @@ const OUT_OF_BOUNDS_2: isize = {
//~| pointer to 10 bytes starting at offset 0 is out-of-bounds
};

const OUT_OF_BOUNDS_SAME: isize = {
let start_ptr = &4 as *const _ as *const u8;
let length = 10;
let end_ptr = (start_ptr).wrapping_add(length);
// Out-of-bounds is fine as long as the range between the pointers is empty.
unsafe { ptr_offset_from(end_ptr, end_ptr) }
};

pub const DIFFERENT_ALLOC_UNSIGNED: usize = {
let uninit = std::mem::MaybeUninit::<Struct>::uninit();
let base_ptr: *const Struct = &uninit as *const _ as *const Struct;
Expand Down Expand Up @@ -122,4 +108,23 @@ pub const OFFSET_VERY_FAR2: isize = {
//~^ inside
};

// If the pointers are the same, OOB/null/UAF is fine.
pub const OFFSET_FROM_NULL_SAME: isize = {
let ptr = 0 as *const u8;
unsafe { ptr_offset_from(ptr, ptr) }
};
const OUT_OF_BOUNDS_SAME: isize = {
let start_ptr = &4 as *const _ as *const u8;
let length = 10;
let end_ptr = (start_ptr).wrapping_add(length);
unsafe { ptr_offset_from(end_ptr, end_ptr) }
};
const UAF_SAME: isize = {
let uaf_ptr = {
let x = 0;
&x as *const i32
};
unsafe { ptr_offset_from(uaf_ptr, uaf_ptr) }
};

fn main() {}
20 changes: 10 additions & 10 deletions tests/ui/consts/offset_from_ub.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -24,49 +24,49 @@ LL | unsafe { ptr_offset_from(field_ptr, base_ptr as *const u16) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ exact_div: 1_isize cannot be divided by 2_isize without remainder

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

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

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

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

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

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

error[E0080]: evaluation of constant value failed
--> $DIR/offset_from_ub.rs:99:14
--> $DIR/offset_from_ub.rs:85:14
|
LL | unsafe { ptr_offset_from_unsigned(p, p.add(2) ) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from_unsigned` called when first pointer has smaller offset than second: 0 < 8

error[E0080]: evaluation of constant value failed
--> $DIR/offset_from_ub.rs:106:14
--> $DIR/offset_from_ub.rs:92:14
|
LL | unsafe { ptr_offset_from_unsigned(ptr2, ptr1) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ptr_offset_from_unsigned` called when first pointer is too far ahead of second
Expand All @@ -79,7 +79,7 @@ error[E0080]: evaluation of constant value failed
note: inside `std::ptr::const_ptr::<impl *const u8>::offset_from`
--> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
note: inside `OFFSET_VERY_FAR1`
--> $DIR/offset_from_ub.rs:115:14
--> $DIR/offset_from_ub.rs:101:14
|
LL | unsafe { ptr2.offset_from(ptr1) }
| ^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -92,7 +92,7 @@ error[E0080]: evaluation of constant value failed
note: inside `std::ptr::const_ptr::<impl *const u8>::offset_from`
--> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
note: inside `OFFSET_VERY_FAR2`
--> $DIR/offset_from_ub.rs:121:14
--> $DIR/offset_from_ub.rs:107:14
|
LL | unsafe { ptr1.offset_from(ptr2.wrapping_offset(1)) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand Down

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