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Don't call assume_init on Deferred's Data #779

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merged 1 commit into from
Feb 5, 2022

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saethlin
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@saethlin saethlin commented Feb 5, 2022

This situation can be observed by running MIRIFLAGS="-Zmiri-check-number-validity" cargo miri test in crossbeam/crossbeam-deque:

test is_empty ... error: Undefined Behavior: type validation failed at .value[0]: encountered uninitialized bytes
  --> /home/ben/crossbeam/crossbeam-epoch/src/deferred.rs:49:27a
   |
49 |                     data: data.assume_init(),
   |                           ^^^^^^^^^^^^^^^^^^ type validation failed at .value[0]: encountered uninitialized bytes
   |

In the crossbeam-deque test suite, a Deferred was created from a FnOnce which is smaller than the Data. This makes the call to MaybeUninit::assume_init() immediate UB (the reference to it created upon call is probably UB too). The docs for MaybeUninit::assume_init() say this:

It is up to the caller to guarantee that the MaybeUninit really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior. The type-level documentation contains more information about this initialization invariant.

Since Data doesn't have a Drop impl, we can just leave it in the MaybeUninit wrapper. This removes all issues about type validity.

In the crossbeam-deque test suite, a Deferred is created from a FnOnce
which is smaller than the Data. This makes the call to assume_init()
immediate UB (the reference to it created upon call is probably UB too).
Since Data doesn't have a Drop impl, we can just leave it in the MaybeUninit
wrapper.
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Thanks!

bors r+

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bors bot commented Feb 5, 2022

Build succeeded:

@bors bors bot merged commit be6ff29 into crossbeam-rs:master Feb 5, 2022
bors bot added a commit that referenced this pull request Feb 5, 2022
786: Prepare for the next release r=taiki-e a=taiki-e

- crossbeam-epoch 0.9.6 -> 0.9.7
  - Fix Miri error when `-Zmiri-check-number-validity` is enabled. (#779)
- crossbeam-queue 0.3.3 -> 0.3.4
  - Implement `IntoIterator` for `ArrayQueue` and `SegQueue`. (#772)
- crossbeam-utils 0.8.6 -> 0.8.7
  - Add `AtomicCell<i*,u*>::{fetch_max,fetch_min}`. (#785)
  - Add `AtomicCell<i*,u*,bool>::fetch_nand`. (#785)
  - Fix unsoundness of `AtomicCell<{i,u}64>` arithmetics on 32-bit targets that support `Atomic{I,U}64` (#781)


Co-authored-by: Taiki Endo <[email protected]>
bors bot added a commit that referenced this pull request Feb 5, 2022
786: Prepare for the next release r=taiki-e a=taiki-e

- crossbeam-epoch 0.9.6 -> 0.9.7
  - Fix Miri error when `-Zmiri-check-number-validity` is enabled. (#779)
- crossbeam-queue 0.3.3 -> 0.3.4
  - Implement `IntoIterator` for `ArrayQueue` and `SegQueue`. (#772)
- crossbeam-utils 0.8.6 -> 0.8.7
  - Add `AtomicCell<{i*,u*}>::{fetch_max,fetch_min}`. (#785)
  - Add `AtomicCell<{i*,u*,bool}>::fetch_nand`. (#785)
  - Fix unsoundness of `AtomicCell<{i,u}64>` arithmetics on 32-bit targets that support `Atomic{I,U}64` (#781)


Co-authored-by: Taiki Endo <[email protected]>
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2 participants