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Possible UB from use of uninitialized [&T; N] #126
Description
Creating an array with mem::uninitialized may be UB if the array's elements contain non-null types (like &T) or uninhabited types (like !) or other types that have invalid values.
This can result in undefined behavior even if you never read the elements directly while they are uninitialized. For example, this program prints "true" if optimizations are enabled, when built with current rustc:
fn main() {
let x: Option<[&u8; 1]> = unsafe { Some(std::mem::uninitialized()) };
println!("{}", x.is_none());
}SmallVec isn't eligible for null pointer optimization, so it isn't affected by this. However, it's possible that a future Rust compiler could add other optimizations that break if SmallVec uses mem::uninitialized to create arrays of such types.
The ideal solution is to switch from uninitialized to the new MaybeUninit union (rust-lang/rfcs#1892) when it becomes stable.
Activity
mbrubeck commentedon Sep 27, 2018
See also rust-lang/rust#54542.
bluss commentedon Dec 3, 2018
ManuallyDrop<T>changed to allow enum layout optimization using the interior value, so it regressed this crate in that way. rust-lang/rust/pull/52711I can't make the test fail, but I think we have a problem hiding in here, as long as an optional smallvec of references is the same size as the plain type:
https://play.rust-lang.org/?version=stable&mode=release&edition=2015&gist=471955af6ec77850bf98446aba013c35
mbrubeck commentedon Dec 3, 2018
SmallVec contains an enum, so I think the enum layout optimization is using spare bits from that enum's tag to store the Option discriminant. If this is true then we're not seeing incorrect behavior in practice yet.
llogiq commentedon Mar 30, 2019
I recently looked into the use of
MaybeUninit<T>in arrays. We'd need to extend theArraytrait with an associated type oftype InternalArray = [MaybeUninit<$ty>; $n]and use that internally. However that would require theheapvariant to become (*mut MaybeUninit<A::Item>, usize) to ensure UB-freedom.Auto merge of #162 - thomcc:maybe-uninit, r=emilio