Re: [PATCH v2] rust: alloc: fix dangling pointer in VecExt<T>::reserve()

[Benno Lossin <benno.lossin@proton.me>]

On 30.04.24 14:13, Danilo Krummrich wrote:
> Currently, a Vec<T>'s ptr value, after calling Vec<T>::new(), is
> initialized to Unique::dangling(). Hence, in VecExt<T>::reserve(), we're
> passing a dangling pointer (instead of NULL) to krealloc() whenever a
> new Vec<T> is created through VecExt<T> extension functions.
> 
> This only works as long as align_of::<T>(), used by Unique::dangling() to
> derive the dangling pointer, resolves to a value between 0x0 and
> ZERO_SIZE_PTR (0x10) and krealloc() hence treats it the same as a NULL
> pointer however.
> 
> This isn't a case we should rely on, since there may be types whose
> alignment may exceed the range still covered by krealloc(), plus other
> kernel allocators are not as tolerant either.
> 
> Instead, pass a real NULL pointer to krealloc_aligned() if Vec<T>'s
> capacity is zero.
> 
> Fixes: 5ab560ce12ed ("rust: alloc: update `VecExt` to take allocation flags")
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>

Nice find!

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

I don't mind the `match`, but an `if` is also fine.

-- 
Cheers,
Benno