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

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Apr 29, 2024 at 9:24 PM Danilo Krummrich <dakr@redhat.com> 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.

Good catch!

> This only works since it happens that Unique::dangling()'s value (0x1)
> falls within the range between 0x0 and ZERO_SIZE_PTR (0x10) and
> krealloc() hence treats it the same as a NULL pointer however.

This isn't quite true. The value of Unique::dangling() is actually
align_of::<T>() rather than 0x1. So for types that have an alignment
of 0x20 or greater, it would not work today.

> This isn't a case we should rely on, especially since other kernel
> allocators are not as tolerant. 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")
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>