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From: Valentin Obst <kernel@valentinobst.de>
To: aliceryhl@google.com
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Subject: Re: [PATCH v2 3/4] rust: uaccess: add typed accessors for userspace pointers
Date: Fri,  9 Feb 2024 18:18:41 +0100
Message-ID: <20240209171841.3494-1-kernel@valentinobst.de>
In-Reply-To: <CAH5fLgg3C79DHz0FJ1LOhvetv4THHNAnu6XPx0vycFCb_txkfg@mail.gmail.com>
References: <CAH5fLgg3C79DHz0FJ1LOhvetv4THHNAnu6XPx0vycFCb_txkfg@mail.gmail.com>
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> > > +/// If a struct implements this trait, then it is okay to copy it byte-for-byte
> > > +/// to userspace. This means that it should not have any padding, as padding
> > > +/// bytes are uninitialized. Reading uninitialized memory is not just undefined
> > > +/// behavior, it may even lead to leaking sensitive information on the stack to
> > > +/// userspace.
> >
> > This feels a bit too restrictive to me. Isn't it okay to copy types with
> > padding if it is ensured that the padding is always initialized?
> >
> > I recall that in C one occasionally does a `memset` for structs that are
> > copied to user space. I imagine that one could have a Rust
> > abstraction/macro that makes it easy to define custom types that can
> > always guarantee that all padding bytes are initialized. Such types
> > would then qualify for being copied to user space if all field do so as
> > well.
> >
> > This could be a significant quality-of-life improvement for drivers
> > as it can be tedious to define struct without padding.
>
> I don't think we should go that route. For example:
>
> let struct_1 = ..;
> memset(&mut struct_1, 0);
> let struct_2 = struct_1;
>
> Even though struct_1 has its padding zeroed here, that is not the case
> for struct_2. When Rust performs a typed copy/move, the padding is not
> copied.
>
> Anyway, there is a work-around. Define your struct with MaybeUninit:
>
> // INVARIANT: All bytes always initialized.
> struct MyWrapper(MaybeUninit<bindings::c_struct>);
>
> impl Default for MyWrapper {
>     fn default() -> Self {
>         MyWrapper(MaybeUninit::zeroed())
>     }
> }
>
> Unlike the bare struct, things wrapped in MaybeUninit always have
> their padding preserved. Then, you can implement the trait for this
> wrapper, since its padding is always initialized even if that is not
> true for the wrapped struct.

Yea, I see the issue posed by moving values around. What I had in mind
was some library code that makes achieving the above behavior more
ergonomic and semantically clearer (in the sense that without the
surrounding comments it might not be immediately clear to someone
reading the code why you are doing things that way). My reply was
mainly about gauging interest into such a feature.

However, this really is independent from this patch and could always be
added later. I agree with mentioning padding in the comment and see
nothing blocking here.

	- Best Valentin

>
> Alice