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Pick two. >> >> Zerocopy makes zero-cost memory manipulation effortless. We write >> `unsafe` so you don't have to. > > I tried applying this and using it with Binder. I ran into one > challenge, which is this uapi struct: > > struct binder_transaction_data { > /* The first two are only used for bcTRANSACTION and brTRANSACTION, > * identifying the target and contents of the transaction. > */ > union { > /* target descriptor of command transaction */ > __u32 handle; > /* target descriptor of return transaction */ > binder_uintptr_t ptr; > } target; > binder_uintptr_t cookie; /* target object cookie */ > ... > } > > The problem is that when the union contains a handle, there are 4 bytes > of padding in the union. Currently Rust Binder handles this by wrapping > the uapi struct in MaybeUninit and using MaybeUninit::zeroed() to > construct it, ensuring that even if padding is present, it is zeroed. > > However, this trick relies on unsafely implementing AsBytes for > BinderTransactionData with the safety comment being that the MaybeUninit > actually always contains initialized data. > > To translate this to zerocopy, I'd have to do this: > > unsafe impl zerocopy::IntoBytes for $newname { > fn only_derive_is_allowed_to_implement_this_trait() {} > } > > One fix could be to update the uapi header by explicitly adding the > padding, but that's kind of awkward for a union like this, since I'd > have to do it like this with an extra struct: > > union { > /* target descriptor of command transaction */ > struct { > __u32 handle; > __u32 _pad; > }; > /* target descriptor of return transaction */ > binder_uintptr_t ptr; > } target; > > It's not clear to me if changing the uapi headers like this is even > allowed to begin with. It's a somewhat non-trivial change. > > Any better ideas? > > Alice We can patch out `only_derive_is_allowed_to_implement_this_trait`? I don't = like this "extra safeguarding" in the first place. I think the "unsafe" there is sufficient. Best, Gary