Re: [PATCH v17 4/5] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

Hi Andy,

On Mon, Jun 17, 2024 at 05:06:22PM -0700, Andy Lutomirski wrote:
> On Fri, Jun 14, 2024 at 12:08 PM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> >
> > Provide a generic C vDSO getrandom() implementation, which operates on
> > an opaque state returned by vgetrandom_alloc() and produces random bytes
> > the same way as getrandom(). This has a the API signature:
> >
> >   ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);
> 
> Last time around, I mentioned some potential issues with this function
> signature, and I didn't see any answer.  My specific objection was to
> the fact that the caller passes in a pointer but not a length, and
> this potentially makes reasoning about memory safety awkward,
> especially if anything like CRIU is involved.

Oh, I understood this backwards last time - I thought you were
criticizing the size_t len argument, which didn't make any sense.

Re-reading now, what you're suggesting is that I add an additional
argument called `size_t opaque_len`, and then the implementation does
something like:

    if (opaque_len != sizeof(struct vgetrandom_state))
    	goto fallback_syscall;

With the reasoning that falling back to syscall is better than returning
-EINVAL, because that could happen in a natural way due to CRIU. In
contrast, your objection to opaque_state not being aligned falling back
to the syscall was that it should never happen ever, so -EFAULT is more
fitting.

Is that correct?

If I've gotten you right this time, I'll add that argument as described.
Seems straight forward to do. It's a bit annoying from a libc
perspective, as the length has to be stored, but that's not impossible.

Jason