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

Jason A. Donenfeld
2024-06-18T19:27:56Z

On Tue, Jun 18, 2024 at 10:55:17AM -0700, Andy Lutomirski wrote:
> On Mon, Jun 17, 2024 at 5:12 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > 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@zx2c4.com> 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?
>
> My alternative suggestion, which is far less well formed, would be to
> make the opaque argument be somehow not pointer-like and be more of an
> opaque handle. So it would be uintptr_t instead of void *, and the
> user API would be built around the user getting a list of handles
> instead of a block of memory.
>
> The benefit would be a tiny bit less overhead (potentially), but the
> API would need substantially more rework. I'm not convinced that this
> would be worthwhile.

I'd thought about this too -- a Windows-style handle system -- but
it seemed complicated and just not worth it, so the simplicity here
seems more appealing. I'm happy to take your suggestion of an opaque_len
argument (and it's already implemented in my "vdso" branch), and
leaving it at that.

Jason