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@xxxxxxxxx> 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@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? > > 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
