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 [off-list ref] wrote:

quoted

Hi Andy,

On Mon, Jun 17, 2024 at 05:06:22PM -0700, Andy Lutomirski wrote:

quoted

On Fri, Jun 14, 2024 at 12:08 PM Jason A. Donenfeld [off-list ref] wrote:

quoted

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