Re: recvmsg() "short receive" after FIONREAD

From: Andriy Gapon <>
Date: Sat, 11 Sep 2021 14:16:23 UTC
On 11/09/2021 17:13, Mark Johnston wrote:
> I think the semantic change is ok.  Did you change FIONREAD to lock the
> sockbuf?  I think it would be necessary to avoid races with pulseaudio:
> sb_acc is modified before sb_ctl, so there could be windows where
> sbavail(sb) - sb->sb_ctl gives a larger.
> And, it is not really safe to lock the sockbuf itself, since it may be
> overwritten by a listen(2) call.  SOCK_RECVBUF_LOCK(so) should be used
> instead.

I didn't think about the locking, so I didn't add it.
My current patch is trivial:
@@ -210,7 +210,7 @@ soo_ioctl(struct file *fp, u_long cmd, void *data, struct 
ucred *active_cred,
                 if (SOLISTENING(so)) {
                         error = EINVAL;
                 } else {
-                       *(int *)data = sbavail(&so->so_rcv);
+                       *(int *)data = sbavail(&so->so_rcv) - so->so_rcv.sb_ctl;

Let me try adding the lock.

Thank you again!

Andriy Gapon