Re: recvmsg() "short receive" after FIONREAD

From: Andriy Gapon <avg_at_FreeBSD.org>
Date: Sat, 11 Sep 2021 17:28:11 +0300
On 11/09/2021 17:16, Andriy Gapon wrote:
> 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:
> _at__at_ -210,7 +210,7 _at__at_ 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;
>                  }
>                  break;
> 
> Let me try adding the lock.

By the way, soo_stat() seems to be another good example to follow.


-- 
Andriy Gapon
Received on Sat Sep 11 2021 - 14:28:11 UTC

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