Re: [PATCH net v3 2/2] vsock/bpf: fix zero-length, MSG_OOB and MSG_ERRQUEUE reads in recvmsg

[jrmmhm . kernel]

On Fri, Oct 30, 2026 at 03:25:26PM +0100, Stefano Garzarella wrote:
> Ditto.

Same as for 1/2, I'll rewrite it.

> >+    if (unlikely(flags & MSG_ERRQUEUE))
> >+            return __vsock_recvmsg(sk, msg, len, flags);
>
> If the psock ingress queue is not empty, wouldn't recvmsg(MSG_ERRQUEUE)
> (e.g. a MSG_ZEROCOPY user reading completions) also consume stream bytes
> through sk_msg_recvmsg(), since it only looks at MSG_PEEK? If so, maybe
> worth mentioning in the commit message.

Yes, when the read passes a buffer. I reproduced it: without this patch
a 1-byte MSG_ERRQUEUE read with stream data in the psock ingress queue
returns that byte. A zero-length read, as MSG_ZEROCOPY users do, takes
none, but never reaches the error queue either: it gets -EAGAIN instead
of the completion. Routing MSG_ERRQUEUE first fixes both; I'll mention
it in the message.

> >+    if (flags & MSG_OOB)
> >+            return -EOPNOTSUPP;
> >+
> >+    if (!len)
> >+            return 0;
>
> Is the order of the checks important? We should mention it in the commit
> description (if it's important or not and why).

Yes. MSG_ERRQUEUE must come first, or a zero-length error-queue read
returns 0. MSG_OOB goes before len == 0 so that recv(fd, buf, 0, MSG_OOB)
returns -EOPNOTSUPP as in the native path on a connected socket. I'll say
so in the message.

Thanks,
Jerome