Message-ID: <66fadc698746d_187400294a5@willemb.c.googlers.com.notmuch>
Date: Mon, 30 Sep 2024 13:14:17 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 shuah@...nel.org, 
 willemb@...gle.com, 
 linux-kselftest@...r.kernel.org, 
 netdev@...r.kernel.org, 
 Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next 1/3] net-timestamp: add strict check when setting
 tx flags

Jason Xing wrote:
> On Mon, Sep 30, 2024 at 7:49 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Jason Xing wrote:
> > > On Mon, Sep 30, 2024 at 6:39 PM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > Jason Xing wrote:
> > > > > From: Jason Xing <kernelxing@...cent.com>
> > > > >
> > > > > Even though this case is unlikely to happen, we have to avoid such
> > > > > a case occurring at an earlier point: the sk_rmem_alloc could get
> > > > > increased because of inserting more and more skbs into the errqueue
> > > > > when calling __skb_complete_tx_timestamp(). This bad case would stop
> > > > > the socket transmitting soon.
> > > >
> > > > It is up to the application to read from the error queue frequently
> > > > enough and/or increase SO_RCVBUF.
> > >
> > > Sure thing. If we test it without setting SOF_TIMESTAMPING_SOFTWARE on
> > > the loopback, it will soon stop. That's the reason why I tried to add
> > > the restriction just in case.
> >
> > I don't follow at all.
> >
> > That bit does not affect the core issue: that the application is not
> > clearing its error queue quickly enough.
> >
> > > >
> > > > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > > > > ---
> > > > >  net/core/sock.c | 4 ++++
> > > > >  1 file changed, 4 insertions(+)
> > > > >
> > > > > diff --git a/net/core/sock.c b/net/core/sock.c
> > > > > index fe87f9bd8f16..4bddd6f62e4f 100644
> > > > > --- a/net/core/sock.c
> > > > > +++ b/net/core/sock.c
> > > > > @@ -905,6 +905,10 @@ int sock_set_timestamping(struct sock *sk, int optname,
> > > > >       if (val & ~SOF_TIMESTAMPING_MASK)
> > > > >               return -EINVAL;
> > > > >
> > > > > +     if (val & SOF_TIMESTAMPING_TX_RECORD_MASK &&
> > > > > +         !(val & SOF_TIMESTAMPING_SOFTWARE))
> > > > > +             return -EINVAL;
> > > > > +
> > > >
> > > > This breaks hardware timestamping
> > >
> > > Yes, and sorry about that. I'll fix this.
> >
> > As is I don't understand the purpose of this patch. Please do not
> > just resubmit with a change, but explain the problem and suggested
> > solution first.
> >
> 
> I will drop this patch because I just tested with my program in the
> local machine and found there is one mistake I made about calculating
> the diff between those two . Sorry for the noise.
> 
> Well, I only need to send a V2 patch of patch [3/3] in the next few days.
> 
> BTW, please allow me to ask one question unrelated to this patch
> again. I do wonder: if we batch the recv skbs from the errqueue when
> calling tcp_recvmsg() -> inet_recv_error(), it could break users,
> right?

Analogous to __msg_zerocopy_callback with __msg_zerocopy_callback.

Only here we cannot return range-based results and thus cannot just
expand the range of the one outstanding notification.

This would mean in ip(v6)_recv_error calling sock_dequeue_err_skb,
sock_recv_timestamp and put_cmsg IP_RECVERR multiple times. And
ip_cmsg_recv if needed.

Existing applications do not have to expect multiple results per
single recvmsg call. So enabling that unconditionally could break
them.

Adding this will require a new flag. An sk_tsflag is the obvious
approach.

Interpreting a MSG_* flag passed to recvmsg would be
another option. If there is a bit that can be set with MSG_ERRQUEUE
and cannot already be set currently. But I don't think that's the
case. We allow all bits and ignore any undefined ones.

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