netdev - Re: send/sendmsg ENOMEM errors WAS(Re: [PATCH net 6/6] sctp: not return ENOMEM err back in sctp_packet

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Date:   Sun, 23 Oct 2016 15:52:08 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Vlad Yasevich <vyasevic@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Michael Tuexen <Michael.Tuexen@...chi.franken.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Brenda Butler <bjb@...atatu.com>, gabor@...atatu.com
Subject: Re: send/sendmsg ENOMEM errors WAS(Re: [PATCH net 6/6] sctp: not
 return ENOMEM err back in sctp_packet_transmit

On 16-10-23 02:20 PM, Xin Long wrote:

> This patch doesn't ignore all the ENOMEN cases, only after msg is
> enqueued in out queue/send queue, in the lower layer, when alloc
> new skb and copy data from old skb, if it fails to alloc new skb, sctp
> will ignore this ENOMEM, as this msg will be taken care by retransmit
> mechanism, it's reasonable and also safe, user can't feel that.
>

Yes, that part i got.

> But for the cases before enqueue, like in sctp_sendmsg,
> sctp_datamsg_from_user may return ENOMEM, this err will return
> back to user, and can't be ignored.
>

The hard part is distinguishing between the above case and real
failure.
I am assuming in the case above user is _not_ required to send
again. But in the general case they are required to send again.
Correct?

> So I don't really think we should change something in manpage, what
> do you think ? maybe a little explanation there is also nice, :)

Yes, that would help. In particular it should be clear what user space
is expected to do. While this is about sctp - I am assuming equivalent
behavior for all callers of sendxxx() regardless of protocol.

cheers,
jamal