Re: [RFC PATCH 1/4] net: Introduce read_sock_cmsg proto_ops for control message delivery

Hannes Reinecke
2026-02-19T08:10:14Z

On 2/18/26 17:12, Chuck Lever wrote:
>
>
> On Wed, Feb 18, 2026, at 10:52 AM, Hannes Reinecke wrote:
>> On 2/18/26 15:33, Chuck Lever wrote:
>>> On 2/18/26 2:29 AM, Hannes Reinecke wrote:
>
>>>> The way I see it, ->read_sock() has an advantage when you can work
>>>> with skbuffs (ie network packets) directly. Then it has the benefit
>>>> of being paced with the packets arriving as they do on the network.
>>>>
>>>> But for TLS this is no longer true; the TLS skbuffs are synthesized
>>>> on top of the TCP stream, and the boundary of a TLS skbuff is completely
>>>> decoupled from the TCP skbuff (in theory, at least).
>>>>
>>>> This can result in TLS skbuff stalls when waiting for the remainder
>>>> of the TCP data stream to arrive.
>>>
>>> An entire TLS Record has to be present before it can be decrypted and
>>> passed to the socket consumer. Is that what you mean?
>>>
>> Yes. And that makes ->read_sock() a bit pointless (from my POV); looking
>> at the network stack the skbuffs are assembled/decoded by the TLS layer,
>> only to be converted into skbuffs again for ->read_sock().
>> Which seems a bit pointless.
>
> Is this true only of the software kTLS implementation? For
> offload, I'd think the decrypted records are placed directly
> in the skbs that are handed to consumers.
>
>
>> But the main point with ->read_sock() is that we can only process
>> skbuffs, ie we have to wait for the TLS layer to assemble the entire
>> record.
>> With recvmsg() we at least have a theoretical choice of returning
>> a short read, allowing the code to do something clever in the meantime.
>
> Do you mean that read_sock is always synchronous? Perhaps
> judicious construction of the consumer can help there; but
> I expect that plain TCP has similar pathologies during, for
> instance, a network partition.
>
>
>> Mind you, that is all conjecture. Might well be that ->read_sock()
>> is beneficial for TLS, too. Or, at least, doesn't do any harm.
>> It's just that I don't trust the author of the TLS read_sock()
>> implementation; his network stack knowledge isn't _that_ great.
>
> Time to get out our measuring sticks, I suppose.
>
>
>> (Problem is that my performance measurements always ran into
>> some obnoxious occasional stall, rendering the entire measurement
>> pretty worthless. So I couldn't tell whether the recvmsg()
>> implementation delivers a benefit or not)
>
> Generally speaking (ie, hand-wave mode) that kind of stall arises
> because the consumer expects it will get another data_ready and the
> network layer does not conform with the implementer's expectations.
>
Naa. Tested that one. I've seen stalls affecting _all_ queues at
the same time, in regular intervals spaced with the power of 2.
Current reasoning is HW-based LLDP frames handling.

> So I like the read_sock_cmsg design here because with recvmsg,
> the CMSG payload can appear in the consumer's data buffer. For
> direct I/O, that's bad. We also really don't want to place the
> TLS Alerts in page cache pages. With read_sock_cmsg, the CMSG
> payload can be efficiently directed to an independent buffer
> that is never visible to user space.
>
> And I prefer having a single receive path for both non-TLS and
> TLS processing. That's so much less code to deal with.
>
> This approach seems to me to be more architecturally sound.
> We just have to get over the implementation bumps IMO. I think
> we can certainly examine ->read_sock with software kTLS to see
> if there's room for improvement.
>
And that, I guess, is the real argument. We've seen with the
recent TLS alert CVE that posting TLS messages in the recvmsg
payload is a _bad_ idea, as this puts far too many restrictions
on the format of the message.

So you see me convinced. I'll give it a go to convert nvme-tcp.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@suse.de +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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