linux-kernel - Re: [PATCH net-next v8 1/2] net/tls: support setting the maximum payload size

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Message-ID: <cd557c5b11b04da060f07d3849dc46e7b3625ed1.camel@gmail.com>
Date: Fri, 24 Oct 2025 12:11:11 +1000
From: Wilfred Mallawa <wilfred.opensource@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>
Cc: netdev@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, "David S .
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>,  Jonathan Corbet	 <corbet@....net>, Simon Horman
 <horms@...nel.org>, John Fastabend	 <john.fastabend@...il.com>, Shuah Khan
 <shuah@...nel.org>
Subject: Re: [PATCH net-next v8 1/2] net/tls: support setting the maximum
 payload size

On Thu, 2025-10-23 at 18:44 -0700, Jakub Kicinski wrote:
> On Wed, 22 Oct 2025 10:19:36 +1000 Wilfred Mallawa wrote:
> > +TLS_TX_MAX_PAYLOAD_LEN
> > +~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +Specifies the maximum size of the plaintext payload for
> > transmitted TLS records.
> > +
> > +When this option is set, the kernel enforces the specified limit
> > on all outgoing
> > +TLS records. No plaintext fragment will exceed this size. This
> > option can be used
> > +to implement the TLS Record Size Limit extension [1].
> > +
> > +* For TLS 1.2, the value corresponds directly to the record size
> > limit.
> > +* For TLS 1.3, the value should be set to record_size_limit - 1,
> > since
> > +  the record size limit includes one additional byte for the
> > ContentType
> > +  field.
> > +
> > +The valid range for this option is 64 to 16384 bytes for TLS 1.2,
> > and 63 to
> > +16384 bytes for TLS 1.3. The lower minimum for TLS 1.3 accounts
> > for the
> > +extra byte used by the ContentType field.
> > +
> > +[1] https://datatracker.ietf.org/doc/html/rfc8449
> 
> Sorry for not paying attention to the last few revisions.
> 
> So we decided to go with the non-RFC definition of the sockopt
> parameter? Is there a reason for that? I like how the "per RFC"
> behavior shifts any blame away from us :)
> 

Hey Jakub,

We've made the change from record_size_limit to max_payload_len mainly
because:

In the previous record_size_limit approach for TLS 1.3, we need to
account for the ContentType byte. Which complicates get/setsockopt()
and tls_get_info(), where in setsockopt() for TLS 1.3 we need to
subtract 1 to the user provided value and in getsockopt() we need add 1
to keep the symmetry between the two (similarly in tls_get_info()). The
underlying assumption was that userspace passes up directly what the
endpoint specified as the record_size_limit.

With this approach we don't need to worry about it and we can pass the
responsibility to user-space as documented, which I think makes the
kernel code simpler.

> > +	err = nla_put_u16(skb, TLS_INFO_TX_MAX_PAYLOAD_LEN,
> > +			  ctx->tx_max_payload_len);
> > +
> 
> nit: unnecessary empty line

Ah! will fixup for V9

Regards,
Wilfred

> 
> > +	if (err)
> > +		goto nla_failure;