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

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Message-ID: <d52862c17afab501453c3fbb5167f78d24b1f807.camel@gmail.com>
Date: Sun, 26 Oct 2025 18:50:19 +1000
From: Wilfred Mallawa <wilfred.opensource@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Sabrina Dubroca <sd@...asysnail.net>, 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 Fri, 2025-10-24 at 16:33 -0700, Jakub Kicinski wrote:
> On Fri, 24 Oct 2025 12:11:11 +1000 Wilfred Mallawa wrote:
> > 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.
> 
> But we haven't managed to avoid that completely:
> 
> +	if (value < TLS_MIN_RECORD_SIZE_LIM - (tls_13 ? 1 : 0) ||
> 
> I understand the motivation, the kernel code is indeed simpler.
> 
> Last night I read the RFC and then this patch, and it took me like
> 10min to get all of it straight in my head. Maybe I was tried but
> I feel like the user space developers will judge us harshly for 
> the current uAPI.

I am open to reverting this to `record_size_limit` in that case. I
think the only trade off is just a bit more complexity in the kernel
side for the additional checks. Does that sound good to you
Jakub/Sabrina?

Regards,
Wilfred