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[ Upstream commit31642e7089] Simon Kapadia reported the following issue: <quote> The Online Amateur Radio Community (OARC) has recently been experimenting with building a nationwide packet network in the UK. As part of our experimentation, we have been testing out packet on 300bps HF, and playing with net/rom. For HF packet at this baud rate you really need to make sure that your MTU is relatively low; AX.25 suggests a PACLEN of 60, and a net/rom PACLEN of 40 to go with that. However the Linux net/rom support didn't work with a low PACLEN; the mkiss module would truncate packets if you set the PACLEN below about 200 or so, e.g.: Apr 19 14:00:51 radio kernel: [12985.747310] mkiss: ax1: truncating oversized transmit packet! This didn't make any sense to me (if the packets are smaller why would they be truncated?) so I started investigating. I looked at the packets using ethereal, and found that many were just huge compared to what I would expect. A simple net/rom connection request packet had the request and then a bunch of what appeared to be random data following it: </quote> Simon provided a patch that I slightly revised: Not only we must not use skb_tailroom(), we also do not want to count NR_NETWORK_LEN twice. Fixes:1da177e4c3("Linux-2.6.12-rc2") Co-Developed-by: Simon Kapadia <szymon@kapadia.pl> Signed-off-by: Simon Kapadia <szymon@kapadia.pl> Signed-off-by: Eric Dumazet <edumazet@google.com> Tested-by: Simon Kapadia <szymon@kapadia.pl> Reviewed-by: Simon Horman <simon.horman@corigine.com> Link: https://lore.kernel.org/r/20230524141456.1045467-1-edumazet@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.