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Message-ID: <20240424173114.035ddd7b@gato.skoll.ca>
Date: Wed, 24 Apr 2024 17:31:14 -0400
From: Dianne Skoll <dianne@...ll.ca>
To: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby
<jirislaby@...nel.org>
Subject: N_HDLC line discipline: Race condition
Hi,
I'm reposting here (originally emailed GregKH and Jiri Slaby directly
as they are listed as tty maintainers) as well as responding to a
reply from Greg.
Original mail:
Some people have been reporting bugs using synchronous PPP with the
rp-pppoe user-space program. See for example
https://github.com/dfskoll/rp-pppoe/issues/32
I've narrowed this down to the N_HDLC line discipline sometimes
concatenating two packets on the write side into a single read on the
other side. I have attached a proof-of-concept program illustrating
the problem.
If you run:
/test_n_hdlc
with no arguments, then the program makes two writes in quick succession
to the tty followed by a read. It generally only takes one or two tries
on my computer before I see both writes being combined in a single read.
On the other hand, if you run:
/test_n_hdlc foo
with a single argument, then the program sleeps for 0.1s between writes,
and I never see them being combined in a single read, even after 20 tries.
I'm running mainline kernel 6.8.7 on amd64; the processor model name
per cpuinfo is: AMD Ryzen Threadripper 3970X 32-Core Processor
Regards,
Dianne.
Greg's reply with my responses inline:
> What is wrong with that? Does the N_HDLC line discipline somewhere
> state that this is not possible to happen? Normal write combining
> happens for other tty ldiscs. Userspace should be able to handle
> this, unless again, the N_HDLC ldisc somehow says that this should
> never happen.
> Ok, I looked at the comments at the top of the the ldisc, and it says
> this should not happen, so something is odd, I agree.
Right. It's done that way so a PPP 'pty' helper always gets exactly one
frame when it reads from the tty file dscriptor.
> Did this change recently? Or has this always been the case? Meaning
> did something in the kernel change to cause this to break?
I haven't used this mode of PPP in over a decade, so I know it used to work,
but no longer does. I'm sorry I can't narrow down more precisely as to
when it stopped working.
> Also, please cc: the linux-serial list for tty issues, doing stuff in
> private is generally not a good idea.
OK, sorry about that... I misread the bug reporting instructions and
forgot to Cc the lists.
Regards,
Dianne.
Download attachment "test_n_hdlc.tar.gz" of type "application/gzip" (1085 bytes)
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