Re: [PATCH] n_tty: Remove LINEMODE support

From: Howard Chu
Date: Mon Jan 19 2015 - 14:43:39 EST

Peter Hurley wrote:
On 01/19/2015 11:36 AM, Howard Chu wrote:
Peter Hurley wrote:
Thanks, Howard.

[ adding Ted too ]

On 01/19/2015 07:46 AM, Howard Chu wrote:
Peter Hurley wrote:
On 01/18/2015 05:45 PM, Howard Chu wrote:
Peter Hurley wrote:
Commit 26df6d13406d1 ("tty: Add EXTPROC support for LINEMODE") added
the undocumented EXTPROC input processing mode, which ignores the ICANON
setting and forces pty slave input to be processed in non-canonical
mode.

What's the motivation to remove this code, rather than improve it if it
needs fixing? It has been removed from the Linux kernel at least once
before already, and that was a mistake back then too.

It is a significant maintenance burden, and I have concerns about the
level of support it's receiving. Here's some outstanding issues:

1. No man page documentation. At a minimum, tty_ioctl(4) and termios(3)
need the userspace visible definitions and behavior documented. Better
would be a LINEMODE (7) description of how this implementation works
wrt supporting RFC 1116.

That can be added easily enough.

Is this you signing up? :)

Sure. I suppose the obvious place to add it is near the TIOCPKT doc in tty_ioctl(4). But, about your further comment:

Thanks.

Which brings up another point: only a pty master should be able to set EXTPROC
mode. Right now, any tty can be set to EXTPROC and the pty slave can even
accidentally unset it. This argues for a new pty ioctl() to set EXTPROC in
termios. pty_set_termios() can silently merge the bit.

Historically EXTPROC could also be set on regular ttys, for lines that were hooked up to special terminal interfaces. E.g. X.29 PADs that did their own input processing. I'm not sure how relevant this is now, but this is the reason EXTPROC is not a pty-specific ioctl.

How does the remote end find out when non-canon mode is selected?
And who does the EXTPROC setting, because if it's the program that opened the
tty, then it can just select raw mode instead.

There's a few significant differences in semantics between raw mode and EXTPROC mode. The point of EXTPROC is to maintain the tty driver state as if cooked mode were still in effect. One obvious difference is that VMIN/VTIME are overlaid on VEOL/VEOF in raw mode, so the two are not directly interchangeable.

The fact that EXTPROC can be manually unset is by design. Quoting from the original again:

stty.diff:
This file contains the changes needed for the stty(1) program
to report on the current status of the TS_EXTPROC bit. It also
allows the user to turn on/off the TS_EXTPROC bit. This is useful
because it allows the user to say "stty -extproc", and the
LINEMODE option will be automatically disabled, and saying "stty
extproc" will re-enable the LINEMODE option.


6. EXTPROC still does some input processing on the server. For example,
7-bit mode (ISTRIP), tolower mode (IUCLC) and processing while
closing; if input processing is being done on the local/client side,
why the extra work here?

That's defensive, on the assumption that something else might break if e.g. the tty expected only 7-bit input but 8-bit characters were sent to it.

Ok, is that because RFC 1116 doesn't specify ISTRIP and IUCLC handling so
the server can't be sure the client did it? If so, that should be documented
so that refactors don't remove that handling.

Could you get back to me about this, as well?

The telnet protocol (RFC854) defines a Network Virtual Terminal (NVT) as using 7-bit USASCII in an 8-bit field. As such, it expects the client to be able to generate both upper and lower case itself, so there's no analogue to IUCLC, and there would be no reason to use ISTRIP.

RFC5198 updates the protocols to use UTF8. So again, it assumes full octets are being transmitted.

Perhaps we can drop these special cases from the driver.

Regular ptys dump input while closing so this should too.

7. This needs a reference userspace implementation which for the moment
could double as regression testing. A library with unit tests would
be ideal.

telnet/telnetd can probably used as a starting point for this.

Except telnet is unmaintained except by each distro. Your patches don't apply
to my distro telnet; they had been applied but are now ifdef'd out. I messed
around with rebuilding it but never got telnetd to actually set EXTPROC.

My point is there is no straightforward way to test this, and that's a problem.

In fact, this is my main concern: no matter how useful this might be, it's
pointless if the complementary userspace puzzle piece is unsupported.
I know you have written patches but that's not the same as support; for example
your OpenSSH fork is 4 years old, which is a non-starter because I'm sure
there's known exploits which have been fixed in those 4 years.

Why doesn't Ubuntu/Debian telnet even compile for line mode support? Did the
package maintainer shut it off because it was causing bug reports?

I don't know. Looking at the package history it appears they have never enabled it. https://launchpad.net/ubuntu/+source/netkit-telnet/+changelog

I also don't know where the actual VCS repo is for this code.

There isn't one. The base code is from downloaded tarball.

I don't know what distro you use (or if you even do), but for Ubuntu/Debian
the base package information is here https://packages.debian.org/wheezy/telnetd
including maintainer email, etc.

Typically, to get changes merged requires making a patch(es) against the package
source and sending/uploading those.

This was already done, and the package maintainer said he was going to integrate it. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=585527

--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
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