linux-kernel - Re: [PATCH 0/7] TTY Keyboard Status Request

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Date:   Sun, 9 Jun 2019 23:56:10 +0300
From:   Arseny Maslennikov <ar@...msu.ru>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        "Vladimir D . Seleznev" <vseleznv@...linux.org>
Subject: Re: [PATCH 0/7] TTY Keyboard Status Request

On Sun, Jun 09, 2019 at 09:51:32PM +0200, Pavel Machek wrote:
> Hi!
> 
> > > > This patch series introduces TTY keyboard status request, a feature of
> > > > the n_tty line discipline that reserves a character in struct termios
> > > > (^T by default) and reacts to it by printing a short informational line
> > > > to the terminal and sending a Unix signal to the tty's foreground
> > > > process group. The processes may, in response to the signal, output a
> > > > textual description of what they're doing.
> > > > 
> > > > The feature has been present in a similar form at least in
> > > > Free/Open/NetBSD; it would be nice to have something like this in Linux
> > > > as well. There is an LKML thread[1] where users have previously
> > > > expressed the rationale for this.
> > > > 
> > > > The current implementation does not break existing kernel API in any
> > > > way, since, fortunately, all the architectures supported by the kernel
> > > > happen to have at least 1 free byte in the termios control character
> > > > array.
> > > 
> > > I like the idea... I was often wondering "how long will this dd take". (And in
> > > case of dd, SIGUSR1 does the job).
> > > 
> > > I assume this will off by default, so that applications using ^T today will not
> > > get surprise signals?
> > 
> > If any of isig, icanon and iexten is disabled on the tty, the signal is
> > not sent.
> 
> As expected.
> 
> > Any application that wants to handle raw terminal input events itself,
> > e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn
> > off the tty's cooked mode, i.e. at least icanon. This means those
> > applications are unaffected.
> 
> Agreed, those are unaffected.
> 
> But if I have an application doing read() from console (without
> manipulating tty), am I going to get surprise signal when user types
> ^T?
> 
> 	     	      	      	     	      	     	       	     Pavel

As of now, that application will indeed receive a signal that is
guaranteed to be ignored by default.

This is similar to SIGWINCH, which is default-ignored as well: if the
terminal width/height changes (like when a terminal emulator window is
resized), its foreground pgrp gets a surprise signal as well, and the
processes that don't care about WINCH (and thus have default
disposition) do not get confused.
E.g. 'strace cat' demonstrates this quite clearly.


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