Re: [PATCH] vt: detect and ignore OSC codes.

Adam Borowski
2014-02-16T03:01:07Z

On Thu, Feb 13, 2014 at 10:39:12AM -0800, Greg Kroah-Hartman wrote:
> On Wed, Jan 15, 2014 at 07:21:04AM +0100, Adam Borowski wrote:
> > These can be used to send commands consisting of an arbitrary string to the
> > terminal, most often used to set a terminal's window title or to redefine
> > the colour palette. Our console doesn't use OSC, unlike everything else,
> > which can lead to junk being displayed if a process sends such a code
> > unconditionally.
> >
> > Not following Ecma-48, this commit recognizes 7-bit forms (ESC ] ... 0x07,
> > ESC ] .. ESC \) but not 8-bit (0x9D ... 0x9C).
> >
> > Signed-off-by: Adam Borowski <kilobyte@angband.pl>
>
> Where is this documented?

It's a mix of Ecma-48 and undocumented practice. Ecma-48 says:

# 8.3.89
# OSC - OPERATING SYSTEM COMMAND
# Notation: (C1)
# Representation: 0x9D or ESC 0x5D (])
#
# OSC is used as the opening delimiter of a control string for operating
# system use. The command string following may consist of a sequence of
# bit combinations in the range 0x08 to 0x0D and 0x20 to 0x7E. The
# control string is closed by the terminating delimiter STRING TERMINATOR
# (ST). The interpretation of the command string depends on the relevant
# operating system.

# 8.3.143
# ST - STRING TERMINATOR
# Notation: (C1)
# Representation: 0x9C or ESC 0x5C (\)
#
# ST is used as the closing delimiter of a control string opened by
# APPLICATION PROGRAM COMMAND (APC), DEVICE CONTROL STRING (DCS),
# OPERATING SYSTEM COMMAND (OSC), PRIVACY MESSAGE (PM), or START OF STRING
# (SOS).

... which doesn't define the behaviour for characters 0x00..0x07, 0x0E..0x1F
or 0x7F..0xFF. Somehow, using 0x07 for termination became a widespread
idiom, used more often than proper ESC \. For this reason, implementations
I know all recognize 0x07 as a terminator. The behaviour for other
characters differs, ie, is truly undefined.

As, unlike what Ecma-48 says, using 8-bit characters in a window title is a
reasonable thing to do, I'd allow 0x80..0xFF as non-terminators. I have no
idea what to do with remaining control characters: the current patch allows
them to be interpreted, as it's usually the case for control characters
inside terminal codes. I did not research other implementation here.

I did not recognize 0x9C as ST for two reasons: 1. it'd break non-ASCII
characters that happen to include this byte, and 2. Linux already fails to
recognize 8-bit control codes (with one exception: 0x9B stands for ESC [).

Should I put the above explanation somewhere? As a comment? In the commit
message? Or does it need to be elaborated even further?

> > ---
> > drivers/tty/vt/vt.c | 14 +++++++++++---
> > 1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index 61b1137..0377c52 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -1590,7 +1590,7 @@ static void restore_cur(struct vc_data *vc)
> >
> > enum { ESnormal, ESesc, ESsquare, ESgetpars, ESgotpars, ESfunckey,
> > EShash, ESsetG0, ESsetG1, ESpercent, ESignore, ESnonstd,
> > - ESpalette };
> > + ESpalette, ESosc };
> >
> > /* console_lock is held (except via vc_init()) */
> > static void reset_terminal(struct vc_data *vc, int do_clear)
> > @@ -1650,11 +1650,15 @@ static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, int c)
> > * Control characters can be used in the _middle_
> > * of an escape sequence.
> > */
> > + if (vc->vc_state == ESosc && c>=8 && c<=13) /* ... except for OSC */
> > + return;
> > switch (c) {
> > case 0:
> > return;
> > case 7:
> > - if (vc->vc_bell_duration)
> > + if (vc->vc_state == ESosc)
> > + vc->vc_state = ESnormal;
> > + else if (vc->vc_bell_duration)
> > kd_mksound(vc->vc_bell_pitch, vc->vc_bell_duration);
> > return;
> > case 8:
> > @@ -1765,7 +1769,9 @@ static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, int c)
> > } else if (c=='R') { /* reset palette */
> > reset_palette(vc);
> > vc->vc_state = ESnormal;
> > - } else
> > + } else if (c>='0' && c<='9')
> > + vc->vc_state = ESosc;
> > + else
> > vc->vc_state = ESnormal;
> > return;
> > case ESpalette:
> > @@ -2023,6 +2029,8 @@ static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, int c)
> > return;
> > default:
> > vc->vc_state = ESnormal;
> > + case ESosc:
> > + return;
>
> Why below the default: case?

Just to shave a line and a return statement. From your objection, I guess
this goes against the coding standards, right?

Meow!
--
A tit a day keeps the vet away.