On Mon, Feb 07, 2022 at 12:25:21AM +0300, Arseny Maslennikov wrote:
> On Sun, Feb 06, 2022 at 07:48:54AM -0800, Walt Drummond wrote:
> > @@ -2430,6 +2459,11 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
> > retval = read_cnt(ldata);
> > up_write(&tty->termios_rwsem);
> > return put_user(retval, (unsigned int __user *) arg);
> > + case TIOCSTAT:
>
> Perhaps we want to guard this (example pseudocode follows):
>
> if (*our ldisc is not n_tty*)
> return an error like -ENOTTY;
>
> ...since kerninfo is useless for non-UI ttys, e. g. serial device
> drivers, and this ioctl could mess them up if this code path can be
> taken. (I have not verified this kind of breakage is possible.) Please
> see the complete rationale below, this paragraph is an illustrational
> note for it.
Oh wait, this *is* n_tty_ioctl(), so the ioctl is n_tty-specific. This
makes the case below even clearer.
I've been clumsy, sorry about that.
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index 6616d4a0d41d..f2f4f48ea502 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -125,7 +125,7 @@ struct ktermios tty_std_termios = { /* for the benefit of tty drivers */
> > .c_oflag = OPOST | ONLCR,
> > .c_cflag = B38400 | CS8 | CREAD | HUPCL,
> > .c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK |
> > - ECHOCTL | ECHOKE | IEXTEN,
> > + ECHOCTL | ECHOKE | IEXTEN | NOKERNINFO,
>
> Does this mean that nokerninfo is on by default? Do we have a reason to
> do that?
>
> As of this patch we require icanon and iexten to be set for the message
> to be composed and printed. An experiment shows PTY encapsulation
> programs like openssh turn off both those flags on the tty they run on
> before they take control (contrary to what has been said in LWN), so
> they are unimpacted.
>
> The termios(3) page from man-pages states:
> Raw mode
> cfmakeraw() sets the terminal to something like the "raw" mode
> of the old Version 7 terminal driver: input is available char‐
> acter by character, echoing is disabled, and all special pro‐
> cessing of terminal input and output characters is disabled.
> The terminal attributes are set as follows:
>
> termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
> | INLCR | IGNCR | ICRNL | IXON);
> termios_p->c_oflag &= ~OPOST;
> termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
> termios_p->c_cflag &= ~(CSIZE | PARENB);
> termios_p->c_cflag |= CS8;
>
> So any program which uses this API effectively turns off kerninfo as
> implemented here.
>
> There are 2 ways n_tty_status() can be called as of this patch: either
> from inside n_tty or via TIOCSTAT. The first path can't be taken on ttys
> whose ldisc is not N_TTY, ...
The second path is OK as well.
> Given all this, is there any other reason to enable nokerninfo (i. e.
> disable status message) by default?
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