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:

quoted

@@ -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.

quoted

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?