Subject: Re: [PATCH v2 3/3] vstatus: Display an informational message when the VSTATUS character is pressed or TIOCSTAT ioctl is called.

From: Greg KH <[email-masked]>
Date: Sun, 6 Feb 2022 18:16:07 +0100

On Sun, Feb 06, 2022 at 07:48:54AM -0800, Walt Drummond wrote:
> When triggered by pressing the VSTATUS key or calling the TIOCSTAT
> ioctl, the n_tty line discipline will display a message on the user's
> tty that provides basic information about the system and an
> 'interesting' process in the current foreground process group, eg:
> 
>   load: 0.58  cmd: sleep 744474 [sleeping] 0.36r 0.00u 0.00s 0% 772k
> 
> The status message provides:
>  - System load average
>  - Command name and process id (from the perspective of the session)
>  - Scheduler state
>  - Total wall-clock run time
>  - User space run time
>  - System space run time
>  - Percentage of on-cpu time
>  - Resident set size

This should be documented somewhere, and not buried in a changelog text
like this.  Can you also add this information somewhere in the
Documentation/ directory so that people have a hint as to what is going
on here?

> The message is only displayed when the tty has the VSTATUS character
> set, the local flags ICANON and IEXTEN are enabled and NOKERNINFO is
> disabled; it is always displayed when TIOCSTAT is called regardless of
> tty settings.
> 
> Signed-off-by: Walt Drummond <[email-masked]>
> ---
>  drivers/tty/Makefile       |   2 +-
>  drivers/tty/n_tty.c        |  34 +++++++
>  drivers/tty/n_tty_status.c | 181 +++++++++++++++++++++++++++++++++++++
>  drivers/tty/tty_io.c       |   2 +-
>  include/linux/tty.h        |   5 +
>  5 files changed, 222 insertions(+), 2 deletions(-)
>  create mode 100644 drivers/tty/n_tty_status.c

Also, any chance for a test to be added so that we can ensure that this
doesn't change over time in ways that confuse/break people?

Is this now a new user/kernel api format that we must preserve for
forever?  Can we add/remove items over time that make sense or are
programs (not just people), going to parse this?

> 
> diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
> index a2bd75fbaaa4..3539d7ab77e5 100644
> --- a/drivers/tty/Makefile
> +++ b/drivers/tty/Makefile
> @@ -2,7 +2,7 @@
>  obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \
>  				   tty_buffer.o tty_port.o tty_mutex.o \
>  				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o \
> -				   n_null.o
> +				   n_null.o n_tty_status.o
>  obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
>  obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
>  obj-$(CONFIG_AUDIT)		+= tty_audit.o
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 64a058a4c63b..fd70efc333d7 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -80,6 +80,7 @@
>  #define ECHO_BLOCK		256
>  #define ECHO_DISCARD_WATERMARK	N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)
>  
> +#define STATUS_LINE_LEN 160   /* tty status line will truncate at this length */

Tabs please.

>  
>  #undef N_TTY_TRACE
>  #ifdef N_TTY_TRACE
> @@ -127,6 +128,8 @@ struct n_tty_data {
>  	struct mutex output_lock;
>  };
>  
> +static void n_tty_status(struct tty_struct *tty);
> +
>  #define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1))
>  
>  static inline size_t read_cnt(struct n_tty_data *ldata)
> @@ -1334,6 +1337,11 @@ static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
>  			commit_echoes(tty);
>  			return;
>  		}
> +		if (c == STATUS_CHAR(tty) && L_IEXTEN(tty)) {
> +			if (!L_NOKERNINFO(tty))
> +				n_tty_status(tty);
> +			return;
> +		}
>  		if (c == '\n') {
>  			if (L_ECHO(tty) || L_ECHONL(tty)) {
>  				echo_char_raw('\n', ldata);
> @@ -1763,6 +1771,7 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
>  			set_bit(EOF_CHAR(tty), ldata->char_map);
>  			set_bit('\n', ldata->char_map);
>  			set_bit(EOL_CHAR(tty), ldata->char_map);
> +			set_bit(STATUS_CHAR(tty), ldata->char_map);
>  			if (L_IEXTEN(tty)) {
>  				set_bit(WERASE_CHAR(tty), ldata->char_map);
>  				set_bit(LNEXT_CHAR(tty), ldata->char_map);
> @@ -2413,6 +2422,26 @@ static unsigned long inq_canon(struct n_tty_data *ldata)
>  	return nr;
>  }
>  
> +static void n_tty_status(struct tty_struct *tty)
> +{
> +	struct n_tty_data *ldata = tty->disc_data;
> +	char *msg;
> +	size_t len;
> +
> +	msg = kzalloc(STATUS_LINE_LEN, GFP_KERNEL);

Please check for memory failures.

> +
> +	if (ldata->column != 0) {
> +		*msg = '\n';
> +		len = n_tty_get_status(tty, msg + 1, STATUS_LINE_LEN - 1);
> +	} else {
> +		len = n_tty_get_status(tty, msg, STATUS_LINE_LEN);
> +	}
> +
> +	do_n_tty_write(tty, NULL, msg, len);
> +
> +	kfree(msg);
> +}
> +
>  static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
>  		       unsigned int cmd, unsigned long arg)
>  {
> @@ -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:
> +		down_read(&tty->termios_rwsem);
> +		n_tty_status(tty);
> +		up_read(&tty->termios_rwsem);
> +		return 0;
>  	default:
>  		return n_tty_ioctl_helper(tty, file, cmd, arg);
>  	}
> diff --git a/drivers/tty/n_tty_status.c b/drivers/tty/n_tty_status.c
> new file mode 100644
> index 000000000000..f0e053651368
> --- /dev/null
> +++ b/drivers/tty/n_tty_status.c
> @@ -0,0 +1,181 @@
> +// SPDX-License-Identifier: GPL-1.0+

We can not take GPL-1.0 code into the kernel anymore, sorry.  Please
consider using a sane license :)

> +/*
> + * n_tty_status.c --- implements VSTATUS and TIOCSTAT from BSD
> + *
> + * Display a basic status message containing information about the
> + * foreground process and system load on the users tty, triggered by
> + * the VSTATUS character or TIOCSTAT. Ex,
> + *
> + *   load: 14.11  cmd: tcsh 19623 [running] 185756.62r 88.00u 17.50s 0% 4260k
> + *
> + */
> +
> +#include <linux/tty.h>
> +#include <linux/mm.h>
> +#include <linux/sched/loadavg.h>
> +#include <linux/sched/mm.h>
> +
> +/* Convert nanoseconds into centiseconds */
> +static inline long ns_to_cs(long l)
> +{
> +	return l / (NSEC_PER_MSEC * 10);
> +
> +}

Unneded blank line.

> +
> +/* We want the pid from the context of session */
> +static inline pid_t __get_pid(struct task_struct *tsk, struct tty_struct *tty)
> +{
> +	struct pid_namespace *ns;
> +
> +	spin_lock_irq(&tty->ctrl.lock);
> +	ns = ns_of_pid(tty->ctrl.session);
> +	spin_unlock_irq(&tty->ctrl.lock);
> +
> +	return __task_pid_nr_ns(tsk, PIDTYPE_PID, ns);
> +}
> +
> +/* This is the same odd "bitmap" described in
> + * fs/proc/array.c:get_task_state().  Consistency with standard
> + * implementations of VSTATUS requires a different set of state
> + * names.
> + */
> +static const char * const task_state_name_array[] = {
> +	"running",
> +	"sleeping",
> +	"disk sleep",
> +	"stopped",
> +	"tracing stop",
> +	"dead",
> +	"zombie",
> +	"parked",
> +	"idle",
> +};

How often is this going to get out-of-sync?  Should we use a real
enumerated type here?  Put the string somewhere else to keep this only
in one place?

> +
> +static inline const char *get_task_state_name(struct task_struct *tsk)
> +{
> +	BUILD_BUG_ON(1 + ilog2(TASK_REPORT_MAX) != ARRAY_SIZE(task_state_name_array));

What is this protecting from?  What is going to change that requires
this to be increased?

> +	return task_state_name_array[task_state_index(tsk)];
> +}
> +
> +static inline struct task_struct *compare(struct task_struct *new,
> +					  struct task_struct *old)
> +{
> +	unsigned int ostate, nstate;
> +
> +	if (old == NULL)
> +		return new;
> +
> +	ostate = task_state_index(old);
> +	nstate = task_state_index(new);
> +
> +	if (ostate == nstate) {
> +		if (old->start_time > new->start_time)
> +			return old;
> +		return new;
> +	}
> +
> +	if (ostate < nstate)
> +		return old;
> +
> +	return new;
> +}
> +
> +static struct task_struct *pick_process(struct tty_struct *tty)
> +{
> +	struct task_struct *new, *winner = NULL;
> +
> +	read_lock(&tasklist_lock);
> +	spin_lock_irq(&tty->ctrl.lock);
> +
> +	do_each_pid_task(tty->ctrl.pgrp, PIDTYPE_PGID, new) {
> +		winner = compare(new, winner);
> +	} while_each_pid_task(tty->ctrl.pgrp, PIDTYPE_PGID, new);
> +
> +	spin_unlock_irq(&tty->ctrl.lock);
> +
> +	if (winner)
> +		winner = get_task_struct(winner);
> +
> +	read_unlock(&tasklist_lock);
> +
> +	return winner;
> +}

What are these two functions trying to do?  A comment would be nice to
give us a hint as I am guessing I am going to have to maintain this for
forever :)

> +
> +size_t n_tty_get_status(struct tty_struct *tty, char *msg, size_t msglen)
> +{
> +	struct task_struct *p;
> +	struct mm_struct *mm;
> +	struct rusage rusage;
> +	unsigned long loadavg[3];
> +	uint64_t pcpu, cputime, wallclock;
> +	struct timespec64 utime, stime, rtime;
> +	char tname[TASK_COMM_LEN];
> +	unsigned int pid;
> +	char *state;
> +	unsigned long rss = 0;
> +	size_t len = 0;
> +
> +	get_avenrun(loadavg, FIXED_1/200, 0);

Why 200?

> +	len = scnprintf(msg + len, msglen - len, "load: %lu.%02lu  ",
> +			LOAD_INT(loadavg[0]), LOAD_FRAC(loadavg[0]));
> +
> +	if (tty->ctrl.session == NULL) {
> +		len += scnprintf(msg + len, msglen - len,
> +				 "not a controlling terminal\n");
> +		goto out;
> +	}
> +
> +	if (tty->ctrl.pgrp == NULL) {
> +		len += scnprintf(msg + len, msglen - len,
> +				 "no foreground process group\n");
> +		goto out;
> +	}
> +
> +	/* Note that if p is refcounted */
> +	p = pick_process(tty);
> +	if (p == NULL) {
> +		len += scnprintf(msg + len, msglen - len,
> +				 "empty foreground process group\n");
> +		goto out;
> +	}
> +
> +	mm = get_task_mm(p);
> +	if (mm) {
> +		rss = get_mm_rss(mm) * PAGE_SIZE / 1024;
> +		mmput(mm);
> +	}
> +	get_task_comm(tname, p);
> +	getrusage(p, RUSAGE_BOTH, &rusage);
> +	pid = __get_pid(p, tty);
> +	state = (char *) get_task_state_name(p);
> +	wallclock = ktime_get_ns() - p->start_time;
> +	put_task_struct(p);
> +
> +	/* After this point, any of the information we have on p might
> +	 * become stale.  It's OK if the status message is a little bit
> +	 * lossy.
> +	 */
> +
> +	utime.tv_sec = rusage.ru_utime.tv_sec;
> +	utime.tv_nsec = rusage.ru_utime.tv_usec * NSEC_PER_USEC;
> +	stime.tv_sec = rusage.ru_stime.tv_sec;
> +	stime.tv_nsec = rusage.ru_stime.tv_usec * NSEC_PER_USEC;
> +	rtime = ns_to_timespec64(wallclock);
> +
> +	cputime = timespec64_to_ns(&utime) + timespec64_to_ns(&stime);
> +	pcpu = div64_u64(cputime * 100, wallclock);
> +
> +	len += scnprintf(msg + len, msglen - len,
> +			 /* task, PID, task state */
> +			 "cmd: %s %d [%s] "
> +			 /* rtime,    utime,      stime,      %cpu   rss */
> +			 "%llu.%02lur %llu.%02luu %llu.%02lus %llu%% %luk\n",
> +			 tname,	pid, state,
> +			 rtime.tv_sec, ns_to_cs(rtime.tv_nsec),
> +			 utime.tv_sec, ns_to_cs(utime.tv_nsec),
> +			 stime.tv_sec, ns_to_cs(stime.tv_nsec),
> +			 pcpu, rss);
> +
> +out:
> +	return len;
> +}
> 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,
>  	.c_cc = INIT_C_CC,
>  	.c_ispeed = 38400,
>  	.c_ospeed = 38400,
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index cbe5d535a69d..2e483708608c 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -49,6 +49,7 @@
>  #define WERASE_CHAR(tty) ((tty)->termios.c_cc[VWERASE])
>  #define LNEXT_CHAR(tty)	((tty)->termios.c_cc[VLNEXT])
>  #define EOL2_CHAR(tty) ((tty)->termios.c_cc[VEOL2])
> +#define STATUS_CHAR(tty) ((tty)->termios.c_cc[VSTATUS])
>  
>  #define _I_FLAG(tty, f)	((tty)->termios.c_iflag & (f))
>  #define _O_FLAG(tty, f)	((tty)->termios.c_oflag & (f))
> @@ -114,6 +115,7 @@
>  #define L_PENDIN(tty)	_L_FLAG((tty), PENDIN)
>  #define L_IEXTEN(tty)	_L_FLAG((tty), IEXTEN)
>  #define L_EXTPROC(tty)	_L_FLAG((tty), EXTPROC)
> +#define L_NOKERNINFO(tty) _L_FLAG((tty), NOKERNINFO)
>  
>  struct device;
>  struct signal_struct;
> @@ -389,6 +391,9 @@ extern void __init n_tty_init(void);
>  static inline void n_tty_init(void) { }
>  #endif
>  
> +/* n_tty_status.c */
> +size_t n_tty_get_status(struct tty_struct *tty, char *msg, size_t msglen);

No need for this to be in include/linux/tty.h, put it in the .h file in
drivers/tty/ please.

thanks,

greg k-h