Date:	Mon, 21 Jul 2014 15:57:03 +0200
From:	Petr Mládek <pmladek@...e.cz>
To:	Alex Elder <elder@...aro.org>
Cc:	akpm@...ux-foundation.org, kay@...y.org, bp@...e.de,
	john.stultz@...aro.org, jack@...e.cz, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] printk: insert newline for truncated records

On Mon 2014-07-21 07:32:18, Alex Elder wrote:
> On 07/21/2014 06:57 AM, Petr Mládek wrote:
> > On Fri 2014-07-18 16:28:04, Alex Elder wrote:
> >> If a log record has LOG_PREFIX set, its predecessor record should be
> >> terminated if it was marked LOG_CONT.
> >>
> >> In devkmsg_read(), this condition was being ignored, which would
> >> lead to such records showing up combined when reading /dev/kmsg.
> >> Fix this oversight.
> >>
> >> We should similarly insert a newline in msg_print_text() in this
> >> case, to avoid formatted records getting merged.
> >>
> >> Suggested-by: Petr Mládek <pmladek@...e.cz>
> >> Signed-off-by: Alex Elder <elder@...aro.org>
> >> ---
> >>  kernel/printk/printk.c | 18 +++++++++++++++---
> >>  1 file changed, 15 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index e9f0632..a5ad81c 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -575,6 +575,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
> >>  	char cont;
> >>  	size_t len;
> >>  	ssize_t ret;
> >> +	bool insert_newline;
> >>  
> >>  	if (!user)
> >>  		return -EBADF;
> >> @@ -626,7 +627,10 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
> >>  	else
> >>  		cont = '-';
> >>  
> >> -	len = sprintf(user->buf, "%u,%llu,%llu,%c;",
> >> +	/* Insert a newline if the previous line was not terminated properly */
> >> +	insert_newline = (user->prev & LOG_CONT) && (msg->flags & LOG_PREFIX);
> >> +	len = sprintf(user->buf, "%s%u,%llu,%llu,%c;",
> >> +		      insert_newline ? "\n" : "",
> >>  		      (msg->facility << 3) | msg->level,
> >>  		      user->seq, ts_usec, cont);
> >>  	user->prev = msg->flags;
> >> @@ -999,10 +1003,12 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
> >>  {
> >>  	const char *text = log_text(msg);
> >>  	size_t text_size = msg->text_len;
> >> +	size_t len = 0;
> >> +	bool insert_newline;
> >>  	bool prefix = true;
> >>  	bool newline = true;
> >> -	size_t len = 0;
> >>  
> >> +	insert_newline = (prev & LOG_CONT) && (msg->flags & LOG_PREFIX);
> >>  	if ((prev & LOG_CONT) && !(msg->flags & LOG_PREFIX))
> >>  		prefix = false;
> >>  
> >> @@ -1023,9 +1029,13 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
> >>  
> >>  		if (buf) {
> >>  			if (print_prefix(msg, syslog, NULL) +
> >> -			    text_len + 1 >= size - len)
> >> +			    text_len + 2 >= size - len)
> > 
> > It counts the '\n' even when it is not used.
> > I think that it is even wrong that it calculates prefix when it is not used.
> 
> That's true, and I have yet another un-posted patch that
> addresses this problem (well the second one).  I am not
> going to fix this problem in this patch, but the fix is
> coming.
> 
> Now that you're looking at the code I'm touching, you're
> seeing the same things I did...
> 
> I think I'll start posting that series later today or
> tomorrow.   I just hate to get too far ahead of myself.

I suggest to always wait at least 24 hours before sending another
version. I think that it is very hard for others to follow if there
are too many versions in the wild and the code is too changing.
This is why you get comments basically only from me.

Also a night usually helps to sort ideas and go the right direction.
In fact, I was too fast myself as well. I am going to comment only one
version per-day from now on :-)


> >>  				break;
> >>  
> >> +			if (insert_newline) {
> >> +				insert_newline = false;
> >> +				buf[len++] = '\n';
> >> +			}
> >>  			if (prefix)
> >>  				len += print_prefix(msg, syslog, buf + len);
> >>  			memcpy(buf + len, text, text_len);
> >> @@ -1034,6 +1044,8 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
> >>  				buf[len++] = '\n';
> >>  		} else {
> >>  			/* SYSLOG_ACTION_* buffer size only calculation */
> >> +			if (insert_newline)
> >> +				len++;
> > 
> > You forgot "insert_newline = false" here.
> 
> Yes you're right.  It's good that you're reviewing this.
> (The patches I have not yet posted affect this area of
> code, and should eliminate this block...)
> 
> >>  			if (prefix)
> >>  				len += print_prefix(msg, syslog, NULL);
> >>  			len += text_len;
> > 
> > It is just matter of personal style but I would suggest to do this
> > before the do-while cycle:
> > 
> > 	/* Force newline if the previous text was not properly finished */
> > 	if ((prev & LOG_CONT) && (msg->flags & LOG_PREFIX) && (len < size)) {
> > 		if (buf)
> > 			buf[len++] = '\n';
> > 		else
> > 			len++;
> > 	}
> > 
> > IMHO, it is more clear. The do-while cycle already is complex enough.
> 
> I agree with this.  It's a one-time thing and doesn't belong in
> the loop.  When you suggested inserting the newline I think I
> didn't think it through completely.  I will do this.
> 
> > BTW: This is relared to the first patch. I would either patch all
> > three locations in one patch or better split it into three patches.
> 
> I am keeping the first patch separate from this one.  I think
> the first is related (in that we improve readability by inserting
> some newlines) but it's really addressing a different problem.

You are right that the problem is different but it is quite similar.
In each case, I would keep the patches closer.


> Meanwhile, this patch is addressing essentially the same problem
> in two spots, so I'd like to keep these together rather than
> splitting it in two.

I do not have strong opinion here.


> I will move this patch earlier in the series, however, making
> it follow patch 1.

Sounds good.

Best Regards,
Petr
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