linux-kernel - Re: [PATCH] printk: inject caller information into the body of message

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Date:   Thu, 13 Sep 2018 23:28:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        penguin-kernel@...ove.sakura.ne.jp,
        kbuild test robot <fengguang.wu@...el.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of
 message

On (09/13/18 14:26), Petr Mladek wrote:
> > +
> > +int vpr_line(struct pr_line *pl, const char *fmt, va_list args)
> > +{
> > +	struct seq_buf *s = &pl->sb;
> > +	int ret, len;
> > +
> > +	if (fmt[0] == '\n') {
> > +		pr_line_flush(pl);
> > +		return 0;
> > +	}
> 
> You would need to check if fmt[1] == '\0'. But then you would need
> to be careful about a possible buffer overflow. I would personally
> avoid this optimization.

Good call. It was a fast path for pr_cont("\n").
But it made me wondering and I did some grepping

arch/m68k/kernel/traps.c:                               pr_cont("\n       ");
arch/m68k/kernel/traps.c:                       pr_cont("\n       ");
kernel/trace/ftrace.c:          pr_cont("\n expected tramp: %lx\n", ip);

Lovely.
It will take us some time.

> > +	ret = seq_buf_vprintf(s, fmt, args);
> > +
> > +	len = seq_buf_used(s);
> > +	if (len && s->buffer[len - 1] == '\n')
> > +		pr_line_flush(pl);
> 
> This would cause that pr_line_flush() won't be strictly needed.
> Also it would encourage people to use this feature a more
> complicated way (for more lines). Do we really want this?

Not that I see any problems with pr_line_flush(). But can drop it, sure.
pr_line() is a replacement for pr_cont() and as such it's not for multi-line
buffering.

	-ss