linux-kernel - Re: [PATCH 2/2] tracing: Use trace_seq_used() and seq_buf

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Date:	Wed, 19 Nov 2014 08:48:00 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 2/2] tracing: Use trace_seq_used() and seq_buf_used()
 instead of len

On Wed, 19 Nov 2014 12:40:17 +0100
Petr Mladek <pmladek@...e.cz> wrote:

> > > 
> > > There is one more dangerous usage in trace_printk_seq(). It is on
> > > three lines there.
> > 
> > You totally confused me. What usage in trace_printk_seq(), and what
> > three lines?
> > 
> > In this patch, trace_printk_seq() looks like this:
> > 
> > int trace_print_seq(struct seq_file *m, struct trace_seq *s)
> > {
> >         int ret;
> > 
> >         __trace_seq_init(s);
> > 
> >         ret = seq_buf_print_seq(m, &s->seq);
> > 
> >         /*
> >          * Only reset this buffer if we successfully wrote to the
> >          * seq_file buffer. This lets the caller try again or
> >          * do something else with the contents.
> >          */
> >         if (!ret)
> >                 trace_seq_init(s);
> > 
> >         return ret;
> > }
> 
> The confusion is caused by the 'k' ("print" vs. "printk") in the
> function name. I was talking about the following function from
> kernel/trace/trace.c:

Silly 'k', Trix are for kids!

> 
> void
> trace_printk_seq(struct trace_seq *s)
> {
> 	/* Probably should print a warning here. */
> 	if (s->seq.len >= TRACE_MAX_PRINT)
> 		s->seq.len = TRACE_MAX_PRINT;
> 
> 	/* should be zero ended, but we are paranoid. */
> 	s->buffer[s->seq.len] = 0;
> 
> 	printk(KERN_TRACE "%s", s->buffer);
> 
> 	trace_seq_init(s);
> }
> 
> I found it when checking the applied patches in origin/rfc/seq-buf
> branch. I hope that it was the correct place.

Yes, that's the working branch for this code.

Anyway, I saw this and thought about using trace_seq_used(), but then I
realized that this is trace_seq code which has a hard coded buffer
length of PAGE_SIZE which on all archs is more than 1000
(TRACE_MAX_PRINT).

Regardless of overflow or not (or even if trace_seq is full), that if
statement will prevent this from doing any buffer overflows.

s->seq.len will never be more than s->seq.size after the test against
TRACE_MAX_PRINT. So I see no harm here.

trace_printk_seq() is for dumping the ring buffer to console, which is
usually something done on panic. It's special.

-- Steve
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