linux-kernel - Re: [RFC][PATCH 03/12 v3] tracing: Create seq_buf layer in trace

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Message-ID: <20141105161720.71abdbdb@gandalf.local.home>
Date:	Wed, 5 Nov 2014 16:17:20 -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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 03/12 v3] tracing: Create seq_buf layer in
 trace_seq

On Wed, 5 Nov 2014 15:00:07 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 5 Nov 2014 13:41:47 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > 
> > > > + */
> > > > +int seq_buf_bitmask(struct seq_buf *s, const unsigned long *maskp,
> > > > +		    int nmaskbits)
> > > > +{
> > > > +	unsigned int len = SEQ_BUF_LEFT(s);
> > > >
> > > > +	int ret;
> > > > +
> > > > +	WARN_ON(s->size == 0);
> > > > +
> > > > +	if (s->len < s->size) {
> > > > +		ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);
> > > 
> > > It writes to the beginning of the buffer. It should be
> > > 
> > > 		ret = bitmap_scnprintf(s->buffer + s->len, len,
> > > 				       maskp, nmaskbits);
> > >
> > 
> > Yep thanks. Luckily its only user didn't care.
> > 
> > Will fix.
> >  
> > > 
> > > > +		if (s->len + ret < s->size) {
> > > 
> > > This will always happen because bitmap_scnprintf() is limited by SEQ_BUF_LEFT(s)
> > > and it currently returns the remaining size - len - 1.
> > 
> > Hmm, that's correct, as bitmap_scnprintf() returns the amount written
> > instead of the amount that it would write like snprintf() would.
> > 
> > 
> > > 
> > > You might want to use "s->size - s->len" instead of SEQ_BUF_LEFT(s).
> > 
> > That wont help when we make overflow len > size.
> > 
> > Probably should see if ret == the amount of bits required for the
> > bitmask.
> 
> Here's the new version:
> 

Take 2:

int seq_buf_bitmask(struct seq_buf *s, const unsigned long *maskp,
		    int nmaskbits)
{
	unsigned int len = seq_buf_buffer_left(s);
	int ret;

	WARN_ON(s->size == 0);

	if (s->len < s->size) {
		ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);
		/*
		 * Note, because bitmap_scnprintf() only returns the
		 * number of bytes written and not the number that
		 * would be written, we use the last byte of the buffer
		 * to let us know if we overflowed. There's a small
		 * chance that the bitmap could have fit exactly inside
		 * the buffer, but it's not that critical if that does
		 * happen.
		 */
		if (s->len + ret < s->size) {
			s->len += ret;
			return 0;
		}
	}
	seq_buf_set_overflow(s);
	return -1;
}

-- Steve
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