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

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Date:	Thu, 6 Nov 2014 17:33:54 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
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 2014-11-05 16:21:46, Steven Rostedt wrote:
> On Wed, 5 Nov 2014 16:17:20 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > 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);
> 
> Bah, I need to make this: len = s->size - s->len.
> 
> As this would work for both cases of what a overflowed buffer is.

Hmm, this would produce -1 (UNIT_MAX) when the overflow is
(s->len = s->size + 1). I would keep seq_buf_buffer_left(s);


> > 	int ret;
> > 
> > 	WARN_ON(s->size == 0);
> > 
> > 	if (s->len < s->size) {

It does not make sense to try if there is only one byte left. I would do:

	if (len > 1)

together with the change below.

> > 		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.
> > 		 */

This is great explanation. You might want to move it above the
"if (len > 1)" if you agree with it.

> > 		if (s->len + ret < s->size) {

This should work with both variants of the overflow if "len" is really
the space left.

		if (ret < len)

I mean that we fail if we wrote the buffer until the last possible byte.

> > 			s->len += ret;
> > 			return 0;
> > 		}
> > 	}
> > 	seq_buf_set_overflow(s);
> > 	return -1;
> > }


Best Regards,
Petr
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