linux-kernel - Re: [RFC][PATCH 08/12 v3] tracing: Add seq_buf_get_buf() and seq_buf

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Date:	Mon, 10 Nov 2014 14:23:10 -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 08/12 v3] tracing: Add seq_buf_get_buf() and
 seq_buf_commit() helper functions

On Mon, 10 Nov 2014 19:33:29 +0100
Petr Mladek <pmladek@...e.cz> wrote:

> On Fri 2014-11-07 13:39:29, Steven Rostedt wrote:
> > More updates. Hmm, maybe I should have posted the full series ;-)
> > 
> > -- Steve
> > 
> > From 41a3f3f5e772ca26ef4441a0312d3f108693d7dc Mon Sep 17 00:00:00 2001
> > From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> > Date: Wed, 29 Oct 2014 17:30:50 -0400
> > Subject: [PATCH] tracing: Add seq_buf_get_buf() and seq_buf_commit() helper
> >  functions
> > 
> > Add two helper functions; seq_buf_get_buf() and seq_buf_commit() that
> > are used by seq_buf_path(). This makes the code similar to the
> > seq_file: seq_path() function, and will help to be able to consolidate
> > the functions between seq_file and trace_seq.
> > 
> > Link: http://lkml.kernel.org/r/20141104160222.644881406@goodmis.org
> > 
> > Tested-by: Jiri Kosina <jkosina@...e.cz>
> > Acked-by: Jiri Kosina <jkosina@...e.cz>
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> Reviewed-by: Petr Mladek <pmladek@...e.cz>
> 
> Well, I am curious about the BUG_ONs, see below.
> 
> > ---
> >  include/linux/seq_buf.h | 40 ++++++++++++++++++++++++++++++++++++++++
> >  kernel/trace/seq_buf.c  |  7 +++----
> >  2 files changed, 43 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> > index 4aab47d10760..7dacdc791225 100644
> > --- a/include/linux/seq_buf.h
> > +++ b/include/linux/seq_buf.h
> > @@ -61,6 +61,46 @@ seq_buf_buffer_left(struct seq_buf *s)
> >  	return s->size - s->len;
> >  }
> >  
> > +/**
> > + * seq_buf_get_buf - get buffer to write arbitrary data to
> > + * @s: the seq_buf handle
> > + * @bufp: the beginning of the buffer is stored here
> > + *
> > + * Return the number of bytes available in the buffer, or zero if
> > + * there's no space.
> > + */
> > +static inline size_t seq_buf_get_buf(struct seq_buf *s, char **bufp)
> > +{
> > +	BUG_ON(s->len > s->size + 1);
> 
> I just wonder if the BUG_ON() is appropriate here. There is used
> WARN_ON() for the other similar checks.

That should be a WARN_ON(). Thanks.

It's probably a BUG_ON as that was code that was used internally for
earlier iterations, and the BUG_ON() was for me to see it quickly.
Could have also been that previous versions could access memory that it
should not.

But as it's intended to be used by others, it should be a warning
instead of a bug.


> 
> On one hand. This function will be used by a code that manipulates
> the buffer its own way. Therefore the BUG() would help to debug
> potential problems.
> 
> On the other hand, this function is used just to get the buffer.
> Therefore the BUG() might come too late. The buffer was broken
> somewhere else.
> 
> > +
> > +	if (s->len < s->size) {
> > +		*bufp = s->buffer + s->len;
> > +		return s->size - s->len;
> > +	}
> > +
> > +	*bufp = NULL;
> > +	return 0;
> > +}
> > +
> > +/**
> > + * seq_buf_commit - commit data to the buffer
> > + * @s: the seq_buf handle
> > + * @num: the number of bytes to commit
> > + *
> > + * Commit @num bytes of data written to a buffer previously acquired
> > + * by seq_buf_get.  To signal an error condition, or that the data
> > + * didn't fit in the available space, pass a negative @num value.
> > + */
> > +static inline void seq_buf_commit(struct seq_buf *s, int num)
> > +{
> > +	if (num < 0) {
> > +		seq_buf_set_overflow(s);
> > +	} else {
> > +		BUG_ON(s->len + num > s->size + 1);
> 
> I agree that the BUG_ON makes sense here. If someone passed too big
> "num", she probably also wrote too many bytes and the memory is
> corrupted at this point.

Yeah, this one is worse than the other one and should bug to prevent
memory corruption.

Thanks for reviewing this,

-- Steve

> 
> > +		s->len += num;
> > +	}
> > +}
> > +
> 
> Best Regards,
> Petr

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