Re: [PATCH 18/26 v5] seq_buf: Create seq_buf_used() to find out how much was written

[Petr Mladek]

On Wed 2014-11-19 10:49:56, Steven Rostedt wrote:
> On Tue, 18 Nov 2014 16:02:04 +0100
> Petr Mladek <pmladek@xxxxxxx> wrote:
> 
> > On Fri 2014-11-14 23:59:05, Steven Rostedt wrote:
> > > From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
> > > 
> > > Add a helper function seq_buf_used() that replaces the SEQ_BUF_USED()
> > > private macro to let callers have a method to know how much of the
> > > seq_buf was written to.
> > > 
> > > Link: http://lkml.kernel.org/r/20141114011412.170377300@xxxxxxxxxxx
> > > Link: http://lkml.kernel.org/r/20141114011413.321654244@xxxxxxxxxxx
> > > 
> > > Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > ---
> > >  include/linux/seq_buf.h | 6 ++++++
> > >  kernel/trace/seq_buf.c  | 5 +----
> > >  2 files changed, 7 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> > > index 5d91262433e2..93718e570d4c 100644
> > > --- a/include/linux/seq_buf.h
> > > +++ b/include/linux/seq_buf.h
> > > @@ -64,6 +64,12 @@ seq_buf_buffer_left(struct seq_buf *s)
> > >  	return (s->size - 1) - s->len;
> > >  }
> > >  
> > > +/* How much buffer was written? */
> > > +static inline unsigned int seq_buf_used(struct seq_buf *s)
> > > +{
> > > +	return min(s->len, s->size);
> > 
> > To be precise, it should do
> > 
> > 	return min(s->len, s->size - 1);
> > 
> > at this stage and switch to the above code in ("[PATCH 21/26 v5] tracing: Have
> > seq_buf use full buffer"). Well, it does not cause any big harm. Feel
> > free to add:
> 
> I actually thought about it, but realized that this only replaces the
> original use of s->len, and we are only doing this to avoid buffer
> overflows. If you get garbage, then so be it, as the original code
> would give garbage too. Remember, some of the functions did have real
> content in that last byte, even though it was considered "overflowed".

I thought that it was based on ftrace_seq code that had

#define TRACE_SEQ_BUF_USED(s) min((s)->len, (unsigned int)(PAGE_SIZE -
 1))

But you are right that the callers used s->len because the macro was
local in trace_seq.c.

Well, I think that it does not matter much. As you said, there might be
a garbage a the end of the buffer. Callers used "len" directly.
Also it will be min(s->len, s->size); after the commit "Have seq_buf
use full buffer".

Let's leave it as is.

Best Regards,
Petr
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