linux-kernel - Re: [PATCH 1/6] : bug fix, remove partial zero out

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Message-ID: <20100130212652.GJ5675@nowhere>
Date:	Sat, 30 Jan 2010 22:26:54 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/6] : bug fix, remove partial zero out

On Tue, Jan 26, 2010 at 11:31:58AM +0800, Lai Jiangshan wrote:
> Frederic Weisbecker wrote:
> > On Tue, Jan 19, 2010 at 03:33:56PM +0800, Lai Jiangshan wrote:
> >> partial-zero-out a struct is very dangerous, we should zero out
> >> field by field directly when need.
> >>
> >> partial-zero-out for struct trace_iterator exists when ftrace
> >> was first introduced into mainline kernel. But in this few years,
> >> the code of ftrace is changed a lot, and:
> >>
> >> 1) partial-zero-out for struct trace_iterator has a bug now,
> >>    cpumask_var_t started should not be zeroed out.
> >>
> >> 2) I viewed the codes and found that fields below
> >>    "/* The below is zeroed out in pipe_read */"
> >>    don't need to be zeroed out or initialized now.
> >>
> >> So, we remove the code of "partial zero out"
> >>
> >> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> >> ---
> >> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> >> index 3ca9485..c6d0e1a 100644
> >> --- a/include/linux/ftrace_event.h
> >> +++ b/include/linux/ftrace_event.h
> >> @@ -54,7 +54,6 @@ struct trace_iterator {
> >>  	struct ring_buffer_iter	*buffer_iter[NR_CPUS];
> >>  	unsigned long		iter_flags;
> >>  
> >> -	/* The below is zeroed out in pipe_read */
> >>  	struct trace_seq	seq;
> >>  	struct trace_entry	*ent;
> >>  	int			leftover;
> >> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> >> index 5314c90..27fecf8 100644
> >> --- a/kernel/trace/trace.c
> >> +++ b/kernel/trace/trace.c
> >> @@ -3124,12 +3124,6 @@ waitagain:
> >>  	if (cnt >= PAGE_SIZE)
> >>  		cnt = PAGE_SIZE - 1;
> >>  
> >> -	/* reset all but tr, trace, and overruns */
> >> -	memset(&iter->seq, 0,
> >> -	       sizeof(struct trace_iterator) -
> >> -	       offsetof(struct trace_iterator, seq));
> >> -	iter->pos = -1;
> >> -
> > 
> > 
> > 
> > I'm not sure exaclty why we needed to zero the seq here.
> > We already reset it in trace_seq_init().
> > 
> > We might do it again on waitagain. I lost track how we could
> > ever need to goto waitagain. It was about a tricky bug to fix
> > but I'm don't remember exactly the details.
> > 
> > That said, if trace_seq_to_user returns -EBUSY, we
> > re-init the seq buffer, so it should be fine I guess.
> 
> Yes, -EBUSY is strange here.
> but any way, trace_seq_init() is called.
> 
> > 
> > But concerning the need of setting iter->pos to -1, I'm not
> > sure we need to remove it. Shouldn't it be set to 0 btw?
> > 
> 
> ->pos is not used here, ->idx is just increased here,
> so we don't need to initialize them.


Ok.

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