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

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Message-ID: <20100120175205.GA5017@nowhere>
Date:	Wed, 20 Jan 2010 18:52:09 +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 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.

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?

Steve?

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