linux-kernel - Re: [PATCH] tracing: fix referencing after memory freeing and refactors code

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Date:	Sat, 19 Oct 2013 16:18:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Geyslan Gregório Bem <geyslan@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	kernel-br <kernel-br@...glegroups.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	open list <linux-kernel@...r.kernel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [PATCH] tracing: fix referencing after memory freeing and
	refactors code

On 10/19, Geyslan Gregório Bem wrote:
>
> 2013/10/19 Oleg Nesterov <oleg@...hat.com>:
> > On 10/17, Steven Rostedt wrote:
> >>
> >> I'm thinking of just nuking the tracing_open_generic() here. The only
> >> thing it does here is the tracing_disabled check. The assignment of
> >> inode->i_private to filp->private_data is pointless
> >
> > The same for ftrace_enable_fops() and ftrace_event_filter_fops() at
> > least. The users of event_file_data() do not use ->private_data.
> >
>
> Aren't "ftrace_enable_fops" and "ftrace_event_filter_fops" structures?

I meant, their ->open() methods.

> About event_file_data() I think that the callers uses the
> private_data. So, we have to analyze better.

No, event_file_data() uses ->i_private, filp->private_data is not used.
And it can't be used, it can point to the already destroyed/freed data.

but, as for seq_open() users,

> static int trace_format_open(struct inode *inode, struct file *file)
> {
>     struct seq_file *m;
>     int ret;
>
>     ret = seq_open(file, &trace_format_seq_ops);
>     if (ret < 0)
>         return ret;
>
>     m = file->private_data;
>     m->private = file;
>
>     return 0;
> }
>
> I really got confused here. The 'm' assignments are, to me, pointless.

I confused too... Why do you think it is pointless?

Just in case, not that after seq_open() ->private_data points to seq_file
but it is still "void *". And in this case ->private_data has nothing to
do with ->private_data  set by tracing_open_generic().

Oleg.

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