From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [BUG] ext4 trace events cause NULL pointer dereferences
Date: Wed, 21 Jul 2010 10:16:06 -0400
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Cc: Li Zefan <lizf@cn.fujitsu.com>, Theodore Ts'o <tytso@mit.edu>,
	LKML <linux-kernel@vger.kernel.org>, linux-ext4@vger.kernel.org,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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On Wed, 2010-07-21 at 22:31 +0900, KOSAKI Motohiro wrote:
> Hi Steven,

>         if (ac)
> 	        trace_ext4_mb_release_group_pa(ac, pa);
> 
> But, I don't think this is proper fix because we don't want any overhead
> if the tracepoint is disabled.
> 
> So, How do we check NULL in TP_fast_assign()?

You could do:

	TP_fast_assign(
		if (ac) {
	                __entry->dev            = ac->ac_sb->s_dev;
        	        __entry->ino            = ac->ac_inode->i_ino;
        	        __entry->pa_pstart      = pa->pa_pstart;
        	        __entry->pa_len         = pa->pa_len;
		}
        ),

But this just makes the __entry null and wastes the ring buffer.

I may be able to add a __discard_entry that may help. Then we could do
something like this:

	if (ac) {
                __entry->dev            = ac->ac_sb->s_dev;
                __entry->ino            = ac->ac_inode->i_ino;
                __entry->pa_pstart      = pa->pa_pstart;
                __entry->pa_len         = pa->pa_len;
	} else
		__discard_entry;

Does this seem reasonable?

But for now, the wasting the entry seems to be the only choice we have,
or to do as you suggested and have the "if (ac) trace_...", but I don't
like that.

-- Steve