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SubjectRe: [PATCH] ftrace: Avoid double-free of dyn_ftrace
Date
* From: "Steven Rostedt" <rostedt@goodmis.org>
> On Fri, 13 Mar 2009, Zhaolei wrote:
>
>> * From: "Ingo Molnar" <mingo@elte.hu>
>> >
>> > * Zhaolei <zhaolei@cn.fujitsu.com> wrote:
>> >
>> >> If dyn_ftrace is free before ftrace_release(),
>> >> ftrace_release() will free it again and make
>> >> ftrace_free_records wrong.
>> >>
>> >> Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
>> >> ---
>> >> kernel/trace/ftrace.c | 3 ++-
>> >> 1 files changed, 2 insertions(+), 1 deletions(-)
>> >>
>> >> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> >> index d33d306..26c45aa 100644
>> >> --- a/kernel/trace/ftrace.c
>> >> +++ b/kernel/trace/ftrace.c
>> >> @@ -356,7 +356,8 @@ void ftrace_release(void *start, unsigned long size)
>> >>
>> >> mutex_lock(&ftrace_lock);
>> >> do_for_each_ftrace_rec(pg, rec) {
>> >> - if ((rec->ip >= s) && (rec->ip < e))
>> >> + if ((rec->ip >= s) && (rec->ip < e) &&
>> >> + !(rec->flags & FTRACE_FL_FREE))
>> >> ftrace_free_rec(rec);
>> >
>> > Applied to tip:tracing/ftrace, thanks!
>> >
>> > I'm wondering, did you trigger this in practice (if yes, how?),
>> > or have you found it via code review?
>> Hello, Ingo
>>
>> It is found via code review.
>
> Hmm, could you explain this more. I'm thinking that this scenario should
> not happen, and if it does, it should probably be a bug.
>
> Because when we call ftrace_free_rec we change the rec->ip to point to the
> next record in the chain. Something is very wrong if rec->ip >= s &&
> rec->ip < e and the record is already free.
Hello, Steven

Thanks for your comment.
I got your meaning, and I agree that if rec->ip >= s && rec->ip < e,
this record is not freed.
But IMHO, "if rec->ip >= s && rec->ip < e" is used to select records in the module,
and function of ignore "freed record" is only its side-effect.
So, add a special judgement to avoid "freed record" is not a bad idea.
And I also agree your suggestion of add a WARN_ON, because this should not happened.

B.R.
Zhaolei

>
> We can add a:
>
> WARN_ON(rec->flags & FTRACE_FL_FREE);
>
> in ftrace_free_rec if you are worried about this happening.
>
> -- Steve
>
>
>
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