Re: [PATCH] ftrace: Avoid double-free of dyn_ftrace

[Zhaolei]

* From: "Steven Rostedt" <rostedt@xxxxxxxxxxx>
> On Fri, 13 Mar 2009, Zhaolei wrote:
> 
>> * From: "Ingo Molnar" <mingo@xxxxxxx>
>> > 
>> > * Zhaolei <zhaolei@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> >> ---
>> >>  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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