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

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.
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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