netdev - Re: [PATCH net-next] net_sched: fix a regression in tcf_proto_lookup

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Date:	Fri, 20 Dec 2013 13:40:27 -0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jamal Hadi Salim <jhs@...atatu.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next] net_sched: fix a regression in tcf_proto_lookup_ops()

On Fri, Dec 20, 2013 at 12:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2013-12-20 at 11:57 -0800, Cong Wang wrote:
>
>> Then why mirred_cleanup_module() appears in the top of your backtrace?
>
> Because you have not understood the trace ?
>
> Thats exactly why its not useful at all.
>
>
> [   76.656061] RIP: 0010:[<ffffffffa008d882>]  [<ffffffffa008d882>] mirred_cleanup_module+0x13a/0x2a [act_mirred]^M
>
>
> This means RIP is _after_ mirred_cleanup_module() body, as this function size if 0x2a bytes
>
> We were trying to execute a data portion which happens to be in
> the neighboring of innocent mirred_cleanup_module()
>
> When not using module, trace was different and made no sense anyway :
>
...
>
> We actually try to execute code at ffffffff818a4b80 == &act_mirred_ops
>
> ffffffff818a4b40 d cls_mod_lock
> ffffffff818a4b50 d tcf_proto_base
> ffffffff818a4b60 d act_mod_lock
> ffffffff818a4b70 d act_base
> ffffffff818a4b80 d act_mirred_ops
> ffffffff818a4bf0 d mirred_device_notifier
> ffffffff818a4c10 d mirred_list
> ffffffff818a4c20 d _rs.42828
> ffffffff818a4c40 d ematch_mod_lock
> ffffffff818a4c50 d ematch_ops
>
> So only a bisection actually gave me the faulty commit.
>
> Stacktrace was absolutely useless.
>
>

Thanks for explaining this. Then:

Reviewed-by: Cong Wang <xiyou.wangcong@...il.com>
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