netdev - Re: [PATCH net] netlink: access nlk groups safely in netlink bind and getname

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Message-ID: <CAM_iQpXP7ANA3EWUyy3c9iJ2zsRfM7aaA7bR=QG+fW5t6q9qSg@mail.gmail.com>
Date:   Mon, 11 Sep 2017 10:35:04 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        David Herrmann <dh.herrmann@...il.com>
Subject: Re: [PATCH net] netlink: access nlk groups safely in netlink bind and getname

On Sun, Sep 10, 2017 at 4:45 AM, Xin Long <lucien.xin@...il.com> wrote:
> On Sat, Sep 9, 2017 at 7:35 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> On Tue, Sep 5, 2017 at 8:53 PM, Xin Long <lucien.xin@...il.com> wrote:
>>> Now there is no lock protecting nlk ngroups/groups' accessing in
>>> netlink bind and getname. It's safe from nlk groups' setting in
>>> netlink_release, but not from netlink_realloc_groups called by
>>> netlink_setsockopt.
>>>
>>> netlink_lock_table is needed in both netlink bind and getname when
>>> accessing nlk groups.
>>
>> This looks very odd.
>>
>> netlink_lock_table() should be protecting nl_table, why
>> it also protects nlk->groups?? For me it looks like you
>> need lock_sock() instead.
> I believe netlink_lock_table might be only used to protect nl_table
> at the beginning and surely lock_sock is better here. Thanks.
>
> But can you explain why  netlink_lock_table() was also used in
> netlink_getsockopt NETLINK_LIST_MEMBERSHIPS ? or it
> was just a mistake ?

No, it is fine but not necessary, because netlink_realloc_groups()
doesn't change nl_table, it only changes nlk->groups. So we
don't have take the global write lock, the lock sock makes more
sense here, same for your bind() and getname() case.