linux-kernel - Re: [PATCH net-next] net: sock: add the case if sk is NULL

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Date:   Mon, 9 Aug 2021 11:34:31 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     yajun.deng@...ux.dev, Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: sock: add the case if sk is NULL



On 8/9/21 8:12 AM, yajun.deng@...ux.dev wrote:
> August 6, 2021 9:11 PM, "Jakub Kicinski" <kuba@...nel.org> wrote:
> 
>> On Fri, 6 Aug 2021 14:38:15 +0800 Yajun Deng wrote:
>>
>>> Add the case if sk is NULL in sock_{put, hold},
>>> The caller is free to use it.
>>>
>>> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
>>
>> The obvious complaint about this patch (and your previous netdev patch)
>> is that you're spraying branches everywhere in the code. Sure, it may
> 
> Sorry for that, I'll be more normative in later submission.
>> be okay for free(), given how expensive of an operation that is but
>> is having refcounting functions accept NULL really the best practice?
>>
>> Can you give us examples in the kernel where that's the case?
> 
> 0   include/net/neighbour.h         neigh_clone()
> 1   include/linux/cgroup.h          get_cgroup_ns() and put_cgroup_ns()  (This is very similar to my submission)
> 2   include/linux/ipc_namespace.h   get_ipc_ns()
> 3   include/linux/posix_acl.h       posix_acl_dup()
> 4   include/linux/pid.h             get_pid()
> 5   include/linux/user_namespace.h  get_user_ns()
> 

These helpers might be called with NULL pointers by design.

sock_put() and sock_hold() are never called with NULL.

Same for put_page() and hundreds of other functions.

By factorizing a conditional in the function, hoping to remove one in few callers,
we add more conditional branches (and increase code size)