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Date:   Mon, 9 Aug 2021 13:28:02 +0300
From:   Leon Romanovsky <leon@kernel.org>
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     yajun.deng@linux.dev, Jakub Kicinski <kuba@kernel.org>,
        davem@davemloft.net, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next] net: sock: add the case if sk is NULL
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On Mon, Aug 09, 2021 at 11:34:31AM +0200, Eric Dumazet wrote:
> 
> 
> On 8/9/21 8:12 AM, yajun.deng@linux.dev wrote:
> > August 6, 2021 9:11 PM, "Jakub Kicinski" <kuba@kernel.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@linux.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)

You can add into your list that such "if NULL" checks add false
impression that NULL can be there and it is valid.

Thanks

>