linux-kernel - Re: [PATCH] RDMA: remove null check after call container

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Date:   Tue, 17 May 2022 21:42:41 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Haowen Bai <baihaowen@...zu.com>
Subject: Re: [PATCH] RDMA: remove null check after call container_of()

Le 17/05/2022 à 20:03, Jason Gunthorpe a écrit :
> On Tue, May 17, 2022 at 07:54:38PM +0200, Christophe JAILLET wrote:
>> Le 17/05/2022 à 14:16, Jason Gunthorpe a écrit :
>>> On Tue, May 17, 2022 at 09:33:28AM +0800, Haowen Bai wrote:
>>>> container_of() will never return NULL, so remove useless code.
>>>
>>> It is confusing here, but it can be null.
>>
>> Hi,
>>
>> out of curiosity, can you elaborate how it can be NULL?
> 
> It is guarented/required that that container_of is a 0 offset. The
> normal usage of the ib_alloc_device macro:
> 
> #define ib_alloc_device(drv_struct, member)                                    \
> 	container_of(_ib_alloc_device(sizeof(struct drv_struct) +              \
> 				      BUILD_BUG_ON_ZERO(offsetof(              \
> 					      struct drv_struct, member))),    \
> 		     struct drv_struct, member)
> 
> Enforces this property with a BUILD_BUG_ON
> 
> So, if the input pointer to container_of is reliably NULL or ERR_PTR
> then the output pointer will be the same.
> 
> The rvt code here open codes the call because it is a mid-layer and
> the sizeof() calculation above is not correct for it.
> 
> Jason
> 

Crystal clear.
Thank you for the explanation.

CJ