linux-kernel - Re: [PATCH] IB/qib: Fix null pointer subtraction compiler warning

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Date:   Fri, 3 Sep 2021 13:24:41 -0400
From:   Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>
To:     Jason Gunthorpe <jgg@...dia.com>, linux-rdma@...r.kernel.org
Cc:     Leon Romanovsky <leonro@...dia.com>,
        kernel test robot <lkp@...el.com>,
        Mike Marciniszyn <mike.marciniszyn@...nelisnetworks.com>,
        llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/qib: Fix null pointer subtraction compiler warning

On 9/3/21 9:07 AM, Jason Gunthorpe wrote:
>>> drivers/infiniband/hw/qib/qib_sysfs.c:411:1: warning: performing pointer subtraction with a null pointer has undefined behavior
> +[-Wnull-pointer-subtraction]
>    QIB_DIAGC_ATTR(rc_resends);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~
>    drivers/infiniband/hw/qib/qib_sysfs.c:408:51: note: expanded from macro 'QIB_DIAGC_ATTR'
>                    .counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \
> 
> Use offsetof and accomplish the type check using static_assert.
> 
> Fixes: 4a7aaf88c89f ("RDMA/qib: Use attributes for the port sysfs")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> ---
>  drivers/infiniband/hw/qib/qib_sysfs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/hw/qib/qib_sysfs.c b/drivers/infiniband/hw/qib/qib_sysfs.c
> index d57e49de6650be..452e2355d24eeb 100644
> --- a/drivers/infiniband/hw/qib/qib_sysfs.c
> +++ b/drivers/infiniband/hw/qib/qib_sysfs.c
> @@ -403,9 +403,11 @@ static ssize_t diagc_attr_store(struct ib_device *ibdev, u32 port_num,
>  }
>  
>  #define QIB_DIAGC_ATTR(N)                                                      \
> +	static_assert(&((struct qib_ibport *)0)->rvp.n_##N != (u64 *)NULL);    \
>  	static struct qib_diagc_attr qib_diagc_attr_##N = {                    \
>  		.attr = __ATTR(N, 0664, diagc_attr_show, diagc_attr_store),    \
> -		.counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \
> +		.counter =                                                     \
> +			offsetof(struct qib_ibport, rvp.n_##N) / sizeof(u64)   \
>  	}
>  
>  QIB_DIAGC_ATTR(rc_resends);
> 
> base-commit: 6a217437f9f5482a3f6f2dc5fcd27cf0f62409ac
> 

Looks fine.

Acked-by: Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>