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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 1 May 2020 09:54:19 +0200
Message-ID: <CAK8P3a1=5dmgB+k9B_jk2qBhBPnMSFMWrByP4jRvyvaJwBo94A@mail.gmail.com>
Subject: Re: [PATCH 13/15] scsi: sas: avoid gcc-10 zero-length-bounds warning
To:     John Garry <john.garry@huawei.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        James Bottomley <James.Bottomley@steeleye.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Hannes Reinecke <hare@suse.com>,
        linux-scsi <linux-scsi@vger.kernel.org>
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On Fri, May 1, 2020 at 9:48 AM John Garry <john.garry@huawei.com> wrote:
> On 30/04/2020 22:30, Arnd Bergmann wrote:

> > This should really be a flexible-array member, but the structure
> > already has such a member, swapping it out with sense_data[] would
> > cause many more warnings elsewhere.
> >
>
>
> Hi Arnd,
>
> If we really prefer flexible-array members over zero-length array
> members, then could we have a union of flexible-array members? I'm not
> sure if that's a good idea TBH (or even permitted), as these structures
> are defined by the SAS spec and good practice to keep as consistent as
> possible, but just wondering.

gcc does not allow flexible-array members inside of a union, or more than
one flexible-array member at the end of a structure.

I found one hack that would work, but I think it's too ugly and likely not
well-defined either:

struct ssp_response_iu {
...
        struct {
                u8      dummy[0]; /* a struct must have at least one
non-flexible member */
                u8      resp_data[]; /* allowed here because it's at
the one of a struct */
        };
        u8     sense_data[];
} __attribute__ ((packed));

> Apart from that:
>
> Reviewed-by: John Garry <john.garry@huawei.com>

Thanks!

     Arnd