linux-kernel - Re: [PATCH 13/15] scsi: sas: avoid gcc-10 zero-length-bounds warning

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Date:   Fri, 01 May 2020 07:53:38 -0700
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Arnd Bergmann <arnd@...db.de>, John Garry <john.garry@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        James Bottomley <James.Bottomley@...eleye.com>,
        Hannes Reinecke <hare@...e.com>,
        linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 13/15] scsi: sas: avoid gcc-10 zero-length-bounds warning

On Fri, 2020-05-01 at 09:54 +0200, Arnd Bergmann wrote:
> On Fri, May 1, 2020 at 9:48 AM John Garry <john.garry@...wei.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 */

If gcc is now warning about zero length members, why isn't it warning
about this one ... are unions temporarily excluded?

>                 u8      resp_data[]; /* allowed here because it's at
> the one of a struct */
>         };
>         u8     sense_data[];
> } __attribute__ ((packed));

Let's go back to what the standard says:  we want the data beyond the
ssp_response_iu to be addressable either as sense_data if it's an error
return or resp_data if it's a real response.  What about trying to use
an alias attribute inside the structure ... will that work on gcc-10?

James