linux-kernel - Re: [PATCH][next] scsi: pm80xx: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <30d62858-0ce5-41cc-9599-23d5c38785ae@oracle.com>
Date: Thu, 4 Sep 2025 14:49:25 +0100
From: John Garry <john.g.garry@...cle.com>
To: James Bottomley <James.Bottomley@...senPartnership.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Jack Wang <jinpu.wang@...ud.ionos.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Cc: linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] scsi: pm80xx: Avoid -Wflex-array-member-not-at-end
 warning

On 04/09/2025 13:39, James Bottomley wrote:
> On Thu, 2025-09-04 at 07:52 +0100, John Garry wrote:
>> On 03/09/2025 19:44, Gustavo A. R. Silva wrote:
>>> diff --git a/drivers/scsi/pm8001/pm8001_hwi.h
>>> b/drivers/scsi/pm8001/pm8001_hwi.h
>>> index fc2127dcb58d..7dc7870a8f86 100644
>>> --- a/drivers/scsi/pm8001/pm8001_hwi.h
>>> +++ b/drivers/scsi/pm8001/pm8001_hwi.h
>>> @@ -339,8 +339,10 @@ struct ssp_completion_resp {
>>>    	__le32	status;
>>>    	__le32	param;
>>>    	__le32	ssptag_rescv_rescpad;
>>> -	struct ssp_response_iu  ssp_resp_iu;
>>>    	__le32	residual_count;
>>> +
>>> +	/* Must be last --ends in a flexible-array member. */
>>> +	struct ssp_response_iu  ssp_resp_iu;
>> this is a HW structure, right? I did not think that it is ok to
>> simply re-order them...
> Agreed, this is a standards defined information unit corresponding to
> an on the wire data structure.  The patch is clearly wrong.
> 
> That being said, the three things the flexible member can contain are
> no data, response data or sense data.  None of them has a residual
> count at the beginning and, indeed, this field is never referred to in
> the driver, so it looks like it can simply be deleted to fix the
> warning.

Seems reasonable. I don't see how the sizeof(struct ssp_completion_resp) 
is relevant, as the size of the memory to hold this structure (and other 
response types) will be defined elsewhere.

> 
> That being said, this pattern of adding fields after flexible members
> to represent data that's common to all content types of the union is
> not unknown in SCSI so if you want to enable this warning, what are we
> supposed to do when we encounter a genuine use case?

Such a problem was solved in commit cd6856d38881, but I can't say that 
it is a good example as we simply dropped the flex array usage.

Thanks,
John