Re: [PATCH v2][next] scsi: megaraid_sas: Avoid a couple -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 10/7/25 23:56, James Bottomley wrote:
> On Tue, 2025-10-07 at 15:18 +0100, Gustavo A. R. Silva wrote:
>>
>>
>> On 10/7/25 12:59, James Bottomley wrote:
>>> On Tue, 2025-10-07 at 11:43 +0100, Gustavo A. R. Silva wrote:
>>>> Hi all,
>>>>
>>>> Friendly ping: who can take this, please?
>>>
>>> After what happened with the qla2xxx driver, everyone is a bit wary
>>> of these changes, particularly when they affect structures shared
>>> with the hardware. Megaraid is a broadcom acquisition so although
>>> maintained it might take them a while to check this.
>>
>> I've been in constant communication with the people involved. So far,
>> none of them has expressed any concerns about this to me. However, I
>> appreciate your feedback.
>>
>> In any case, I promptly submitted a bugfix minutes after getting the
>> report.
> 
> I'm not criticizing the response, just saying that problems like this
> cause me to think that someone who understands and can test the
> hardware needs to look at this.  However ...

That's true. I agree.

> 
>>> However, you could help us with this: as I understand it (there is
>>> a bit of a no documentation problem here), the TRAILING_OVERLAP
>>> formalism merely gets the compiler not to warn about the situation
>>> rather than actually changing anything in the layout of the
>>> structure?  In which case you should be able to demonstrate the
>>> binary produced before and after this patch is the same, which
>>> would very much reduce the risk of taking it.
>>
>> This is quite simple. Here you go the pahole output before and after
>> changes.
>>
>> BEFORE CHANGES:
>>
>> pahole -C MR_FW_RAID_MAP_ALL drivers/scsi/megaraid/megaraid_sas_fp.o
>> struct MR_FW_RAID_MAP_ALL {
>>           struct MR_FW_RAID_MAP      raidMap;              /*     0
>> 10408 */
>>           /* --- cacheline 162 boundary (10368 bytes) was 40 bytes ago
>> --- */
>>           struct MR_LD_SPAN_MAP      ldSpanMap[64];        /* 10408
>> 161792 */
>>
>>           /* size: 172200, cachelines: 2691, members: 2 */
>>           /* last cacheline: 40 bytes */
>> };
>>
>> AFTER CHANGES:
>>
>> pahole -C MR_FW_RAID_MAP_ALL drivers/scsi/megaraid/megaraid_sas_fp.o
>> struct MR_FW_RAID_MAP_ALL {
>>           union {
>>                   struct MR_FW_RAID_MAP raidMap;           /*     0
>> 10408 */
>>                   struct {
>>                           unsigned char __offset_to_FAM[10408]; /*
>> 0 10408 */
>>                           /* --- cacheline 162 boundary (10368 bytes)
>> was 40 bytes ago --- */
>>                           struct MR_LD_SPAN_MAP ldSpanMap[64]; /*
>> 10408 161792 */
>>                   };                                       /*     0
>> 172200 */
>>           };                                               /*     0
>> 172200 */
>>
>>           /* size: 172200, cachelines: 2691, members: 1 */
>>           /* last cacheline: 40 bytes */
>> };
>>
>> As you can see, the size is exactly the same, as are the offsets for
>> both members raidMap and ldSpanMap.
> 
> ... this is good enough to prove that the binary before and after is
> identical and thus there's no change to the structures, which means the
> risk of accepting the patch is significantly lower.
> 
>>   The trick is that, thanks to the union and __offset_to_FAM, the
>> flexible-array member raidMap.ldSpanMap[] now appears as the last
>> member instead of somewhere in the middle.
>>
>> So both ldSpanMap and raidMap.ldSpanMap[] now cleanly overlap, as
>> seems to have been intended.
>>
>> (Exactly the same applies for struct MR_DRV_RAID_MAP_ALL)
>>
>> I can include this explanation to the changelog text if you'd like.
> 
> I'll leave it up to Martin, but I think going forwards it would be
> helpful if you could indicate that you've checked that the binary
> layout before and after is unchanged and thus the risk of merging the
> patch is low.

Absolutely. I'll do that.

Thanks for the feedback.
-Gustavo