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 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.

> 
> 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. 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.

Thanks
-Gustavo