linux-kernel - Re: [PATCH 2/6] scsi: mpt3sas: Use flexible arrays when less obviously possible

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Message-ID: <202307281508.32604C856@keescook>
Date:   Fri, 28 Jul 2023 15:26:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     James Seo <james@...iv.tech>
Cc:     Sathya Prakash <sathya.prakash@...adcom.com>,
        Sreekanth Reddy <sreekanth.reddy@...adcom.com>,
        Suganath Prabu Subramani 
        <suganath-prabu.subramani@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        MPT-FusionLinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] scsi: mpt3sas: Use flexible arrays when less
 obviously possible

On Tue, Jul 25, 2023 at 09:13:27AM -0700, James Seo wrote:
> These old-style 1-length variable arrays can be directly converted
> into C99 flexible array members without any further source changes
> and without any meaningful binary changes. All uses of the affected
> structs were investigated, and the existing code somehow manages to
> weather the reduced sizeof() the affected structs in every case.

Doing build comparisons here, I see a lot of binary changes. They may
be, as you say, harmless, and since you've actually got hardware then
this is a good verification of the changes, but I do wonder if this
needs more detailed commit log (or split up patches).

However, the problem I see is that this code was already doing weird
stuff with structs that appear to not have been using flex arrays
actually. With "pahole" I can see struct MPT3SAS_ADAPTER changes:

-       Mpi2IOUnitPage8_t          iounit_pg8;           /*  3668    40 */
-       Mpi2IOCPage1_t             ioc_pg1_copy;         /*  3708    24 */
+       Mpi2IOUnitPage8_t          iounit_pg8;           /*  3668    16 */
+       Mpi2IOCPage1_t             ioc_pg1_copy;         /*  3684    24 */

struct _MPI2_CONFIG_PAGE_IO_UNIT_8 (Mpi2IOUnitPage8_t) is in the
_middle_ of struct MPT3SAS_ADAPTER.... :|

In the earlier attempts at this conversion, it seemed that some of these
are actually fixed-size:

https://lore.kernel.org/lkml/20210202235118.GA314410@embeddedor/

I think this patch needs to be broken up into per-struct changes, so
they can be reviewed individually.

-Kees

-- 
Kees Cook