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

Kees Cook
2023-07-28T22:27:03Z

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