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SubjectRe: [PATCH v3] lib/bch: Remove VLA usage
Hi Kees,

On Tue, 19 Jun 2018 16:48:17 -0700
Kees Cook <keescook@chromium.org> wrote:

> On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic <ivan.djelic@parrot.com> wrote:
> > On Thu, May 31, 2018 at 11:45:25AM -0700, Kees Cook wrote:
> >> In the quest to remove all stack VLA usage from the kernel[1], this
> >> allocates a fixed size stack array to cover the range needed for
> >> bch. This was done instead of a preallocation on the SLAB due to
> >> performance reasons, shown by Ivan Djelic:
> >>
> >> little-endian, type sizes: int=4 long=8 longlong=8
> >> cpu: Intel(R) Core(TM) i5 CPU 650 @ 3.20GHz
> >> calibration: iter=4.9143µs niter=2034 nsamples=200 m=13 t=4
> >>
> >> Buffer allocation | Encoding throughput (Mbit/s)
> >> ---------------------------------------------------
> >> on-stack, VLA | 3988
> >> on-stack, fixed | 4494
> >> kmalloc | 1967
> >>
> >> So this change actually improves performance too, it seems.
> >>
> >> The resulting stack allocation can get rather large; without
> >> CONFIG_BCH_CONST_PARAMS, it will allocate 4096 bytes, which
> >> trips the stack size checking:
> >>
> >> lib/bch.c: In function ‘encode_bch’:
> >> lib/bch.c:261:1: warning: the frame size of 4432 bytes is larger than 2048 bytes [-Wframe-larger-than=]
> >>
> >> Even the default case for "allmodconfig" (with CONFIG_BCH_CONST_M=14 and
> >> CONFIG_BCH_CONST_T=4) would have started throwing a warning:
> >>
> >> lib/bch.c: In function ‘encode_bch’:
> >> lib/bch.c:261:1: warning: the frame size of 2288 bytes is larger than 2048 bytes [-Wframe-larger-than=]
> >>
> >> But this is how large it's always been; it was just hidden from
> >> the checker because it was a VLA. So the Makefile has been adjusted to
> >> silence this warning for anything smaller than 4500 bytes, which should
> >> provide room for normal cases, but still low enough to catch any future
> >> pathological situations.
> >>
> >> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
> >>
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> ---
> >> v3: fix r_bytes to whole-word size
> >> v2: switch to fixed-size stack array
> >> ---
> >> lib/Makefile | 1 +
> >> lib/bch.c | 23 +++++++++++++++--------
> >> 2 files changed, 16 insertions(+), 8 deletions(-)
> >
> >
> > The patch looks good to me. It also passed my regression tests.
> >
> > Reviewed-by: Ivan Djelic <ivan.djelic@parrot.com>
> > Tested-by: Ivan Djelic <ivan.djelic@parrot.com>
>
> Thanks for the review and testing!
>
> Who's the best person to carry this patch?

Looks like all users of this lib are in drivers/mtd, so I can take the
patches if you want, but I can also let you take them if you prefer.

Here is my

Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>

in case you were waiting for it.

Regards,

Boris

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