linux-kernel - Re: [PATCH v3] lib/bch: Remove VLA usage

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Date:   Wed, 20 Jun 2018 09:38:05 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ivan Djelic <ivan.djelic@...rot.com>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        Linux mtd <linux-mtd@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] lib/bch: Remove VLA usage

Hi Kees,

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

> On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic <ivan.djelic@...rot.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@...omium.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@...rot.com>
> > Tested-by: Ivan Djelic <ivan.djelic@...rot.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@...tlin.com>

in case you were waiting for it.

Regards,

Boris