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

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Date:   Fri, 1 Jun 2018 13:09:21 +0200
From:   Ivan Djelic <ivan.djelic@...rot.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Boris Brezillon <boris.brezillon@...tlin.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

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,
--
Ivan