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

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Date:   Tue, 19 Jun 2018 16:48:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ivan Djelic <ivan.djelic@...rot.com>
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 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?

-Kees

-- 
Kees Cook
Pixel Security