From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-db5eur01on0069.outbound.protection.outlook.com ([104.47.2.69] helo=EUR01-DB5-obe.outbound.protection.outlook.com) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1fOhwC-0004Lr-HM for linux-mtd@lists.infradead.org; Fri, 01 Jun 2018 11:09:59 +0000 Date: Fri, 1 Jun 2018 13:09:21 +0200 From: Ivan Djelic To: Kees Cook Cc: Boris Brezillon , Brian Norris , David Woodhouse , Marek Vasut , Richard Weinberger , Linux mtd , LKML Subject: Re: [PATCH v3] lib/bch: Remove VLA usage Message-ID: <20180601110921.GC4695@parrot.com> References: <20180531184525.GA11068@beast> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180531184525.GA11068@beast> List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 > --- > 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 Tested-by: Ivan Djelic Thanks, -- Ivan