From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.bootlin.com ([62.4.15.54]) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1fVXhD-0007jv-Sj for linux-mtd@lists.infradead.org; Wed, 20 Jun 2018 07:38:30 +0000 Date: Wed, 20 Jun 2018 09:38:05 +0200 From: Boris Brezillon To: Kees Cook Cc: Ivan Djelic , Brian Norris , David Woodhouse , Marek Vasut , Richard Weinberger , Linux mtd , LKML Subject: Re: [PATCH v3] lib/bch: Remove VLA usage Message-ID: <20180620093805.6d9c602e@bbrezillon> In-Reply-To: References: <20180531184525.GA11068@beast> <20180601110921.GC4695@parrot.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Hi Kees, On Tue, 19 Jun 2018 16:48:17 -0700 Kees Cook wrote: > On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic wrot= e: > > On Thu, May 31, 2018 at 11:45:25AM -0700, Kees Cook wrote: =20 > >> 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=3D4 long=3D8 longlong=3D8 > >> cpu: Intel(R) Core(TM) i5 CPU 650 @ 3.20GHz > >> calibration: iter=3D4.9143=C2=B5s niter=3D2034 nsamples=3D200 m=3D13 = t=3D4 > >> > >> 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 =E2=80=98encode_bch=E2=80=99: > >> lib/bch.c:261:1: warning: the frame size of 4432 bytes is larger than = 2048 bytes [-Wframe-larger-than=3D] > >> > >> Even the default case for "allmodconfig" (with CONFIG_BCH_CONST_M=3D14= and > >> CONFIG_BCH_CONST_T=3D4) would have started throwing a warning: > >> > >> lib/bch.c: In function =E2=80=98encode_bch=E2=80=99: > >> lib/bch.c:261:1: warning: the frame size of 2288 bytes is larger than = 2048 bytes [-Wframe-larger-than=3D] > >> > >> 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=3DqP= XydAacU1RqZWA@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(-) =20 > > > > > > The patch looks good to me. It also passed my regression tests. > > > > Reviewed-by: Ivan Djelic > > Tested-by: Ivan Djelic =20 >=20 > Thanks for the review and testing! >=20 > 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 in case you were waiting for it. Regards, Boris