On Wed, Jun 20, 2018 at 12:38 AM, Boris Brezillon <boris.brezil...@bootlin.com> wrote: > Hi Kees, > > On Tue, 19 Jun 2018 16:48:17 -0700 > Kees Cook <keesc...@chromium.org> wrote: > >> On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic <ivan.dje...@parrot.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=qpxydaacu1rq...@mail.gmail.com >> >> >> >> Signed-off-by: Kees Cook <keesc...@chromium.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.dje...@parrot.com> >> > Tested-by: Ivan Djelic <ivan.dje...@parrot.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.
I'd love it if you could take it; thank you! -Kees -- Kees Cook Pixel Security
