Hi Kees, On Tue, 19 Jun 2018 16:48:17 -0700 Kees Cook <[email protected]> wrote:
> On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic <[email protected]> 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 <[email protected]> > >> --- > >> 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 <[email protected]> > > Tested-by: Ivan Djelic <[email protected]> > > 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. Here is my Acked-by: Boris Brezillon <[email protected]> in case you were waiting for it. Regards, Boris
