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, -- Ivan
