Re: [PATCH v3] lib/bch: Remove VLA usage

[Boris Brezillon]

On Wed, 20 Jun 2018 11:16:11 -0700
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> On Wed, Jun 20, 2018 at 12:38 AM, Boris Brezillon
> <boris.brezillon@xxxxxxxxxxx> wrote:
> > Hi Kees,
> >
> > On Tue, 19 Jun 2018 16:48:17 -0700
> > Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >  
> >> On Fri, Jun 1, 2018 at 4:09 AM, Ivan Djelic <ivan.djelic@xxxxxxxxxx> 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=4.9143Âs 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 âencode_bchâ:
> >> >> 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 âencode_bchâ:
> >> >> 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@xxxxxxxxxxxxxx
> >> >>
> >> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >> >> ---
> >> >> 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.djelic@xxxxxxxxxx>
> >> > Tested-by: Ivan Djelic <ivan.djelic@xxxxxxxxxx>  
> >>
> >> 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!

Applied to mtd/next.

Thanks,

Boris