[Cake] Cake on openwrt - falling behind

[Dave Taht]

On Mon, Jul 2, 2018 at 12:23 PM Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>
> Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> writes:
>
> >> On 2 Jul 2018, at 19:39, Dave Taht <dave.taht at gmail.com> wrote:
> >>
> >>>
> >>
> >> This seems like it will introduce problems with stuff that isn't or is
> >> legitimately broken in the first place, pointing to potentially random
> >> data in the wrong place.
> >>
> >> would a workaround be adding more padding to the cake stats output so
> >> it's always even?
> >>
> >> why does it work as written on arm?
> >
> > If I understand correctly: This will only be a problem on
> > architectures that require alignment of 64 bit values to 8 byte
> > boundaries which is achieved by padding the structure by a dummy (4
> > byte) value if required. So to hit this bug we need kernel symbol
> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS undefined *and* we need a
> > netlink stats structure that needs a 4 byte dummy pad value to align
> > to 8 bytes. Of the architectures tested, MIPS is the only one that
> > DOES NOT set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus may be
> > exposed to the bug.
> >
> > arm sets CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus no padding is
> > ever required/added, thus pointers always point to the correct data
> > location.
>
> Yup, exactly. This has always been broken on MIPS, I assume, but because
> most other qdiscs send their stats output as a serialised struct, tc
> just automatically falls back to the legacy data format, and no one has
> noticed. But because we switched to sending each stat as an individual
> netlink attribute (and thus no fallback legacy stats struct), we expose
> the bug...

Well, if you wrap that patch in

#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
bla
#endif

I guess I'd sleep better but I do generally get nervous when
arbitrarily subtracting something
from a pointer

>
> -Toke



-- 

Dave Täht
CEO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-669-226-2619