Re: [Cake] Cake on openwrt - falling behind

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk> writes:

>> On 2 Jul 2018, at 19:39, Dave Taht <dave.taht@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...

-Toke