Re: [PATCH v4 net-next 5/5] net: filter: split 'struct sk_filter' into socket and bpf parts

[Alexei Starovoitov]

On Fri, Aug 1, 2014 at 9:06 AM, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote:
> On Thu, Jul 31, 2014 at 02:02:19PM -0700, Alexei Starovoitov wrote:
>> On Thu, Jul 31, 2014 at 12:40 PM, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote:
>> > On Wed, Jul 30, 2014 at 08:34:16PM -0700, Alexei Starovoitov wrote:
>> >> clean up names related to socket filtering and bpf in the following way:
>> >> - everything that deals with sockets keeps 'sk_*' prefix
>> >> - everything that is pure BPF is changed to 'bpf_*' prefix
>> >>
>> >> split 'struct sk_filter' into
>> >> struct sk_filter {
>> >>       atomic_t        refcnt;
>> >>       struct rcu_head rcu;
>> >>       struct bpf_prog *prog;
>> >> };
>> >
>> > I think you can use 'struct bpf_prog prog' instead so the entire
>> > sk_filter remains in the same memory blob (as it is before this
>> > patch).
>> >
>> > You can add an inline function to retrieve the bpg prog from the
>> > filter:
>> >
>> > static inline struct bpf_prog *sk_filter_bpf(struct sk_filter *)
>> >
>> > and use it whenever possible to fetch the bpf program. I'm suggesting
>> > this because we can use the zero array size in the socket filtering
>> > abstraction later on, if the function above is used, this just needs
>> > one line in that function to be updated to fetch the program from the
>> > placeholder.
>>
>> correct. It would speed up SK_RUN_FILTER macro a little and I've
>> considered it, but decided to go with the pointer for two reasons:
>> 1.In sk_attach_filter() the bpf_prog is allocated, then reallocated
>> as part of bpf_prepare_filter(). My patch #1 cleans up that part to
>> avoid 'struct sock *' dependency, so all bpf_* functions work
>> purely with bpf_prog... If bpf_prog is embedded inside sk_filter,
>> bpf_prepare_filter would need to have a callback to reallocate
>> the container struct and pass this callback through the chain
>> of calls, which is ugly
>
> I think you can allocate the sk_filter once you get the final bpf
> program, then you can memcpy() it. This adds some extra overhead in
> the sk_attach_filter(), but that path is executed from user context
> and it's also a rare operation (only once to attach the filter). It's
> still not going to be a beauty, but IMO it's worth to focus on getting
> that little speed up in the packet path at the cost of adding some
> overhead on the socket attach path.

memcpy of 'bpf_prog' is not just 'not a beauty', it won't work, since
bpf_prog is freed via work_queue due to JIT. See bpf_jit_free()
There are few other ways I could think of embedding bpf_prog inside
sk_filter: 1. have a release callback, 2. extra field inside bpf_prog
that will tell outer size of the container (so that all kfree(bpf_prog*)
can adjust the pointer before freeing). 3. extra field which is direct
pointer to the outer container, so that kfree(bpf_prog*) can use it
instead of freeing the bpf_prog itself.
In all cases it means hacking jit compilers for all architectures
in a very ugly way.
Say, we pick #3 (which is the least ugly), see how
kfree(fp) will change to
kfree(fp->pointer_to_outer_container).
it will be not obvious at all to the reader that bpf_prog itself will be
freed by such kfree()... so we'd need to add nasty comments
everywhere...
imo one extra load of fp->prog in critical path of SK_RUN_FILTER
is not worth this mess.

>> 2. having it as a pointer helps nft in the long run, since whole of
>> bpf_prog doesn't stay around inside sk_filter when it's not used.
>
> If you put into place the inline wrapper function that I'm proposing
> above and you use it instead of fp->bpf_prog, I think there should be
> not interference:
>
> static inline struct bpf_prog *sk_filter_bpf(struct sk_filter *sk)
> {
> -       return &sk->bpf_prog;
> +       return (struct bpf_prog *)sk->data;
> }

This is not pretty either, since we'd need to worry that alignment
of sk->data field must match alignment of bpf_prog. Probably can
make it 'long data[0]'...  all of these just feels wrong to avoid
single load in SK_RUN_FILTER().
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