netdev - Re: [RFC PATCH bpf-next] bpf: Introduce bpf

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Date:   Mon, 24 May 2021 21:59:31 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, kernel-team <kernel-team@...com>
Subject: Re: [RFC PATCH bpf-next] bpf: Introduce bpf_timer

On Mon, May 24, 2021 at 8:16 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Sun, May 23, 2021 at 9:01 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Fri, May 21, 2021 at 2:37 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > >
> > > Hi, Alexei
> > >
> > > On Thu, May 20, 2021 at 11:52 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > Introduce 'struct bpf_timer' that can be embedded in most BPF map types
> > > > and helpers to operate on it:
> > > > long bpf_timer_init(struct bpf_timer *timer, void *callback, int flags)
> > > > long bpf_timer_mod(struct bpf_timer *timer, u64 msecs)
> > > > long bpf_timer_del(struct bpf_timer *timer)
> > >
> > > Like we discussed, this approach would make the timer harder
> > > to be independent of other eBPF programs, which is a must-have
> > > for both of our use cases (mine and Jamal's). Like you explained,
> > > this requires at least another program array, a tail call, a mandatory
> > > prog pinning to work.
> >
> > That is simply not true.
>
> Which part is not true? The above is what I got from your explanation.

I tried to write some code sketches to use your timer to implement
our conntrack logic, below shows how difficult it is to use, it does not
even include the user-space part where eBPF programs are put
into the program array.


struct {
       __uint(type, BPF_MAP_TYPE_HASH);
       __uint(max_entries, 1000);
       __type(key, struct tuple);
       __type(value, struct foo);
} conntrack SEC(".maps");

struct map_elem {
       struct bpf_timer timer;
       struct bpf_map *target;
       u32 expires;
};

struct {
       __uint(type, BPF_MAP_TYPE_HASH);
       __uint(max_entries, 1000);
       __type(key, int);
       __type(value, struct map_elem);
} timers SEC(".maps");

struct {
        __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
        __uint(key_size, sizeof(u32));
        __uint(value_size, sizeof(u32));
        __uint(max_entries, 8);
} jmp_table SEC(".maps");

static __u64
cleanup_conntrack(struct bpf_map *map, struct tuple *key, struct foo *val,
                 struct callback_ctx *data)
{
        if (val.expires < now)
                bpf_map_delete_elem(conntrack, key);
}

static int timer_cb(struct bpf_map *map, int *key, struct map_elem *val)
{
       bpf_for_each_map_elem(val->target, cleanup_conntrack, ....);
       /* re-arm the timer again to execute after 1 msec */
       bpf_timer_mod(&val->timer, 1);
       return 0;
}

SEC("prog/0")
int install_timer(void)
{
       struct map_elem *val;
       int key = 0;

       val = bpf_map_lookup_elem(&timers, &key);
       if (val) {
               bpf_timer_init(&val->timer, timer_cb, 0);
               bpf_timer_mod(&val->timer, val->expires);
       }
}

SEC("prog/1")
int mod_timer(void)
{
       struct map_elem *val;
       int key = 0;

       val = bpf_map_lookup_elem(&timers, &key);
       if (val) {
               // XXX: how do we know if a timer has been installed?
               bpf_timer_mod(&val->timer, val->expires);
       }
}

SEC("ingress")
void ingress(struct __sk_buff *skb)
{
        struct tuple tuple;
        // extract tuple from skb

        if (bpf_map_lookup_elem(&timers, &key) == NULL)
                bpf_tail_call(NULL, &jmp_table, 0);
                // here is not reachable unless failure
        val = bpf_map_lookup_elem(&conntrack, &tuple);
        if (val && val->expires < now) {
                bpf_tail_call(NULL, &jmp_table, 1);
                // here is not reachable unless failure
        }
}

SEC("egress")
void egress(struct __sk_buff *skb)
{
        struct tuple tuple;
        // extract tuple from skb

        if (bpf_map_lookup_elem(&timers, &key) == NULL)
                bpf_tail_call(NULL, &jmp_table, 0);
                // here is not reachable unless failure
        val = bpf_map_lookup_elem(&conntrack, &tuple);
        if (val && val->expires < now) {
                bpf_tail_call(NULL, &jmp_table, 1);
                // here is not reachable unless failure
        }
}