Re: [PATCH bpf-next v4 2/3] libbpf: add low level TC-BPF API

[Daniel Borkmann <daniel@iogearbox.net>]

On 4/27/21 11:55 PM, Daniel Borkmann wrote:
> On 4/27/21 8:02 PM, Kumar Kartikeya Dwivedi wrote:
>> On Tue, Apr 27, 2021 at 08:34:30PM IST, Daniel Borkmann wrote:
>>> On 4/23/21 5:05 PM, Kumar Kartikeya Dwivedi wrote:
> [...]
>>>> +/*
>>>> + * @ctx: Can be NULL, if not, must point to a valid object.
>>>> + *     If the qdisc was attached during ctx_init, it will be deleted if no
>>>> + *     filters are attached to it.
>>>> + *     When ctx == NULL, this is a no-op.
>>>> + */
>>>> +LIBBPF_API int bpf_tc_ctx_destroy(struct bpf_tc_ctx *ctx);
>>>> +/*
>>>> + * @ctx: Cannot be NULL.
>>>> + * @fd: Must be >= 0.
>>>> + * @opts: Cannot be NULL, prog_id must be unset, all other fields can be
>>>> + *      optionally set. All fields except replace  will be set as per created
>>>> + *        filter's attributes. parent must only be set when attach_point of ctx is
>>>> + *        BPF_TC_CUSTOM_PARENT, otherwise parent must be unset.
>>>> + *
>>>> + * Fills the following fields in opts:
>>>> + *    handle
>>>> + *    parent
>>>> + *    priority
>>>> + *    prog_id
>>>> + */
>>>> +LIBBPF_API int bpf_tc_attach(struct bpf_tc_ctx *ctx, int fd,
>>>> +                 struct bpf_tc_opts *opts);
>>>> +/*
>>>> + * @ctx: Cannot be NULL.
>>>> + * @opts: Cannot be NULL, replace and prog_id must be unset, all other fields
>>>> + *      must be set.
>>>> + */
>>>> +LIBBPF_API int bpf_tc_detach(struct bpf_tc_ctx *ctx,
>>>> +                 const struct bpf_tc_opts *opts);
>>>
>>> One thing that I find a bit odd from this API is that BPF_TC_INGRESS / BPF_TC_EGRESS
>>> needs to be set each time via bpf_tc_ctx_init(). So whenever a specific program would
>>> be attached to both we need to 're-init' in between just to change from hook a to b,
>>> whereas when you have BPF_TC_CUSTOM_PARENT, you could just use a different opts->parent
>>> without going this detour (unless the clsact wasn't loaded there in the first place).
>>
>> Currently I check that opts->parent is unset when BPF_TC_INGRESS or BPF_TC_EGRESS
>> is set as attach point. But since both map to clsact, we could allow the user to
>> specify opts->parent as BPF_TC_INGRESS or BPF_TC_EGRESS (no need to use
>> TC_H_MAKE, we can detect it from ctx->parent that it won't be a parent id). This
>> would mean that by default attach point is what you set for ctx, but for
>> bpf_tc_attach you can temporarily override to be some other attach point (for
>> the same qdisc). You still won't be able to set anything other than the two
>> though.
> 
> I think the assumption on auto-detecting the parent id in that case might not hold given
> major number could very well be 0. Wrt BPF_TC_UNSPEC ... maybe it's not even needed, back
> to drawing board ...
> 
> Here's how the whole API could look like, usage examples below:
> 
>    enum bpf_tc_attach_point {
>      BPF_TC_INGRESS = 1 << 0,
>      BPF_TC_EGRESS  = 1 << 1,
>      BPF_TC_CUSTOM  = 1 << 2,
>    };
> 
>    enum bpf_tc_attach_flags {
>      BPF_TC_F_REPLACE = 1 << 0,
>    };
> 
>    struct bpf_tc_hook {
>      size_t sz;
>      int    ifindex;
>      enum bpf_tc_attach_point which;
>      __u32  parent;
>      size_t :0;
>    };
> 
>    struct bpf_tc_opts {
>      size_t sz;
>      __u32  handle;
>      __u16  priority;
>      union {
>          int   prog_fd;
>          __u32 prog_id;
>      };
>      size_t :0;
>    };
> 
>    LIBBPF_API int bpf_tc_hook_create(struct bpf_tc_hook *hook);
>    LIBBPF_API int bpf_tc_hook_destroy(struct bpf_tc_hook *hook);
> 
>    LIBBPF_API int bpf_tc_attach(const struct bpf_tc_hook *hook, const struct bpf_tc_opts *opts, int flags);
>    LIBBPF_API int bpf_tc_detach(const struct bpf_tc_hook *hook, const struct bpf_tc_opts *opts);
>    LIBBPF_API int bpf_tc_query(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts);
> 
> So a user could do just:
> 
>    DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex = 42, .which = BPF_TC_INGRESS);
>    DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, .handle = 1, .priority = 1, .prog_fd = fd);
> 
>    err = bpf_tc_attach(&hook, &opts, BPF_TC_F_REPLACE);
>    [...]
> 
> If it's not known whether the hook exists, then a preceding call to:
> 
>    err = bpf_tc_hook_create(&hook);
>    [...]
> 
> The bpf_tc_query() would look like:
> 
>    DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex = 42, .which = BPF_TC_EGRESS);
>    DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, .handle = 1, .priority = 1);
> 
>    err = bpf_tc_query(&hook, &opts);
>    if (!err) {
>           [...]  // gives access to: opts.prog_id
>    }
> 
> The bpf_tc_detach():
> 
>    DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex = 42, .which = BPF_TC_INGRESS);
>    DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, .handle = 1, .priority = 1);
> 
>    err = bpf_tc_detach(&hook, &opts);
>    [...]
> 
> The nice thing would be that hook and opts are kept semantically separate, meaning with
> hook you can iterate though a bunch of devs and ingress/egress locations without changing
> opts, whereas with opts you could iterate on the cls_bpf instance itself w/o changing
> hook. Both are kept extensible via DECLARE_LIBBPF_OPTS().
> 
> Now the bpf_tc_hook_destroy() one:
> 
>    DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex = 42, .which = BPF_TC_INGRESS|BPF_TC_EGRESS);
> 
>    err = bpf_tc_hook_destroy(&hook);
>    [...]
> 
> For triggering a remove of the clsact qdisc on the device, both directions are passed in.
> Combining both is only ever allowed for bpf_tc_hook_destroy().

Small addendum:

     DECLARE_LIBBPF_OPTS(bpf_tc_hook, hook, .ifindex = 42, .which = BPF_TC_INGRESS|BPF_TC_EGRESS);

     err = bpf_tc_hook_create(&hook);
     [...]

... is also possible, of course, and then both bpf_tc_hook_{create,destroy}() are symmetric.

> If /only/ BPF_TC_INGRESS or only BPF_TC_EGRESS is passed, it could flush their lists (aka
> equivalent of `tc filter del dev eth0 ingress` and `tc filter del dev eth0 egress` command).
> 
> For bpf_tc_hook_{create,destroy}() with BPF_TC_CUSTOM, we just return -EINVAL or -EOPNOTSUPP.
> 
> I think the above interface would work nicely and feels intuitive while being extensible.
> Thoughts?
> 
> Thanks,
> Daniel