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Message-ID: <87sg3b8idy.fsf@toke.dk>
Date: Wed, 28 Apr 2021 00:36:25 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Daniel Borkmann <daniel@...earbox.net>,
Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Jesper Dangaard Brouer <brouer@...hat.com>,
netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 2/3] libbpf: add low level TC-BPF API
Daniel Borkmann <daniel@...earbox.net> writes:
> 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.
It should be allowed, but it wouldn't actually make any difference which
combination of TC_INGRESS and TC_EGRESS you specify, as long as one of
them is set, right? I.e., we just attach the clsact qdisc in both
cases...
-Toke
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