netdev - Re: [PATCH bpf-next v3 2/3] libbpf: add low level TC-BPF API

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Date:   Thu, 22 Apr 2021 01:21:55 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        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 v3 2/3] libbpf: add low level TC-BPF API

On 4/22/21 1:08 AM, Kumar Kartikeya Dwivedi wrote:
> On Thu, Apr 22, 2021 at 04:29:28AM IST, Daniel Borkmann wrote:
>> On 4/20/21 9:37 PM, Kumar Kartikeya Dwivedi wrote:
>> [...]
>>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>>> index bec4e6a6e31d..b4ed6a41ea70 100644
>>> --- a/tools/lib/bpf/libbpf.h
>>> +++ b/tools/lib/bpf/libbpf.h
>>> @@ -16,6 +16,8 @@
>>>    #include <stdbool.h>
>>>    #include <sys/types.h>  // for size_t
>>>    #include <linux/bpf.h>
>>> +#include <linux/pkt_sched.h>
>>> +#include <linux/tc_act/tc_bpf.h>
>>>    #include "libbpf_common.h"
>>> @@ -775,6 +777,48 @@ LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker, const char *filen
>>>    LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
>>>    LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);
>>> +/* Convenience macros for the clsact attach hooks */
>>> +#define BPF_TC_CLSACT_INGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_INGRESS)
>>> +#define BPF_TC_CLSACT_EGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_EGRESS)
>>
>> I would abstract those away into an enum, plus avoid having to pull in
>> linux/pkt_sched.h and linux/tc_act/tc_bpf.h from main libbpf.h header.
>>
>> Just add a enum { BPF_TC_DIR_INGRESS, BPF_TC_DIR_EGRESS, } and then the
>> concrete tc bits (TC_H_MAKE()) can be translated internally.
> 
> Ok, will do.
> 
>>> +struct bpf_tc_opts {
>>> +	size_t sz;
>>
>> Is this set anywhere?
> 
> This is needed by the OPTS_* infrastructure.
> 
>>> +	__u32 handle;
>>> +	__u32 class_id;
>>
>> I'd remove class_id from here as well given in direct-action a BPF prog can
>> set it if needed.
> 
> Ok, makes sense.
> 
>>> +	__u16 priority;
>>> +	bool replace;
>>> +	size_t :0;
>>
>> What's the rationale for this padding?
> 
> dde7b3f5f2f4 ("libbpf: Add explicit padding to bpf_xdp_set_link_opts")

Hm, fair enough.

>>> +};
>>> +
>>> +#define bpf_tc_opts__last_field replace
>>> +
>>> +/* Acts as a handle for an attached filter */
>>> +struct bpf_tc_attach_id {
>>
>> nit: maybe bpf_tc_ctx
>>
> 
> Noted.
> 
>>> +	__u32 handle;
>>> +	__u16 priority;
>>> +};
>>> +
>>> +struct bpf_tc_info {
>>> +	struct bpf_tc_attach_id id;
>>> +	__u16 protocol;
>>> +	__u32 chain_index;
>>> +	__u32 prog_id;
>>> +	__u8 tag[BPF_TAG_SIZE];
>>> +	__u32 class_id;
>>> +	__u32 bpf_flags;
>>> +	__u32 bpf_flags_gen;
>>
>> Given we do not yet have any setters e.g. for offload, etc, the one thing
>> I'd see useful and crucial initially is prog_id.
>>
>> The protocol, chain_index, and I would also include tag should be dropped.
> 
> A future user of this API needs to know the tag, so I would like to keep that.
> The rest we can drop, and probably document the default values explicitly.

Couldn't this be added along with the future patch for the [future] user?

The tag should be the tag of the prog itself, so if you have prog_id, you
could also retrieve the same tag from the prog. The tag was basically from
the early days where we didn't have bpf_prog_get_info_by_fd().

What does that future user need to do different here?

Thanks,
Daniel