netdev - Re: [PATCH bpf-next 11/13] bpf: libbpf: Add STRUCT

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Message-ID: <20191218072003.yxilrs4mniy6zgrb@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 18 Dec 2019 07:20:07 +0000
From:   Martin Lau <kafai@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        "Kernel Team" <Kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 11/13] bpf: libbpf: Add STRUCT_OPS support

On Tue, Dec 17, 2019 at 11:03:45PM -0800, Martin Lau wrote:
> On Tue, Dec 17, 2019 at 07:07:23PM -0800, Andrii Nakryiko wrote:
> > On Fri, Dec 13, 2019 at 4:48 PM Martin KaFai Lau <kafai@...com> wrote:
> > >
> > > This patch adds BPF STRUCT_OPS support to libbpf.
> > >
> > > The only sec_name convention is SEC("struct_ops") to identify the
> > > struct ops implemented in BPF, e.g.
> > > SEC("struct_ops")
> > > struct tcp_congestion_ops dctcp = {
> > >         .init           = (void *)dctcp_init,  /* <-- a bpf_prog */
> > >         /* ... some more func prts ... */
> > >         .name           = "bpf_dctcp",
> > > };
> > >
> > > In the bpf_object__open phase, libbpf will look for the "struct_ops"
> > > elf section and find out what is the btf-type the "struct_ops" is
> > > implementing.  Note that the btf-type here is referring to
> > > a type in the bpf_prog.o's btf.  It will then collect (through SHT_REL)
> > > where are the bpf progs that the func ptrs are referring to.
> > >
> > > In the bpf_object__load phase, the prepare_struct_ops() will load
> > > the btf_vmlinux and obtain the corresponding kernel's btf-type.
> > > With the kernel's btf-type, it can then set the prog->type,
> > > prog->attach_btf_id and the prog->expected_attach_type.  Thus,
> > > the prog's properties do not rely on its section name.
> > >
> > > Currently, the bpf_prog's btf-type ==> btf_vmlinux's btf-type matching
> > > process is as simple as: member-name match + btf-kind match + size match.
> > > If these matching conditions fail, libbpf will reject.
> > > The current targeting support is "struct tcp_congestion_ops" which
> > > most of its members are function pointers.
> > > The member ordering of the bpf_prog's btf-type can be different from
> > > the btf_vmlinux's btf-type.
> > >
> > > Once the prog's properties are all set,
> > > the libbpf will proceed to load all the progs.
> > >
> > > After that, register_struct_ops() will create a map, finalize the
> > > map-value by populating it with the prog-fd, and then register this
> > > "struct_ops" to the kernel by updating the map-value to the map.
> > >
> > > By default, libbpf does not unregister the struct_ops from the kernel
> > > during bpf_object__close().  It can be changed by setting the new
> > > "unreg_st_ops" in bpf_object_open_opts.
> > >
> > > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > > ---
> > 
> > This looks pretty good to me. The big two things is exposing structops
> > as real struct bpf_map, so that users can interact with it using
> > libbpf APIs, as well as splitting struct_ops map creation and
> > registration. bpf_object__load() should only make sure all maps are
> > created, progs are loaded/verified, but none of BPF program can yet be
> > called. Then attach is the phase where registration happens.
> Thanks for the review.
> 
> [ ... ]
> 
> > >  static inline __u64 ptr_to_u64(const void *ptr)
> > >  {
> > >         return (__u64) (unsigned long) ptr;
> > > @@ -233,6 +239,32 @@ struct bpf_map {
> > >         bool reused;
> > >  };
> > >
> > > +struct bpf_struct_ops {
> > > +       const char *var_name;
> > > +       const char *tname;
> > > +       const struct btf_type *type;
> > > +       struct bpf_program **progs;
> > > +       __u32 *kern_func_off;
> > > +       /* e.g. struct tcp_congestion_ops in bpf_prog's btf format */
> > > +       void *data;
> > > +       /* e.g. struct __bpf_tcp_congestion_ops in btf_vmlinux's btf
> > 
> > Using __bpf_ prefix for this struct_ops-specific types is a bit too
> > generic (e.g., for raw_tp stuff Alexei used btf_trace_). So maybe make
> > it btf_ops_ or btf_structops_?
> Is it a concern on name collision?
> 
> The prefix pick is to use a more representative name.
> struct_ops use many bpf pieces and btf is one of them.
> Very soon, all new codes will depend on BTF and btf_ prefix
> could become generic also.
> 
> Unlike tracepoint, there is no non-btf version of struct_ops.
May be bpf_struct_ops_?

It was my early pick but it read quite weird,
bpf_[struct]_<ops>_[tcp_congestion]_<ops>.

Hence, I go with __bpf_<actual-name-of-the-kernel-struct> in this series.