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From: John Fastabend <john.fastabend@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	John Fastabend <john.fastabend@gmail.com>
Cc: "Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Edward Cree" <ecree@solarflare.com>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Marek Majkowski" <marek@cloudflare.com>,
	"Lorenz Bauer" <lmb@cloudflare.com>,
	"Alan Maguire" <alan.maguire@oracle.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"David Miller" <davem@davemloft.net>,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: static and dynamic linking. Was: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF
Date: Fri, 15 Nov 2019 08:39:55 -0800	[thread overview]
Message-ID: <5dced4db31be6_5f462b1f489665bcaa@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20191115015001.iyqipxhoqt77iade@ast-mbp.dhcp.thefacebook.com>

Alexei Starovoitov wrote:
> On Tue, Nov 12, 2019 at 09:33:18PM -0800, John Fastabend wrote:
> > 
> > In addition to above flow something like this to load libraries first should
> > also work?
> > 
> >    // here fw2 is a library its never attached to anything but can be
> >    // used to pull functions from
> >    obj = bpf_object__open("fw2.o", attr);
> >    bpf_object__load(obj);
> >    prog = bpf_object__find_program_by_title(obj);
> >    subprog_btf_id0 = libbpf_find_obj_btf_id("name of function", obj);
> >    subprog_btf_id1 = libbpf_find_obj_btf_id("name of function", obj);
> > 
> >    // all pairs of (prog_fd, btf_id) need to be specified at load time
> >    attr.attach[0].prog_fd = fw2_fd;
> >    attr.attach[0].btf_id = subprog_btf_id0;
> >    attr.attach[1].prog_fd = fw2_fd;
> >    attr.attach[1].btf_id = subprog_btf_id1;
> >    obj = bpf_object__open("rootlet.o", attr)
> >    bpf_object__load(obj)
> >    prog = bpf_object__find_program_by_title(obj);
> >    link = bpf_program__replace(prog);
> >    // attach rootlet.o at this point with subprog_btf_id
> 
> The point I'm arguing that these:
>    attr.attach[0].prog_fd = fw2_fd;
>    attr.attach[0].btf_id = subprog_btf_id0;
>    attr.attach[1].prog_fd = fw2_fd;
>    attr.attach[1].btf_id = subprog_btf_id1;
> should not be part of libbpf api. Instead libbpf should be able to adjust
> relocations inside the program. You're proposing to do linking via explicit
> calls, I'm saying such linking should be declarative. libbpf should be able to
> derive the intent from the program and patch calls.

Agree seems cleaner. So when libs are loaded we create a in-kernel table
of functions/symbols and when main program is loaded search for the funcs. 

> 
> Example:
> helpers.o:
> int foo(struct xdp_md *ctx, int var) {...}
> int bar(int *array, bpf_size_t size) {...}
> obj = bpf_object__open("helpers.o", attr)
> bpf_object__load(obj);
> // load and verify helpers. 'foo' and 'bar' are not attachable to anything.
> // These two programs don't have program type.
> // The kernel loaded and verified them.
> main_prog.o:
> int foo(struct xdp_md *ctx, int var);
> int bar(int *array, bpf_size_t size);
> int main_prog(struct xdp_md *ctx) 
> { 
>   int ar[5], ret;
>   ret = foo(ctx, 1) + bar(ar, 5);
> }
> // 'foo' and 'bar' are extern functions from main_prog pov.
> obj = bpf_object__open("main_prog.o", attr)
> bpf_object__load(obj);
> // libbpf finds foo/bar in the kernel and adjusts two call instructions inside
> // main_prog to point to prog_fd+btf_id
> 
> That is the second use case of dynamic linking I've been talking earlier. The

This solves my use case.

> same thing should be possible to do with static linking. Then libbpf will
> adjust calls inside main_prog to be 'call pc+123' and 'foo' and 'bar' will
> become traditional bpf subprograms. main_prog() has single 'struct xdp_md *'
> argument. It is normal attachable XDP program.
> 
> Loading main_prog.o first and then loading helpers.o should be possible as
> well. The verifier needs BTF of extern 'foo' and 'bar' symbols to be able to
> verify main_prog() independently. For example to check that main_prog() is
> passing correct ctx into foo(). That is the main difference vs traditional
> dynamic linking. I think we all agree that we want bpf programs to be verified
> independently. To do that the verifier needs to have BTF (function prototypes)
> of extern symbols. One can argue that it's not necessary and helpers.o can be
> loaded first. I don't think that will work in all cases. There could be many
> dependencies between helpers1.o calling another helpers2.o and so on and there
> will be no good order where calling extern foo() can be avoided.

Agree, its a bit unclear what we would do with a __weak symbol if a helper.o
is loaded after main.o with that symbol. I think you keep using the __weak
symbol instead of automatically reloading it so you would need some explicit
logic to make that happen in libbpf if user wants.

> 
> This thread is getting long :) and sounds like we're converging. I'm thinking
> to combine everything we've discussed so far into dynamic/static linking doc.
> 

Yep seems we are close now. FWIW I just backed into a case where at least
static linking is going to be needed so this moves it up on my list because
I'll be using it as soon as its available.

  reply	other threads:[~2019-11-15 16:39 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-07 17:20 [PATCH bpf-next v3 0/5] xdp: Support multiple programs on a single interface through chain calls Toke Høiland-Jørgensen
2019-10-07 17:20 ` [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other Toke Høiland-Jørgensen
2019-10-07 20:42   ` Alexei Starovoitov
2019-10-08  8:07     ` Toke Høiland-Jørgensen
2019-10-09  1:51       ` Alexei Starovoitov
2019-10-09  8:03         ` Toke Høiland-Jørgensen
2019-10-10  4:41           ` Alexei Starovoitov
2019-10-14 12:35             ` Toke Høiland-Jørgensen
2019-10-14 17:08               ` John Fastabend
2019-10-14 18:48                 ` Toke Høiland-Jørgensen
2019-10-15 16:30                   ` Edward Cree
2019-10-15 16:42                     ` Toke Høiland-Jørgensen
2019-10-15 18:33                       ` Edward Cree
2019-10-17 12:11                         ` Toke Høiland-Jørgensen
2019-10-22 17:27                           ` Edward Cree
2019-10-22 18:07                             ` Toke Høiland-Jørgensen
2019-11-12  2:51                               ` static and dynamic linking. Was: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF Alexei Starovoitov
2019-11-12 16:20                                 ` Toke Høiland-Jørgensen
2019-11-12 19:52                                   ` Alexei Starovoitov
2019-11-12 21:25                                     ` Edward Cree
2019-11-12 23:18                                       ` Alexei Starovoitov
2019-11-13 18:30                                         ` Edward Cree
2019-11-13 18:51                                           ` Andrii Nakryiko
2019-11-15  2:13                                           ` Alexei Starovoitov
2019-11-15 16:56                                             ` John Fastabend
2019-11-12 23:25                                     ` John Fastabend
2019-11-13  0:21                                       ` Alexei Starovoitov
2019-11-13  5:33                                         ` John Fastabend
2019-11-15  1:50                                           ` Alexei Starovoitov
2019-11-15 16:39                                             ` John Fastabend [this message]
2019-11-14 15:41                                     ` Toke Høiland-Jørgensen
2019-11-12 16:32                                 ` Edward Cree
2019-11-15 11:48                                 ` Lorenz Bauer
2019-11-15 23:02                                   ` Alexei Starovoitov
2019-11-18 13:29                                     ` Lorenz Bauer
2019-10-21 23:51                         ` [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other Edward Cree
2019-10-16  2:28               ` Alexei Starovoitov
2019-10-16  8:27                 ` Jesper Dangaard Brouer
2019-10-16 10:35                   ` Daniel Borkmann
2019-10-16 11:16                     ` Toke Høiland-Jørgensen
2019-10-16 13:51                 ` Toke Høiland-Jørgensen
2019-10-19 20:09                   ` bpf indirect calls Alexei Starovoitov
2019-10-20 10:58                     ` Toke Høiland-Jørgensen
2019-10-25 16:30                       ` Alexei Starovoitov
2019-10-27 12:15                         ` Toke Høiland-Jørgensen
2019-10-09 10:19         ` [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other Jesper Dangaard Brouer
2019-10-09 17:57           ` Alexei Starovoitov
2019-10-07 17:20 ` [PATCH bpf-next v3 2/5] bpf: Add support for setting chain call sequence for programs Toke Høiland-Jørgensen
2019-10-07 20:38   ` Daniel Borkmann
2019-10-08  8:09     ` Toke Høiland-Jørgensen
2019-10-07 17:20 ` [PATCH bpf-next v3 3/5] tools: Update bpf.h header for program chain calls Toke Høiland-Jørgensen
2019-10-07 17:20 ` [PATCH bpf-next v3 4/5] libbpf: Add syscall wrappers for BPF_PROG_CHAIN_* commands Toke Høiland-Jørgensen
2019-10-07 17:20 ` [PATCH bpf-next v3 5/5] selftests: Add tests for XDP chain calls Toke Høiland-Jørgensen
2019-10-07 18:58 ` [PATCH bpf-next v3 0/5] xdp: Support multiple programs on a single interface through " John Fastabend
2019-10-08  8:42   ` Toke Høiland-Jørgensen

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