Date:   Fri, 4 Oct 2019 15:47:28 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v3 bpf-next 5/7] libbpf: move bpf_{helpers,endian,tracing}.h
 into libbpf

On Fri, Oct 4, 2019 at 2:58 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On Fri, Oct 04, 2019 at 11:06:13PM +0200, Daniel Borkmann wrote:
> > On Fri, Oct 04, 2019 at 01:21:55PM -0700, Andrii Nakryiko wrote:
> > > On Fri, Oct 4, 2019 at 11:30 AM Jakub Kicinski
> > > <jakub.kicinski@...ronome.com> wrote:
> > > > On Fri, 4 Oct 2019 09:00:42 -0700, Andrii Nakryiko wrote:
> > > > > On Fri, Oct 4, 2019 at 8:44 AM David Ahern <dsahern@...il.com> wrote:
> > > > >> > I'm not following you; my interpretation of your comment seems like you
> > > > > > are making huge assumptions.
> > > > > >
> > > > > > I build bpf programs for specific kernel versions using the devel
> > > > > > packages for the specific kernel of interest.
> > > > >
> > > > > Sure, and you can keep doing that, just don't include bpf_helpers.h?
> > > > >
> > > > > What I was saying, though, especially having in mind tracing BPF
> > > > > programs that need to inspect kernel structures, is that it's quite
> > > > > impractical to have to build many different versions of BPF programs
> > > > > for each supported kernel version and distribute them in binary form.
> > > > > So people usually use BCC and do compilation on-the-fly using BCC's
> > > > > embedded Clang.
> > > > >
> > > > > BPF CO-RE is providing an alternative, which will allow to pre-compile
> > > > > your program once for many different kernels you might be running your
> > > > > program on. There is tooling that eliminates the need for system
> > > > > headers. Instead we pre-generate a single vmlinux.h header with all
> > > > > the types/enums/etc, that are then used w/ BPF CO-RE to build portable
> > > > > BPF programs capable of working on multiple kernel versions.
> > > > >
> > > > > So what I was pointing out there was that this vmlinux.h would be
> > > > > ideally generated from latest kernel and not having latest
> > > > > BPF_FUNC_xxx shouldn't be a problem. But see below about situation
> > > > > being worse.
> > > >
> > > > Surely for distroes tho - they would have kernel headers matching the
> > > > kernel release they ship. If parts of libbpf from GH only work with
> > > > the latest kernel, distroes should ship libbpf from the kernel source,
> > > > rather than GH.
> > > >
> > > > > > > Nevertheless, it is a problem and thanks for bringing it up! I'd say
> > > > > > > for now we should still go ahead with this move and try to solve with
> > > > > > > issue once bpf_helpers.h is in libbpf. If bpf_helpers.h doesn't work
> > > > > > > for someone, it's no worse than it is today when users don't have
> > > > > > > bpf_helpers.h at all.
> > > > > > >
> > > > > >
> > > > > > If this syncs to the github libbpf, it will be worse than today in the
> > > > > > sense of compile failures if someone's header file ordering picks
> > > > > > libbpf's bpf_helpers.h over whatever they are using today.
> > > > >
> > > > > Today bpf_helpers.h don't exist for users or am I missing something?
> > > > > bpf_helpers.h right now are purely for selftests. But they are really
> > > > > useful outside that context, so I'm making it available for everyone
> > > > > by distributing with libbpf sources. If bpf_helpers.h doesn't work for
> > > > > some specific use case, just don't use it (yet?).
> > > > >
> > > > > I'm still failing to see how it's worse than situation today.
> > > >
> > > > Having a header which works today, but may not work tomorrow is going
> > > > to be pretty bad user experience :( No matter how many warnings you put
> > > > in the source people will get caught off guard by this :(
> > > >
> > > > If you define the current state as "users can use all features of
> > > > libbpf and nothing should break on libbpf update" (which is in my
> > > > understanding a goal of the project, we bent over backwards trying
> > > > to not break things) then adding this header will in fact make things
> > > > worse. The statement in quotes would no longer be true, no?
> > >
> > > So there are few things here.
> > >
> > > 1. About "adding bpf_helpers.h will make things worse". I
> > > categorically disagree, bpf_helpers.h doesn't exist in user land at
> > > all and it's sorely missing. So adding it is strictly better
> > > experience already. Right now people have to re-declare those helper
> > > signatures and do all kinds of unnecessary hackery just to be able to
> > > use BPF stuff, and they still can run into the same problem with
> > > having too old kernel headers.
> >
> > Right, so apps tend to ship their own uapi bpf.h header and helper
> > signatures to avoid these issues. But question becomes once they
> > start using soley bpf_helper.h (also in non-tracing context which
> > is very reasonable to assume), then things might break with the patch
> > as-is once they have a newer libbpf with more signatures than their
> > linux/bpf.h defines (and yes, pulling from GH will have this problem),
> > so we'd need to have an answer to that in order to avoid breaking
> > compilation.
> >
> > [...]
> > > 2. As to the problem of running bleeding-edge libbpf against older
> > > kernel. There are few possible solutions:
> > >
> > > a. we hard-code all those BPF_FUNC_ constants. Super painful and not
> > > nice, but will work.
> > >
> > > b. copy/paste enum bpf_func_id definition into bpf_helpers.h itself
> > > and try to keep it in sync with UAPI. Apart from obvious redundancy
> > > that involves, we also will need to make sure this doesn't conflict
> > > with vmlinux.h, so enum name should be different and each value should
> > > be different (which means some wort of additional prefix or suffix).
> > >
> > > c. BPF UAPI header has __BPF_FUNC_MAPPER macro "iterating" over all
> > > defined values for a particular kernel version. We can use that and
> > > additional macro trickery to conditionally define helpers. Again, we
> > > need to keep in mind that w/ vmlinux.h there is no such macro, so this
> > > should work as well.
> > >
> > > I'm happy to hear opinions about these choices (maybe there are some
> > > other I missed), but in any case I'd like to do it in a follow up
> > > patch and land this one as is. It has already quite a lot packed in
> > > it. I personally lean towards c) as it will have a benefit of not
> > > declaring helpers that are not supported by kernel we are compiling
> > > against, even though it requires additional macro trickery.
> > >
> > > Opinions?
> >
> > Was thinking about something like c) as well. So I tried to do a quick
> > hack. Here is how it could work, but it needs a small change in the
> > __BPF_FUNC_MAPPER(), at least I didn't find an immediate way around it:

Well, we are stuck with this comma, so rather than have to support two
bpf.h headers, I'd solve the problem for existing one. It's annoying,
but you can do it with having "/* <your macro> /*" in each FN macro
and then before you apply everything you add /* and after all the
applications you add */.

I'm going to prototype something like what you have below, but will
see we I can minimize amount of extra declarations we need. Do you
think this needs to be done as part of this patch set, or I can defer
that to a follow up patch?

> >
> > static void (*__unspec)(void);
> > static void *(*__map_lookup_elem)(void *map, const void *key);
> > static int (*__map_update_elem)(void *map, const void *key, const void *value, unsigned long long flags);
> > static int (*__map_delete_elem)(void *map, const void *key);
> > static int (*__bpf_probe_read)(void *dst, int size, const void *unsafe_ptr);
> >

[...]

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