Re: [PATCH bpf-next v4 08/11] libbpf: Update gen_loader to emit BTF_KIND_FUNC relocations

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, "Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Network Development" <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf-next v4 08/11] libbpf: Update gen_loader to emit BTF_KIND_FUNC relocations
Date: Tue, 21 Sep 2021 10:20:22 +0530	[thread overview]
Message-ID: <20210921045022.s5mofmkgrkh6inva@apollo.localdomain> (raw)
In-Reply-To: <CAADnVQKjoCLNYBwDvLjgG9cYxrZyhw1Bgvm0yzH0gUWQLNtZnw@mail.gmail.com>

On Tue, Sep 21, 2021 at 06:27:16AM IST, Alexei Starovoitov wrote:
> On Mon, Sep 20, 2021 at 7:15 AM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
> >
> > This change updates the BPF syscall loader to relocate BTF_KIND_FUNC
> > relocations, with support for weak kfunc relocations. The next commit
> > adds bpftool supports to set up the fd_array_sz parameter for light
> > skeleton.
> >
> > A second map for keeping fds is used instead of adding fds to existing
> > loader.map because of following reasons:
>
> but it complicates signing bpf progs a lot.
>

Can you explain this in short? (Just want to understand why it would be
problem).

> > If reserving an area for map and BTF fds, we would waste the remaining
> > of (MAX_USED_MAPS + MAX_KFUNC_DESCS) * sizeof(int), which in most cases
> > will be unused by the program. Also, we must place some limit on the
> > amount of map and BTF fds a program can possibly open.
>
> That is just (256 + 64)*4 bytes of data. Really not much.
> I wouldn't worry about reserving this space.
>

Ok, I'll probably go with this now, I didn't realise a separate fd would be
prohibitive for the signing case, so I thought it would nice to lift the
limiation on number of map_fds by packing fd_array fds in another map.

> > If setting gen->fd_array to first map_fd offset, and then just finding
> > the offset relative to this (for later BTF fds), such that they can be
> > packed without wasting space, we run the risk of unnecessarily running
> > out of valid offset for emit_relo stage (for kfuncs), because gen map
> > creation and relocation stages are separated by other steps that can add
> > lots of data (including bpf_object__populate_internal_map). It is also
> > prone to break silently if features are added between map and BTF fd
> > emits that possibly add more data (just ~128KB to break BTF fd, since
> > insn->off allows for INT16_MAX (32767) * 4 bytes).
>
> I don't follow this logic.
>
> > Both of these issues are compounded by the fact that data map is shared
> > by all programs, so it is easy to end up with invalid offset for BTF fd.
>
> I don't follow this either. There is only one map and one program.
> What sharing are you talking about?

What I saw was that the sequence of calls is like this:
bpf_gen__map_create
add_data - from first emit we add map_fd, we also store gen->fd_array
then libbpf would call bpf_object__populate_internal_map
which calls bpf_gen__map_update_elem, which also does add_data (can be of
arbitrary sizes).

emit_relos happens relatively at the end.
For each program in the object, this sequence can be repeated, such that the
add_data that we do in emit_relos, relative offset from gen->fd_array offset
can end up becoming big enough (as all programs in object add data to same map),
while gen->fd_array comes from first map creation.

However reserving an area works ok (like with the stack).
Thanks.

--
Kartikeya