Re: [PATCH v2 bpf-next 02/12] libbpf: implement BPF CO-RE offset relocation algorithm

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	Yonghong Song <yhs@fb.com>, Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 02/12] libbpf: implement BPF CO-RE offset relocation algorithm
Date: Tue, 30 Jul 2019 18:00:47 -0700	[thread overview]
Message-ID: <CAEf4BzYE9xnyFjmN3+-LgkkOomt383OPNXVhSCO4PncAu20wgw@mail.gmail.com> (raw)
In-Reply-To: <4AB53FC1-5390-4BC7-83B4-7DDBAFD78ABC@fb.com>

On Tue, Jul 30, 2019 at 5:39 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote:
> >
> > This patch implements the core logic for BPF CO-RE offsets relocations.
> > Every instruction that needs to be relocated has corresponding
> > bpf_offset_reloc as part of BTF.ext. Relocations are performed by trying
> > to match recorded "local" relocation spec against potentially many
> > compatible "target" types, creating corresponding spec. Details of the
> > algorithm are noted in corresponding comments in the code.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> > ---
> > tools/lib/bpf/libbpf.c | 915 ++++++++++++++++++++++++++++++++++++++++-
> > tools/lib/bpf/libbpf.h |   1 +
> > 2 files changed, 909 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c

[...]

Please trim irrelevant parts. It doesn't matter with desktop Gmail,
but pretty much everywhere else is very hard to work with.

> > +
> > +     for (i = 1; i < spec->raw_len; i++) {
> > +             t = skip_mods_and_typedefs(btf, id, &id);
> > +             if (!t)
> > +                     return -EINVAL;
> > +
> > +             access_idx = spec->raw_spec[i];
> > +
> > +             if (btf_is_composite(t)) {
> > +                     const struct btf_member *m = (void *)(t + 1);
>
> Why (void *) instead of (const struct btf_member *)? There are a few more
> in the rest of the patch.
>

I just picked the most succinct and non-repetitive form. It's
immediately apparent which type it's implicitly converted to, so I
felt there is no need to repeat it. Also, just (void *) is much
shorter. :)

> > +                     __u32 offset;
> > +
> > +                     if (access_idx >= BTF_INFO_VLEN(t->info))
> > +                             return -EINVAL;
> > +

[...]