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

Andrii Nakryiko
2019-07-31T06:52:47Z

On Tue, Jul 30, 2019 at 10:19 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 30, 2019, at 6:00 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > 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.
>
> This won't be a problem if the patch is shorter. ;)
>
> >
> >>> +
> >>> + 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. :)
>
> _All_ other code in btf.c converts the pointer to the target type.

Most in libbpf.c doesn't, though. Also, I try to preserve pointer
constness for uses that don't modify BTF types (pretty much all of
them in libbpf), so it becomes really verbose, despite extremely short
variable names:

const struct btf_member *m = (const struct btf_member *)(t + 1);

Add one or two levels of nestedness and you are wrapping this line.

> In some cases, it is not apparent which type it is converted to,
> for example:
>
> + m = (void *)(targ_type + 1);
>
> I would suggest we do implicit conversion whenever possible.

Implicit conversion (`m = targ_type + 1;`) is a compilation error,
that won't work.

>
> Thanks,
> Song