linux-kernel - Re: [PATCH bpf-next] libbpf: Add bpf_object_

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Date:   Wed, 23 Aug 2023 11:08:29 -0700
From:   Song Liu <song@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Xu <dxu@...uu.xyz>, ast@...nel.org, andrii@...nel.org,
        daniel@...earbox.net, martin.lau@...ux.dev,
        yonghong.song@...ux.dev, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] libbpf: Add bpf_object__unpin()

On Wed, Aug 23, 2023 at 10:19 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Aug 22, 2023 at 10:44 PM Daniel Xu <dxu@...uu.xyz> wrote:
> >
> > For bpf_object__pin_programs() there is bpf_object__unpin_programs().
> > Likewise bpf_object__unpin_maps() for bpf_object__pin_maps().
> >
> > But no bpf_object__unpin() for bpf_object__pin(). Adding the former adds
> > symmetry to the API.
> >
> > It's also convenient for cleanup in application code. It's an API I
> > would've used if it was available for a repro I was writing earlier.
> >
> > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> > ---
> >  tools/lib/bpf/libbpf.c   | 15 +++++++++++++++
> >  tools/lib/bpf/libbpf.h   |  1 +
> >  tools/lib/bpf/libbpf.map |  1 +
> >  3 files changed, 17 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 4c3967d94b6d..96ff1aa4bf6a 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -8376,6 +8376,21 @@ int bpf_object__pin(struct bpf_object *obj, const char *path)
> >         return 0;
> >  }
> >
> > +int bpf_object__unpin(struct bpf_object *obj, const char *path)
> > +{
> > +       int err;
> > +
> > +       err = bpf_object__unpin_programs(obj, path);
> > +       if (err)
> > +               return libbpf_err(err);
> > +
> > +       err = bpf_object__unpin_maps(obj, path);
> > +       if (err)
> > +               return libbpf_err(err);
> > +
> > +       return 0;
> > +}
> > +
>
> pin APIs predate me, and I barely ever use them, but I wonder if
> people feel fine with the fact that if any single unpin fails, all the
> other programs/maps will not be unpinned? I also wonder if the best
> effort unpinning of everything (while propagating first/last error) is
> more practical? Looking at bpf_object__pin_programs, we try unpin
> everything, even if some unpins fail.

I think the behavior of bpf_object__pin_programs() makes sense for
the error handling path, e.g. undo everything as much as possible.
bpf_object__unpin(), OTOH, is similar to bpf_object__unpin_programs()
and bpf_object__unpin_maps(), if something went wrong, we just give up.

>
> Any thoughts or preferences?

I think current version is ok, as it matches bpf_object__unpin_programs
and bpf_object__unpin_maps behavior. But I guess we can change all
3?

Thanks,
Song