linux-kernel - Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get

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Date:   Wed, 20 Jul 2022 16:26:26 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     Joe Burton <jevburton.kernel@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Joe Burton <jevburton@...gle.com>
Subject: Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get_opts()

On Wed, Jul 20, 2022 at 4:17 PM Roberto Sassu <roberto.sassu@...wei.com> wrote:
>
> > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > Sent: Thursday, July 21, 2022 1:15 AM
> > On Wed, Jul 20, 2022 at 4:12 PM Roberto Sassu <roberto.sassu@...wei.com>
> > wrote:
> > >
> > > > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > > > Sent: Thursday, July 21, 2022 1:09 AM
> > > > On Wed, Jul 20, 2022 at 4:02 PM Roberto Sassu
> > <roberto.sassu@...wei.com>
> > > > wrote:
> > > > >
> > > > > > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > > > > > Sent: Thursday, July 21, 2022 12:48 AM
> > > > > > On Wed, Jul 20, 2022 at 3:44 PM Roberto Sassu
> > > > <roberto.sassu@...wei.com>
> > > > > > wrote:
> > > > > > >
> > > > > > > > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > > > > > > > Sent: Thursday, July 21, 2022 12:38 AM
> > > > > > > > On Wed, Jul 20, 2022 at 3:30 PM Roberto Sassu
> > > > > > <roberto.sassu@...wei.com>
> > > > > > > > wrote:
> > > > > > > > >
> > > > > > > > > > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > > > > > > > > > Sent: Wednesday, July 20, 2022 5:57 PM
> > > > > > > > > > On Wed, Jul 20, 2022 at 1:02 AM Roberto Sassu
> > > > > > > > <roberto.sassu@...wei.com>
> > > > > > > > > > wrote:
> > > > > > > > > > >
> > > > > > > > > > > > From: Stanislav Fomichev [mailto:sdf@...gle.com]
> > > > > > > > > > > > Sent: Tuesday, July 19, 2022 10:40 PM
> > > > > > > > > > > > On Tue, Jul 19, 2022 at 12:40 PM Joe Burton
> > > > > > > > <jevburton.kernel@...il.com>
> > > > > > > > > > > > wrote:
> > > > > > > > > > > > >
> > > > > > > > > > > > > From: Joe Burton <jevburton@...gle.com>
> > > > > > > > > > > > >
> > > > > > > > > > > > > Add an extensible variant of bpf_obj_get() capable of setting
> > the
> > > > > > > > > > > > > `file_flags` parameter.
> > > > > > > > > > > > >
> > > > > > > > > > > > > This parameter is needed to enable unprivileged access to
> > BPF
> > > > > > maps.
> > > > > > > > > > > > > Without a method like this, users must manually make the
> > > > syscall.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Signed-off-by: Joe Burton <jevburton@...gle.com>
> > > > > > > > > > > >
> > > > > > > > > > > > Reviewed-by: Stanislav Fomichev <sdf@...gle.com>
> > > > > > > > > > > >
> > > > > > > > > > > > For context:
> > > > > > > > > > > > We've found this out while we were trying to add support for
> > > > unpriv
> > > > > > > > > > > > processes to open pinned r-x maps.
> > > > > > > > > > > > Maybe this deserves a test as well? Not sure.
> > > > > > > > > > >
> > > > > > > > > > > Hi Stanislav, Joe
> > > > > > > > > > >
> > > > > > > > > > > I noticed now this patch. I'm doing a broader work to add opts
> > > > > > > > > > > to bpf_*_get_fd_by_id(). I also adjusted permissions of bpftool
> > > > > > > > > > > depending on the operation type (e.g. show, dump:
> > > > BPF_F_RDONLY).
> > > > > > > > > > >
> > > > > > > > > > > Will send it soon (I'm trying to solve an issue with the CI, where
> > > > > > > > > > > libbfd is not available in the VM doing actual tests).
> > > > > > > > > >
> > > > > > > > > > Is something like this patch included in your series as well? Can
> > you
> > > > > > > > > > use this new interface or do you need something different?
> > > > > > > > >
> > > > > > > > > It is very similar. Except that I called it bpf_get_fd_opts, as it
> > > > > > > > > is shared with the bpf_*_get_fd_by_id() functions. The member
> > > > > > > > > name is just flags, plus an extra u32 for alignment.
> > > > > > > >
> > > > > > > > We can bikeshed the naming, but we've been using existing
> > conventions
> > > > > > > > where opts fields match syscall fields, that seems like a sensible
> > > > > > > > thing to do?
> > > > > > >
> > > > > > > The only problem is that bpf_*_get_fd_by_id() functions would
> > > > > > > set the open_flags member of bpf_attr.
> > > > > > >
> > > > > > > Flags would be good for both, even if not exact. Believe me,
> > > > > > > duplicating the opts would just create more confusion.
> > > > > >
> > > > > > Wait, that's completely different, right? We are talking here about
> > > > > > BPF_OBJ_GET (which has related BPF_OBJ_PIN).
> > > > > > Your GET_XXX_BY_ID are different so you'll still have to have another
> > > > > > wrapper with opts?
> > > > >
> > > > > Yes, they have different wrappers, just accept the same opts as
> > > > > obj_get(). From bpftool subcommands you want to set the correct
> > > > > permission, and propagate it uniformly to bpf_*_get_fd_by_id()
> > > > > or obj_get(). See map_parse_fds().
> > > >
> > > > I don't think they are accepting the same opts.
> > > >
> > > > For our case, we care about:
> > > >
> > > >         struct { /* anonymous struct used by BPF_OBJ_* commands */
> > > >                 __aligned_u64   pathname;
> > > >                 __u32           bpf_fd;
> > > >                 __u32           file_flags;
> > > >         };
> > > >
> > > > For your case, you care about:
> > > >
> > > >         struct { /* anonymous struct used by BPF_*_GET_*_ID */
> > > >                 union {
> > > >                         __u32           start_id;
> > > >                         __u32           prog_id;
> > > >                         __u32           map_id;
> > > >                         __u32           btf_id;
> > > >                         __u32           link_id;
> > > >                 };
> > > >                 __u32           next_id;
> > > >                 __u32           open_flags;
> > > >         };
> > > >
> > > > So your new _opts libbpf routine should be independent of what Joe is
> > > > doing here.
> > >
> > > It is. Just I use the same opts to set file_flags or open_flags.
> >
> > That seems confusing. Let's have separate calls for separate syscall
> > commands as we do already?
>
> Can you wait one day, I send what I have, so that we see
> everything together?

Sure, CC us both on the patches.