linux-kernel - Re: [RFC PATCH bpf-next 00/13] bpf: Introduce BPF namespace

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Date:   Wed, 5 Apr 2023 20:05:50 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Song Liu <song@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 00/13] bpf: Introduce BPF namespace

On Wed, Apr 5, 2023 at 7:55 PM Yafang Shao <laoar.shao@...il.com> wrote:
>
> It seems that I didn't describe the issue clearly.
> The container doesn't have CAP_SYS_ADMIN, but the CAP_SYS_ADMIN is
> required to run bpftool,  so the bpftool running in the container
> can't get the ID of bpf objects or convert IDs to FDs.
> Is there something that I missed ?

Nothing. This is by design. bpftool needs sudo. That's all.


>
> > > --- a/kernel/bpf/syscall.c
> > > +++ b/kernel/bpf/syscall.c
> > > @@ -3705,9 +3705,6 @@ static int bpf_obj_get_next_id(const union bpf_attr *attr,
> > >         if (CHECK_ATTR(BPF_OBJ_GET_NEXT_ID) || next_id >= INT_MAX)
> > >                 return -EINVAL;
> > >
> > > -       if (!capable(CAP_SYS_ADMIN))
> > > -               return -EPERM;
> > > -
> > >         next_id++;
> > >         spin_lock_bh(lock);
> > >         if (!idr_get_next(idr, &next_id))
> > >
> > > Because the container doesn't have CAP_SYS_ADMIN enabled, while they
> > > only have CAP_BPF and other required CAPs.
> > >
> > > Another possible solution is that we run an agent in the host, and the
> > > user in the container who wants to get the bpf objects info in his
> > > container should send a request to this agent via unix domain socket.
> > > That is what we are doing now in our production environment.  That
> > > said, each container has to run a client to get the bpf object fd.
> >
> > None of such hacks are necessary. People that debug bpf setups with bpftool
> > can always sudo.
> >
> > > There are some downsides,
> > > -  It can't handle pinned bpf programs
> > >    For pinned programs, the user can get them from the pinned files
> > > directly, so he can use bpftool in his case, only with some
> > > complaints.
> > > -  If the user attached the bpf prog, and then removed the pinned
> > > file, but didn't detach it.
> > >    That happened. But this error case can't be handled.
> > > - There may be other corner cases that it can't fit.
> > >
> > > There's a solution to improve it, but we also need to change the
> > > kernel. That is, we can use the wasted space btf->name.
> > >
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index b7e5a55..59d73a3 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -5542,6 +5542,8 @@ static struct btf *btf_parse(bpfptr_t btf_data,
> > > u32 btf_data_size,
> > >                 err = -ENOMEM;
> > >                 goto errout;
> > >         }
> > > +       snprintf(btf->name, sizeof(btf->name), "%s-%d-%d", current->comm,
> > > +                        current->pid, cgroup_id(task_cgroup(p, cpu_cgrp_id)));
> >
> > Unnecessary.
> > comm, pid, cgroup can be printed by bpftool without changing the kernel.
>
> Some questions,
> - What if the process exits after attaching the bpf prog and the prog
> is not auto-detachable?
>   For example, the reuserport bpf prog is not auto-detachable. After
> pins the reuserport bpf prog, a task can attach it through the pinned
> bpf file, but if the task forgets to detach it and the pinned file is
> removed, then it seems there's no way to figure out which task or
> cgroup this prog belongs to...

you're saying that there is a bpf prog in the kernel without
corresponding user space ? Meaning no user space process has an FD
that points to this prog or FD to a map that this prog is using?
In such a case this is truly kernel bpf prog. It doesn't belong to cgroup.

> - Could you pls. explain in detail how to get comm, pid, or cgroup
> from a pinned bpffs file?

pinned bpf prog and no user space holds FD to it?
It's not part of any cgroup. Nothing to print.