netdev - Re: [PATCH net] bpf: expose netns inode to bpf programs

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Date:   Thu, 26 Jan 2017 10:12:28 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <ast@...com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsa@...ulusnetworks.com>,
        Tejun Heo <tj@...nel.org>, Thomas Graf <tgraf@...g.ch>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] bpf: expose netns inode to bpf programs

On Thu, Jan 26, 2017 at 9:46 AM, Alexei Starovoitov <ast@...com> wrote:
> On 1/26/17 8:37 AM, Andy Lutomirski wrote:
>>>
>>> Think of bpf programs as safe kernel modules. They don't have
>>> confined boundaries and program authors, if not careful, can shoot
>>> themselves in the foot. We're not trying to prevent that because
>>> it's impossible to check that the program is sane. Just like
>>> it's impossible to check that kernel module is sane.
>>> But in case of bpf we check that bpf program is _safe_ from the kernel
>>> point of view. If it's doing some garbage, it's program's business.
>>> Does it make more sense now?
>>>
>>
>> With all due respect, I think this is not an acceptable way to think
>> about BPF at all.  If you think of BPF this way, I think there needs
>> to be a real discussion at KS or similar as to whether this is okay.
>> The reason is simple: the kernel promises a stable ABI to userspace
>> but not to kernel modules.  By thinking of BPF as more like a module,
>> you're taking a big shortcut that will either result in ABI breakage
>> down the road or in committing to a problematic stable ABI.
>
>
> you misunderstood the analogy.
> bpf abi is certainly stable. that's why we were careful of not
> exposing anything to it that is not already stable.
>

In that case I don't understand what you're trying to say.  Eric
thinks your patch exposes a bad interface.  A bad interface for
userspace is a very different thing from a bad interface available to
kernel modules.  Are you saying that BPF is kernel-module-like in that
the ABI exposed to BPF programs doesn't need to meet the same quality
standards as userspace ABIs?

--Andy