Re: [RFC PATCH bpf-next 4/8] bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [RFC PATCH bpf-next 4/8] bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link
Date: Tue, 14 Apr 2020 12:32:04 +0200	[thread overview]
Message-ID: <87mu7enysb.fsf@toke.dk> (raw)
In-Reply-To: <CAEf4BzbXCsHCJ6Tet0i5g=pKB_uYqvgiaBNuY-NMdZm8rdZN5g@mail.gmail.com>

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

>> > After that, one can pin bpf_link temporarily and re-open it as
>> > writable one, provided CAP_DAC_OVERRIDE capability is present. All
>> > that works already, because pinned bpf_link is just a file, so one can
>> > do fchmod on it and all that will go through normal file access
>> > permission check code path.
>>
>> Ah, I did not know that was possible - I was assuming that bpffs was
>> doing something special to prevent that. But if not, great!
>>
>> > Unfortunately, just re-opening same FD as writable (which would
>> > be possible if fcntl(fd, F_SETFL, S_IRUSR
>> >  S_IWUSR) was supported on Linux) without pinning is not possible.
>> > Opening link from /proc/<pid>/fd/<link-fd> doesn't seem to work
>> > either, because backing inode is not BPF FS inode. I'm not sure, but
>> > maybe we can support the latter eventually. But either way, I think
>> > given this is to be used for manual troubleshooting, going through few
>> > extra hoops to force-detach bpf_link is actually a good thing.
>>
>> Hmm, I disagree that deliberately making users jump through hoops is a
>> good thing. Smells an awful lot like security through obscurity to me;
>> and we all know how well that works anyway...
>
> Depends on who users are? bpftool can implement this as one of
> `bpftool link` sub-commands and allow human operators to force-detach
> bpf_link, if necessary.

Yeah, I would expect this to be the common way this would be used: built
into tools.

> I think applications shouldn't do this (programmatically) at all,
> which is why I think it's actually good that it's harder and not
> obvious, this will make developer think again before implementing
> this, hopefully. For me it's about discouraging bad practice.

I guess I just don't share your optimism that making people jump through
hoops will actually discourage them :)

If people know what they are doing it should be enough to document it as
discouraged. And if they don't, they are perfectly capable of finding
and copy-pasting the sequence of hoop-jumps required to achieve what
they want, probably with more bugs added along the way.

So in the end I think that all you're really achieving is annoying
people who do have a legitimate reason to override the behaviour (which
includes yourself as a bpftool developer :)). That's what I meant by the
'security through obscurity' comment.

-Toke