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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 06 Apr 2020 13:34:32 +0200

Andrii Nakryiko <[email-masked]> writes:

> Add support to look up bpf_link by ID and iterate over all existing bpf_links
> in the system. GET_FD_BY_ID code handles not-yet-ready bpf_link by checking
> that its ID hasn't been set to non-zero value yet. Setting bpf_link's ID is
> done as the very last step in finalizing bpf_link, together with installing
> FD. This approach allows users of bpf_link in kernel code to not worry about
> races between user-space and kernel code that hasn't finished attaching and
> initializing bpf_link.
>
> Further, it's critical that BPF_LINK_GET_FD_BY_ID only ever allows to create
> bpf_link FD that's O_RDONLY. This is to protect processes owning bpf_link and
> thus allowed to perform modifications on them (like LINK_UPDATE), from other
> processes that got bpf_link ID from GET_NEXT_ID API. In the latter case, only
> querying bpf_link information (implemented later in the series) will be
> allowed.

I must admit I remain sceptical about this model of restricting access
without any of the regular override mechanisms (for instance, enforcing
read-only mode regardless of CAP_DAC_OVERRIDE in this series). Since you
keep saying there would be 'some' override mechanism, I think it would
be helpful if you could just include that so we can see the full
mechanism in context.

-Toke