Re: [PATCH bpf v2 0/6] Fix attach / detach uapi for sockmap and flow_dissector

[Yonghong Song <yhs@fb.com>]

On 6/29/20 2:56 AM, Lorenz Bauer wrote:
> Both sockmap and flow_dissector ingnore various arguments passed to
> BPF_PROG_ATTACH and BPF_PROG_DETACH. We can fix the attach case by
> checking that the unused arguments are zero. I considered requiring
> target_fd to be -1 instead of 0, but this leads to a lot of churn
> in selftests. There is also precedent in that bpf_iter already
> expects 0 for a similar field. I think that we can come up with a

Since bpf_iter is mentioned here, I would like to provide a little
context on how target_fd in link_create is treated there.

Currently, target_fd is always 0 as it is not used. This is
just easier if we want to use it in the future.

In the future, bpf_iter can maintain that target_fd must be 0
or it may not so. For example, it can add a flag value in
link_create such that when flag is set it will take whatever
value in target_fd and use it. Or it may just take a non-0
target_fd as an indication of the flag is set. I have not
finalized patches yet. I intend to do the latter, i.e.,
taking a non-0 target_fd. But we will see once my bpf_iter
patches for map elements are out.

There is another example where 0 and non-0 prog_fd make a difference.
The attach_prog_fd field when doing prog_load.
When attach_prog_fd is 0, it means attaching to vmlinux through
attach_btf_id. If attach_prog_fd is not 0, it means attaching to
another bpf program (replace). So user space (libbpf) may
already need to pay attention to this.

> work around for fd 0 should we need to in the future.
> 
> The detach case is more problematic: both cgroups and lirc2 verify
> that attach_bpf_fd matches the currently attached program. This
> way you need access to the program fd to be able to remove it.
> Neither sockmap nor flow_dissector do this. flow_dissector even
> has a check for CAP_NET_ADMIN because of this. The patch set
> addresses this by implementing the desired behaviour.
> 
> There is a possibility for user space breakage: any callers that
> don't provide the correct fd will fail with ENOENT. For sockmap
> the risk is low: even the selftests assume that sockmap works
> the way I described. For flow_dissector the story is less
> straightforward, and the selftests use a variety of arguments.
> 
> I've includes fixes tags for the oldest commits that allow an easy
> backport, however the behaviour dates back to when sockmap and
> flow_dissector were introduced. What is the best way to handle these?
> 
> This set is based on top of Jakub's work "bpf, netns: Prepare
> for multi-prog attachment" available at
> https://lore.kernel.org/bpf/87k0zwmhtb.fsf@cloudflare.com/T/
> 
> Since v1:
> - Adjust selftests
> - Implement detach behaviour
> 
> Lorenz Bauer (6):
>    bpf: flow_dissector: check value of unused flags to BPF_PROG_ATTACH
>    bpf: flow_dissector: check value of unused flags to BPF_PROG_DETACH
>    bpf: sockmap: check value of unused args to BPF_PROG_ATTACH
>    bpf: sockmap: require attach_bpf_fd when detaching a program
>    selftests: bpf: pass program and target_fd in flow_dissector_reattach
>    selftests: bpf: pass program to bpf_prog_detach in flow_dissector
> 
>   include/linux/bpf-netns.h                     |  5 +-
>   include/linux/bpf.h                           | 13 ++++-
>   include/linux/skmsg.h                         | 13 +++++
>   kernel/bpf/net_namespace.c                    | 22 ++++++--
>   kernel/bpf/syscall.c                          |  6 +--
>   net/core/sock_map.c                           | 53 +++++++++++++++++--
>   .../selftests/bpf/prog_tests/flow_dissector.c |  4 +-
>   .../bpf/prog_tests/flow_dissector_reattach.c  | 12 ++---
>   8 files changed, 103 insertions(+), 25 deletions(-)
>