netdev - Re: Question about the release of BPF_MAP_TYPE_STRUCT

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Message-ID: <20210928183319.vd4q5ydqg6twasqj@kafai-mbp>
Date:   Tue, 28 Sep 2021 11:33:19 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Hou Tao <houtao1@...wei.com>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: Question about the release of BPF_MAP_TYPE_STRUCT_OPS fd

On Tue, Sep 28, 2021 at 10:13:45PM +0800, Hou Tao wrote:
> Hi Martin,
> 
> During the testing of bpf_tcp_ca, I found that if the test program
> aborts before calling bpf_link__detach_struct_ops(), the registered
> bpf_dctcp will not be unregistered, and running bpf_tcp_ca test again
> will fail with -EEXIST error as shown below:
> 
> test_dctcp:PASS:bpf_dctcp__open_and_load 0 nsec
> test_dctcp:FAIL:bpf_map__attach_struct_ops unexpected error: -17
> 
> The root cause is that the release of BPF_MAP_TYPE_STRUCT_OPS fd
> neither put struct_ops programs in maps nor unregister the struct_ops
> from kernel. Was the implementation intentional, or was it an oversight ?
> If it is an oversight, I will post a patch to fix it.
The original use case for the struct_ops is to register and unregister
by command like "bpftool struct_ops (un)register" and then
the kernel sub-system (tcp here) owns it, instead of finding where the maps
may be pinned (could be in multiple locations).  More like the insmod/rmmod.

I think you meant to unregister it in ".map_release_uref"? but it will break
the existing contract above.  I believe a better way to do the auto-unreg
is to create a real bpf_link for struct_ops.  bpf_link has been added to
the xdp and cgroup attachments since then.  I did not give too much
thoughts on this yet.

What bpf-tcp-cc are you implementing
or it is for running the test_progs purpose only ?