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Subject: Re: [PATCH 1/2] Revert "xfrm: interface with if_id 0 should return
 error"
To:     Eyal Birger <eyal.birger@gmail.com>
Cc:     netdev@vger.kernel.org,
        Steffen Klassert <steffen.klassert@secunet.com>
References: <20220301131512.1303-1-kailueke@linux.microsoft.com>
 <CAHsH6Gtzaf2vhSv5sPpBBhBww9dy8_E7c0utoqMBORas2R+_zg@mail.gmail.com>
From:   =?UTF-8?B?S2FpIEzDvGtl?= <kailueke@linux.microsoft.com>
Message-ID: <d5e58052-86df-7ffa-02a0-fc4db5a7bbdf@linux.microsoft.com>
Date:   Tue, 1 Mar 2022 15:17:23 +0100
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Hi,
> Whereas 8dce43919566 ("xfrm: interface with if_id 0 should return error")
> involves xfrm interfaces which don't appear in the pull request.
>
> In which case, why should that commit be reverted?

Correct me if I misunderstood this but reading the commit message it is
explicitly labeled as a behavior change for userspace:

    With this commit:
     ip link add ipsec0  type xfrm dev lo  if_id 0
     Error: if_id must be non zero.

Changing behavior this way is from my understanding a regression because
it breaks programs that happened to work before, even if they worked
incorrect (cf. https://lwn.net/Articles/726021/ "The current process for
Linux development says that kernel patches cannot break programs that
rely on the ABI. That means a program that runs on the 4.0 kernel should
be able to run on the 5.0 kernel, Levin said.").

Regards,
Kai