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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 26 Oct 2018 13:42:01 -0700
Message-ID: <CAHk-=whazvE2VnJiSFeQ_BzU2iUUt=v8BxhgJyq92ecDayJ14g@mail.gmail.com>
Subject: Re: [PATCH] libosd: Remove ignored __weak attribute
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     bvanassche@acm.org, ooo@electrozaur.com, jejb@linux.vnet.ibm.com,
        martin.petersen@oracle.com, linux-scsi@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        Kees Cook <keescook@google.com>
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On Fri, Oct 26, 2018 at 1:06 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Is removing a filesystem considered a userspace breakage?

Yes - if a user notices.

The key word is *USER*.

Note that it's not "user space". It's not about _programs_ noticing,
it's literally about users and their workflows.

If some change breaks a real user workflow, it needs to be reverted.

So this is not about ABI or anything like that. We've had cases where
the ABI stayed the same, but the order of device probing changed, and
that broke peoples setups (because now /dev/sdb and /dev/sda switched
places), and we had to revert.

It's literally about "if a user upgrades a kernel, and something no
longer works, it's a regression".

In general, a good idea is "if you have to wonder about it, just don't
do it".  Because it turns out that users are odd, and often do odd
things much after you'd have thought they'd have long since switched
to more modern hardware or filesystems.

                      Linus