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Date:   Fri, 21 Dec 2018 11:28:13 -0500
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Eric Biggers <ebiggers@kernel.org>,
        linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Victor Hsieh <victorhsieh@google.com>,
        Chandan Rajendra <chandan@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 01/12] fs-verity: add a documentation file
Message-ID: <20181221162813.GB26547@mit.edu>
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        Chandan Rajendra <chandan@linux.vnet.ibm.com>,
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On Fri, Dec 21, 2018 at 07:53:54AM -0800, Matthew Wilcox wrote:
> In contrast to "we'll just fix it up later" (which usually applies
> to in-kernel interfaces), we have a policy of not breaking userspace,
> so accepting this interface means setting it in stone.  We should get
> it right.

I'm not convinced it's a "fix", but my point is that if later on you
want to add extra complexity transforming

     ioctl(fd, FS_IOC_ENABLE_VERITY);

so it does the equivalent of

     ioctl(fd, FS_IOC_ENABLE_VERITY_NOW_WITH_EXTRA_USELESS_COMPLEXITY,
	   fd, sizeof_data, sizeof_verity_data);

it adds essentially no complexity to provide this backwards
compatibility.  But if we need to implement
FS_IOC_ENABLE_VERITY_NOW_WITH_EXTRA_USELESS_COMPLEXITY *now*, we gain
nothing, other than pushing back when fsverity lands upstream.  We'd
have to provide that backwards compatibility interface anyway, since
there are a lot of users for that existing interface.

So why?

				- Ted