linux-ext4 - Re: Proposal: A new fs-verity interface

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Date:   Thu, 24 Jan 2019 16:32:17 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: Proposal: A new fs-verity interface

On Thu, Jan 24, 2019 at 06:22:37PM -0500, Theodore Y. Ts'o wrote:
> The main issue is that for a 129 MB file, the Merkle data is going to
> be a Megabyte.

127MB ... I pointed out this error the last time the documentation
was posted.

> We could store the metadata somewhere else --- for example, we could
> store it in another inode.  But inodes have overhead, and that would
> mean using two inodes for every fs-verity protected files --- and we
> don't need all of the other metadata (mtime, ctime, etc.) for the
> Merkle tree.  So that's how we got to where we were.  I think the
> approach of storing it using the same extent tree where we map logical
> block numbers to physical block numbers make a lot of sense for ext4
> and f2fs.
> 
> It seems that some file system (which may never even implement
> fs-verity) their developers hate that particular approach.  So that's
> where the suggestion of using a separate file descriptor to convey the
> Merkle tree data to the file system came from.  It wasn't my first
> choice.

I'll reiterate an API I suggested on December 21st:

: verity_fd = ioctl(fd, FS_IOC_VERITY_FD);
: write(verity_fd, &merkle_tree);
: close(verity_fd);
: 
: At final close of that verity_fd, the filesystem behaves in the same way
: that it does on receipt of this FS_IOC_ENABLE_VERITY ioctl today.