HEADS DOWN: dm_target_crypt, tcplay and libdm

News mneumann at ntecs.de
Sat Jul 16 18:00:10 PDT 2011

Wow, awesome!



Alex Hornung <ahornung at gmail.com> schrieb:

>Hi all,
>heads down if you use master: problems (maybe) and new features are
>coming your way!
>Over the past few weeks I've been preparing a major series of commits.
>few weeks ago I updated opencrypto and also added support for twofish
>and serpent ciphers. This was the preliminary work needed for tcplay.
>tcplay is a fully featured BSD-licensed TrueCrypt implementation using
>our dm_target_crypt; it is 100% compatible (with recent versions; older
>versions using aes-lrw and similar are not). It supports cipher
>cascades, hidden volumes, system volumes, etc. I started it as an
>experiment to investigate the TrueCrypt header format, but it ended up
>as a full implementation written from scratch. It now is divided into
>tcplay(8), the tool itself, and libtcplay (tcplay(3)), a very simple
>to mount and unmount TrueCrypt volumes. tcplay is now fully integrated
>into DragonFly, including cryptdisks(8) support and root mount support
>(the realroot type is "tcplay", as documented in mkinitrd(8)).
>WARNING: I've done my best in testing it and its compatibility with
>TrueCrypt, but it might well be that I missed something. For now I
>advise you to treat tcplay as unstable/experimental. If you find out
>that some volume is not compatible but works fine in TrueCrypt, it'd be
>good if you could provide me with a sample volume that behaves like
>so I can dig into it.
>While testing tcplay with cipher cascades, I found a bug in
>dm_target_crypt which affects interdependent/stacked volumes. If one
>crypt volume was dependent on another, a situation could occur where
>volume would starve the other of memory I/O buffers provided by a
>mpipe. To avoid this situation I've changed the mpipes to be
>per-instance, to ensure that every instance always has some I/O buffers
>available. It uses now at most 0.2% of physical memory for each
>instance. Previously it used at most 0.5% of physical memory across all
>instances. This is pretty arbitrary, if someone feels it's too
>small/big, please let me know.
>Since tcplay was intended to be BSD licensed but depended on
>libdevmapper which is GPL-licensed, I decided on Friday to rewrite
>libdevmapper under a BSD license; the result of which is libdm. It's a
>fairly simple implementation but has (almost) all public features of
>libdevmapper. It won't work with stuff like dmsetup and lvm since those
>use all sorts of internal pools and trees that libdm doesn't have, but
>have tested it with both tcplay(8) and cryptsetup(8) and it works like
>charm. By default both tcplay(8) and cryptsetup(8) now use libdm
>of libdevmapper. I also switched over cryptsetup after the testing as
>both libtcplay and libcryptsetup are used in cryptdisks(8) and hence
>both have to use the same backend dm library as the namespace collides.
>WARNING: A word of warning about libdm: I wrote it in just a few hours
>and I bet there are still some minor issues. I've tested the main
>functionality of cryptsetup linked against libdm, but there might be
>some cases left that will behave erratically. If that's the case,
>let me know so I can dig into that.
>Just in case someone wonders, I am fully aware of NetBSD's libdm. I did
>not use it because it has a different API that is not directly
>compatible with linux libdevmapper's and would hence require rewriting
>of libdevmapper consumers instead of a plug-in replacement.
>While on it, I also further simplified some of dm(4). I got rid of
>excessive locking in the I/O path and concurrency and performance under
>heavy use should improve. Previously the I/O strategy path was under
>exclusive lock which is now gone. This also fixes some other issues
>could occur when using remove or remove_all, where remove_all would
>the same lock as the strategy routine required, and removal would
>I plan to write some proper documentation on the whole dm story (lvm,
>cryptsetup, tcplay, etc) but I still haven't found the time to do that.
>Alex Hornung

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