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From: Eric Biggers <ebiggers@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Gao Xiang <gaoxiang25@huawei.com>,
	Dave Chinner <david@fromorbit.com>,
	Goldwyn Rodrigues <RGoldwyn@suse.com>, "hch@lst.de" <hch@lst.de>,
	"darrick.wong@oracle.com" <darrick.wong@oracle.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	"ruansy.fnst@cn.fujitsu.com" <ruansy.fnst@cn.fujitsu.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	linux-erofs@lists.ozlabs.org, miaoxie@huawei.com
Subject: Re: [PATCH 10/13] iomap: use a function pointer for dio submits
Date: Fri, 9 Aug 2019 17:17:54 -0700	[thread overview]
Message-ID: <20190810001753.GE100971@gmail.com> (raw)
In-Reply-To: <20190809204517.GR5482@bombadil.infradead.org>

On Fri, Aug 09, 2019 at 01:45:17PM -0700, Matthew Wilcox wrote:
> On Wed, Aug 07, 2019 at 10:49:36PM -0700, Eric Biggers wrote:
> > On Thu, Aug 08, 2019 at 12:26:42PM +0800, Gao Xiang wrote:
> > >     1. decrypt->verity->decompress
> > > 
> > >     2. verity->decompress->decrypt
> > > 
> > >     3. decompress->decrypt->verity
> > > 
> > >    1. and 2. could cause less computation since it processes
> > >    compressed data, and the security is good enough since
> > >    the behavior of decompression algorithm is deterministic.
> > >    3 could cause more computation.
> > > 
> > > All I want to say is the post process is so complicated since we have
> > > many selection if encryption, decompression, verification are all involved.
> > > 
> > > Maybe introduce a core subset to IOMAP is better for long-term
> > > maintainment and better performance. And we should consider it
> > > more carefully.
> > > 
> > 
> > FWIW, the only order that actually makes sense is decrypt->decompress->verity.
> 
> That used to be true, but a paper in 2004 suggested it's not true.
> Further work in this space in 2009 based on block ciphers:
> https://arxiv.org/pdf/1009.1759
> 
> It looks like it'd be computationally expensive to do, but feasible.
> 
> > Decrypt before decompress, i.e. encrypt after compress, because only the
> > plaintext can be compressible; the ciphertext isn't.

It's an interesting paper, but even assuming that "compress after encrypt" could
provide some actual benefit over the usual order (I can't think of any in this
context), it doesn't sound practical.  From what I understand from that paper:

- It assumes the compressor just *knows* a priori some pattern in the plaintext,
  i.e. it can't be arbitrary data.  E.g. the compressor for CBC encrypted data
  assumes that each 128 bits of plaintext is drawn from a distibution much
  smaller than the 2^128 possible values, e.g. at most a certain number of bits
  are set.  If any other data is encrypted+compressed, then the compressor will
  corrupt it, and it's impossible for it to detect that it did so.

  That alone makes it unusable for any use case we're talking about here.

- It only works for some specific encryption modes, and even then each
  encryption mode needs a custom compression algorithm designed just for it.
  I don't see how it could work for XTS, let alone a wide-block mode.

- The decompressor needs access to the encryption key.  [If that's allowed, why
  can't the compressor have access to it too?]

- It's almost certainly *much* slower and won't compress as well as conventional
  compression algorithms (gzip, LZ4, ZSTD, ...) that operate on the plaintext.

Eric

  parent reply	other threads:[~2019-08-10  0:17 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-02 22:00 [PATCH v2 0/13] Btrfs iomap Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 01/13] iomap: Use a IOMAP_COW/srcmap for a read-modify-write I/O Goldwyn Rodrigues
2019-08-03  0:39   ` Darrick J. Wong
2019-08-05  0:06   ` Dave Chinner
2019-08-02 22:00 ` [PATCH 02/13] iomap: Read page from srcmap for IOMAP_COW Goldwyn Rodrigues
2019-08-03  0:23   ` Darrick J. Wong
2019-08-04 23:52   ` Dave Chinner
2019-08-02 22:00 ` [PATCH 03/13] btrfs: Eliminate PagePrivate for btrfs data pages Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 04/13] btrfs: Add a simple buffered iomap write Goldwyn Rodrigues
2019-08-05  0:11   ` Dave Chinner
2019-08-22 15:05     ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 05/13] btrfs: Add CoW in iomap based writes Goldwyn Rodrigues
2019-08-05  0:13   ` Dave Chinner
2019-08-22 15:01     ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 06/13] btrfs: remove buffered write code made unnecessary Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 07/13] btrfs: basic direct read operation Goldwyn Rodrigues
2019-08-12 12:32   ` RITESH HARJANI
2019-08-22 15:00     ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 08/13] btrfs: Carve out btrfs_get_extent_map_write() out of btrfs_get_blocks_write() Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 09/13] btrfs: Rename __endio_write_update_ordered() to btrfs_update_ordered_extent() Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 10/13] iomap: use a function pointer for dio submits Goldwyn Rodrigues
2019-08-03  0:21   ` Darrick J. Wong
2019-08-05 16:08     ` Goldwyn Rodrigues
2019-08-04 23:43   ` Dave Chinner
2019-08-05 16:08     ` Goldwyn Rodrigues
2019-08-05 21:54       ` Dave Chinner
2019-08-08  4:26         ` Gao Xiang
2019-08-08  4:52           ` Gao Xiang
2019-08-08  5:49           ` Eric Biggers
2019-08-08  6:28             ` Gao Xiang
2019-08-08  8:16             ` Dave Chinner
2019-08-08  8:57               ` Gao Xiang
2019-08-08  9:29               ` Gao Xiang
2019-08-08 11:21                 ` Gao Xiang
2019-08-08 13:11                   ` Gao Xiang
2019-08-09 20:45             ` Matthew Wilcox
2019-08-09 23:45               ` Gao Xiang
2019-08-10  0:31                 ` Eric Biggers
2019-08-10  0:50                   ` Eric Biggers
2019-08-10  1:34                     ` Gao Xiang
2019-08-10  1:13                   ` Gao Xiang
2019-08-10  0:17               ` Eric Biggers [this message]
2019-08-02 22:00 ` [PATCH 11/13] btrfs: Use iomap_dio_rw for performing direct I/O writes Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 12/13] btrfs: Remove btrfs_dio_data and __btrfs_direct_write Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 13/13] btrfs: update inode size during bio completion Goldwyn Rodrigues

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