[Gluster-devel] Handling EOBs in CloudFS

[Devon Miller]

How would you handle ->lseek()? Not so much of an issue for Approach 1, but
with interleaved HMACs...

On Thu, Jul 14, 2011 at 5:01 PM, Edward Shishkin <edward at redhat.com> wrote:

> Hello everyone,
> any comments, suggestions are welcome..
>
>
>
>       Handling EOBs (end-of-blocks) for transparent
>    encryption, checking integrity and data authentication
>
>                          DRAFT
>
>
>
> This was designed for CloudFS, which uses 2-level protocol (high and
> low) supported by xlators which reside on server and client sides
> respectively.
>
>
> Definition of EOB. Storage class
>
>
> If file size isn't a multiple of cblock (cipher block) size, then we
> also need to store special padding needed to decrypt its last block
> with some cipher modes like CBC. This padding contains a part of
> ciphertext and must be considered as a part of this file. We'll call
> this padding end-of-file (EOF). If plain text has size a multiple of
> cblock size, then encrypted file won't have (or will have empty) EOF.
>
> Signatures (HMACs, etc) for checking integrity, data authentication,
> etc. have the same nature as EOF. Every such signature is created
> for some logical block in a file. This is not a padding though, as
> in the case of EOF, but anyway such signatures are associated with
> file's data, and we'll consider a class of object, which includes
> EOFs, HMACs, etc, and call them EOBs (end-of-block).
>
> We define storage class of EOBs as "data", i.e. this can be considered
> as part of file's data: we can not read/write data block without
> reading/writing its EOB.
>
>
> Storing EOBs. Approaches and Issues
>
>
> Approach 1: Storing EOBs as xattr values.
>
>
> In this case we store a file in parts which are not adjacent
> from the standpoint of Cloudfs. That said we need to split
> read, and this makes this operation inatomic. This means
> that read(2) will return data compound of parts of different
> "versions".
>
> Example:
>
> Suppose we have a file F stored in 2 different parts F1 and F2.
>
> Process A writes a file F (to be of version 1);
> Process B reads a file F (part F1);
> Process C writes a file F (to be of version 2);
> Process B reads a file F (part F2);
>
> As the result process B returns data compound of
> parts of different versions 1 and 2.
>
> This non-atomicity is different from the non-atomicity that takes
> place in the kernel (local file systems): kernel guarantees
> that all PAGE_SIZE reads with PAGE_SIZE-aligned offsets are
> atomic (this is because reads and writes in kernel acquire
> page locks). Whereas, in our case we'll have that F2 doesn't
> necessarily have PAGE_SIZE-aligned offset.
>
> That said it can happen that we'll get complaints from users,
> who don't expect such non-atomicity. Moreover, in the case when
> EOBs are HMACs for checking integrity, or authentication we'll
> have false positives, as nobody guarantees that versions of HMAC
> and respective data block will coincide.
>
> Solution:
>
> In this approach we need to serialize truncates, appending
> writes and sequences RbRe (read block, read EOB).
>
>
> Approach 2: Storing in file's body.
>
>
> In this case EOBs are stored in file's body (via appending to
> a file in the case of EOF, or interspacing a file with HMACs,
> etc). So file with his EOBs is the whole from the standpoint
> of Cloudfs, and there is no problems with atomicity specific
> to Approach 1.
>
> However, in this case all our files maintained by low-level
> local fs will have increased sizes (added total size of all EOBs).
> So that actual file size must be stored as additional attribute
> (e.g. as xattr value).
>
> ->open() method of the high-level translator loads actual
> file size to the cloudfs-specific part of inode via fetching
> ->getxattr(), so that it is persistent in the memory on server.
>
> Any ->truncate() and appending ->write() of the high-level
> xlator update in-core and on-disk actual sizes simultaneously
> (via fetching ->setxattr() for the last one). This actual size
> is what should be returned to user by ->fstat(), ->lookup(),
> etc. as st_size.
>
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