[GSoC] Implement xattr Support Update

[Shengyu Zhang]

Hi all,

GSoC is coming to an end. It is time to report my work during this summer.

As I said in my proposal, my project has three goals:

- Implement xattr support for ext2fs
- Using xattr to store passive translator
- Port xattr tools to hurd

Now, the first two goals are achieved, I had sumitted two patches for them.
the left work is in progress, I will finish it after gsoc in spare time.

# Implement xattr support for ext2fs

After some discussion[1], we use namespace "gnu." to storing 
hurd-spceifed meta
message. (The anohter reason that doesn't mentioned in aboved discussion is
that: this namespace is already used in glibc[2])

The work about xattr support is based on Cascardo's previous patch[3].
Current ext2fs only supports two xattr namespaces: "user." and "gnu.",
It is no diffcult to add more namespaces, we only add what we need ;-P.

# Using xattr to store passive translator

After implement the xattr support, I modified `diskfs_set_translator' and
`diskfs_get_translator' (functions in ext2fs/inode.c) to add xattr support:

- When setting a passive translator, if xattr is supported on current 
filesystem,
  passive translator will be stored as a xattr value with name 
"gnu.translator".
  Otherwise, the "leagcy translator" will be created.
- When reading a passive translator, if "leagcy translator" was found, 
take it out.
  Otherwise, try to find xattr value with name "gnu.translator"

Note: Although xattr name&value can be created/modified on other system
(such as Linux), passive translator should only work on hurd-created
filesytem (it was discussed in #hurd[4]).

# Port xattr tools to hurd

Until now, libdiskfs & glibc interface has roughly finished.


My code hasn't been merged to upstream yet, you can check them in the 
following
repositories:

https://github.com/lastavenger/gsoc-2016
https://github.com/lastavenger/hurd
https://github.com/lastavenger/glibc

At last, Thank to all the people who gave me helps, especially my two 
mentors
braunr and teythoon :-).

[1] http://lists.gnu.org/archive/html/bug-hurd/2016-07/msg00014.html
[2] 
http://git.savannah.gnu.org/cgit/hurd/glibc.git/tree/hurd/xattr.c?h=baseline#n27
[3] https://savannah.gnu.org/patch/?5126
[4] http://richtlijn.be/~larstiq/hurd/hurd-2016-08-08

--
Best regards
Shengyu Zhang