Re: Feeble attempt at using xattr gnu.translator

[Jan Nieuwenhuizen]

Samuel Thibault writes:

Hello Samuel,

> Did you have a look at Shengyu Zhang's work on xattr? That's about
> summer 2016.

Ah, that's a helpful pointer.  What I notice is that Shengyu's last
message says "not merged yet", to which Richard Braun responds

    https://lists.gnu.org/archive/html/bug-hurd/2016-08/msg00095.html

    "we'll have more time in September to really dive into the code,
    finish the review and merge it."

It got merged, but I'm not sure about the "finish" bit; Justus Winter
disabled it by default (not a great sign), and GSoC 2017 mentions:
"finishing the xattr work"

    https://lists.gnu.org/archive/html/bug-hurd/2017-01/msg00016.html

I cannot find any more meta data, documentation or tests...

> Also see glibc/sysdeps/mach/hurd/setxattr.c which
> translates gnu attributes into RPC calls, and then again in the
> translators's diskfs_set_translator. See for instance in
> hurd/ext2fs/inode.c's diskfs_set_translator, it looks at
>
>  if (EXT2_HAS_COMPAT_FEATURE (sblock, EXT2_FEATURE_COMPAT_EXT_ATTR)
>       && use_xattr_translator_records)

I noticed the --x-xattr-translator-records before but after reading some
more code, I got a little bit further

glibc/hurd/xattr.c has

   gnu.translator       empty if no passive translator
                        translator and arguments: "/hurd/foo\0arg1\0arg2\0"

ah...so there's a nice hack (way??) to also pass arguments...and then

hurd/libdiskfs/file-set-trans.c has

diskfs_S_file_set_translator (struct protid *cred,
[...]
  if (passivelen && passive[passivelen - 1])
    return EINVAL;

hmm; so even when there are no arguments, VALUE must end with 0?!
Okay...

--8<---------------cut here---------------start------------->8---
root@debian:~# touch 1
root@debian:~# setfattr --name=gnu.translator --value=/hurd/pflocal 1
setfattr: 1: Invalid argument
root@debian:~# setfattr --name=gnu.translator --value='/hurd/pflocal\0' 1
root@debian:~# showtrans 1
/hurd/pflocal
--8<---------------cut here---------------end--------------->8---

...so "something" works.

Sadly, this trick does not work on GNU/Linux.  I'm afraid this hasn't
been upstreamed and we would have to debug linux and/or create a patch
for setxattr to accept "gnu." on "Hurd" type file systems.  Thoughts?

Also, some other things also don't work

--8<---------------cut here---------------start------------->8---
root@debian:~# getfattr --name=gnu.translator 1
1: gnu.translator: Operation not supported
root@debian:~# setfattr --name=gnu.translator --value='/geert/pflocal\0' 1
setfattr: 1: Operation not supported
root@debian:~# setfattr --remove=gnu.translator 1
setfattr: 1: Operation not supported
--8<---------------cut here---------------end--------------->8---

Greetings,
janneke

-- 
Jan Nieuwenhuizen <janneke@gnu.org> | GNU LilyPond http://lilypond.org
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