Re: extended attributes and lsextattr/extattr_list_file

[manu%netbsd.org@localhost (Emmanuel Dreyfus)]

YAMAMOTO Takashi <yamt%mwd.biglobe.ne.jp@localhost> wrote:

> > The Linux-flavor API has option for test-and-set operation on attribute.
> > It cannot be done in libc, that really require to be done in the kernel.
> 
> do you think the opposite, implementing freebsd-flavor in userland
> using linux-flavor, is possible?

Yes it would be possible. The only problem woule be to map namespaces.
Linux API uses a dot-separated prefix in the attribute name (e.g.:
system.foo), while the FreeBSD API uses an int that can stand for system
or user.

In order to keep the namespace information, you therefore would have to
rewrite attribute name on set/get/delete operations:
FreeBSD (system, "foo") should become Linux ("system.foo"), and same
thing for user namespace.

But the problem then is listing, when applications using the Linux API
may have stored other namespaces: if you are requested system
attributes, you need to look for system.* attribute and strip the
leading "system.", but what happens if you encounter an odd namespace
savec through the Linux API? Something like that:
Linux ("system.foo") becomes FreeBSD (system, "foo")
Linux ("trusted.foo") becomes FreeBSD (system, "foo")
Linux ("security.foo") becomes FreeBSD (system, "foo")
Linux ("*.foo") becomes FreeBSD (user, "foo")

But here you see a problem when listing user attributes with the FreeBSD
API when you have "user.foo" and "bar.foo": we will see two attributes
with the same name.

A workaround or this would be to use add a special prefix on
set/get/delete operations: 
FreeBSD (system, "foo") would become Linux ("netbsd_system.foo")

That was, when listing attributes, you know you must strip netbsd_system
and netbsd_user namespaces and keep everything else. 
Linux ("netbsd_system.foo") becomes FreeBSD (system, "foo")
Linux ("user_system.foo") becomes FreeBSD (user, "foo")
Linux ("system.foo") becomes FreeBSD (system, "system.foo")
Linux ("trusted.foo") becomes FreeBSD (system, "trusted.foo")
Linux ("security.foo") becomes FreeBSD (system, "security.foo")
Linux ("*.foo") becomes FreeBSD (user, "*.foo")

But then when listing using the Linux API, it would see our weird
netbsd_user and netbsd_system namespaces. A possible fix is to have a
libc stub, unused by the FreeBSD API, that rewrite them to system and
user, e.g.: "netbsd_system.foo" becomes "system.foo"

The answer to your question is therefore yes, but it may introduce
confusion.

-- 
Emmanuel Dreyfus
http://hcpnet.free.fr/pubz
manu%netbsd.org@localhost