file-as-dir vs. dir? (was Re: silent semantic changes with reiser4)

[Andy Lutomirski]

(Apologies for beating on a dead horse here...)

I seems like one confusion here is that everyone has a different idea of 
what the semantics we're talking about are.  I see two main ones:

Hans's: A file, a directory, and an attribute are functionally 
equivalent (except for S_ISxxx and hardlinks).  That is, /usr/bin/metas 
makes sense, and it's not talking about a program called metas.  This 
also means that /foo/metas/metas might exist and needs dealing with.

Linus's (I think):  A directory is just a directory (no attributes and 
no read()able data).  A file can contain attributes, where attributes 
can be "file" attributes or "directory" attributes.  That is, a file is 
also a subtree with posix-like semantics (except for hardlink stuff). 
So doing "touch /tmp/foo; cat /tmp/foo/metas" fails, rather than doing 
something that's probably useless.  "touch /tmp/foo; touch /tmp/foo/bar; 
touch /tmp/foo/bar/baz" fails on the last touch because bar is a file 
attribute and recursive magic is disallowed.

Which one are we talking about?

FWIW, I like the latter version a lot better, as it removes a lot of 
ambiguity.  If I see a path like /tmp/foo/metas/uid, it is either a uid 
attribute (i.e. writing it has security implications) or it is a 
standard file (i.e. writing it  just writes it).  But _I can tell which 
one_ by fstat()ing /tmp/foo!  If it's a directory, than I have either a 
named stream called uid or a genuine file called uid (and I can tell 
which by fstat()ing /tmp/foo/metas), but if it's a file then I have the 
magic uid.  And I'm guaranteed that there's no other funny business, 
because /tmp/foo is a "file with a subtree," which means that it's not 
an attribute.  This way I know exactly what I'm dealing with.

As an added bonus, we could have O_NOMETAS which means that "files" may 
not be traversed.  Then someone who wants to make sure they get a real 
file can do it.  If recursive files-as-dirs were allowed, that might not 
do quite what the caller expected.  If you really wanted a directory 
with some data attached, but it in dir/.mystuff, because that's probably 
what you meant, and any existing tool will do exactly the right thing 
with it.

In the former version, AFAIK, I have a mess.  In UNIX, a path is a path. 
 It's trivial to understand and there's no funny business parsing it. 
But now a path has a slippery meaning where the token "metas" could be a 
plain old token or it could be magic or it could be magic some other 
way.  The security implications and the possbility for something to 
break horribly are scary.

--Andy
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