Re: [9fans] Novice question - run as other

[David Presotto <presotto@closedmind.org>]

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You're confusing name space and semantics.  It is indeed in the name space
but the semantics of writing it is to change the state of the writer not
everyone who can see the name space.  Processes owned by different
users can all share objects in the same name space.  However reading or
writing those objects can have different permissions and results for
the different processes.

If I where to unbind /dev, all processes in that name space
would see that object disappear.

The discontinuity/incongruity in Plan 9 is what an object can see
about the process using it.  Kernel objects (things implemented with #x
for any x) can see the process and can adjust their behavior
on its immediate state (this is especially true of #p).  However
anything that goes through 9P only sees the user's state by way of
the messages sent to it.  It can't tell anything about the particular
process performing an action and can thus only adjust its semantics
according to the user who actually bound the object into its
name space (this action turns to an attach).

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From: David Tolpin <dvd@davidashen.net>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Novice question - run as other
Date: Wed, 17 Mar 2004 22:00:45 +0400 (AMT)
Message-ID: <200403171800.i2HI0jtq086906@adat.davidashen.net>

>
> >@{echo -n none > '#c/user' ; while() {sleep 1} }
>
> >succeeds but does not change the owner of the process. Why?
>
> well, seems it would be bad if all you had to do to change userid was 
> echo into '#c/user'
>
> So, starting from that, the why should be apparent.

The 'Why' was about a different issue.

The question was why the C program behaves differently than
the shell script.  The C program just writes to #c/user to
change the owner of the process.

The answer is that writing to #c/user is special
in the sense that although it is a part of namespace, it changes
the state of the process space, which is only apparent if you look
into the kernel source.

A more general conclusion is that for tasks where writing to
or reading from files reflects or affects process space, and not
name space, rc is not suitable and C code must be written.

I am now into the sources to understand which files in the namespace
are actually not in it, but in the process space.

David Tolpin
http://davidashen.net/