Re: Experiences with compiling Debian

[joost@rulcmc.leidenuniv.nl (joost witteveen)]

> > Well, if we do this, we need to make sure to handle the case where
> > people do something like:
> > 
> >   chown -R 755 debian/tmp/usr/bin
> >   chown g+s debian/tmp/usr/bin/special-binary
> > 
> > i.e. later commands would have to override previous ones.  (probably
> > obvious, but I just wanted to make sure this was kept in mind).
> 
>  The new commands would edit a list of permissions, that would be later
> used by tar. We could set an env var to the path of the list...

Nearly.

What I've done so far:

A programme, "fakeroot", that works as follows:

  $ fakeroot /bin/bash

This will execute /bin/bash[1], and set environmen varable "LD_PRELOAD"
to a libfakeroot.so.0.0. This libfakeroot currently overloads
only chown and lstat[2]. Now, when you type in the thus executed shell:

    $ chown root:users somefile

this will call the wrapper "chown" function. this wrapper calls 
the real chown, _and_ it sends (via SYSV IPC calls) a message
to the (still running) fakeroot "daemon". This daemon records
the new fake onerships of the file in an internal variable.

The idea is now that when you do
   
   $ ls -al somefile

ls will call lstat("somefile", &buf), again the wrapper lstat[2] function
gets executed that sends a message to the daemon, asking if it's
got any extra information about "somefile". The daemon may then
eighther return "no, don't know that file", or (as in this case)
"root:users". Thus, the ls would output that the file is owned
by root:users. Also any other programme that users (l)stat will
think so, so we don't need to modify tar or anything.

The final goal of cource is to be able to do

  $dpkg-buildpackage -rfakeroot

and build that package!

Problems: 
  - see [1], and [2]
  - yes fstat and fchown will be somewhat of a problem, but I'll
    just have to overlod open() etc too, and keep a list of 
    inodes/filenames.
  - I probably will go wrong for pathalogical cases like

       ln file1 file2
       chown mail:sys file2
       rm file2
       ls -al file2

    I doubt whether these are important, and even if they are, I
    guess it will be possible to get it right.

Non-problems:
  The above mentioned problem,
> >   chown -R 755 debian/tmp/usr/bin
> >   chown g+s debian/tmp/usr/bin/special-binary
    isn't a problem: the library function chmod(2) is never
    called with arguments like g+s, but only with the full permissions
    (the user space programme chmod(1) does a stat(2) first).

footnotes:

[1] I currently don't have a libc6 bash, so that doesn't work. I've
    been testing it with "es" (yet another shell that I never heard
    of, but I'm the maintainer).

[2] Well, I've olverloaded lstat all right, but it doesn't seem to
    get called don't know why yet. Why would chown() work and lstat() 
    not?

-- 
joost witteveen, joostje@debian.org
#!/usr/bin/perl -sp0777i<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<j]dsj
$/=unpack('H*',$_);$_=`echo 16dio\U$k"SK$/SM$n\EsN0p[lN*1
lK[d2%Sa2/d0$^Ixp"|dc`;s/\W//g;$_=pack('H*',/((..)*)$/)
#what's this? see http://www.dcs.ex.ac.uk/~aba/rsa/


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