Fakeroot in Scratchbox

Timo Savola

Revision history

Version: Author: Description:
2005-02-06 Savola Based on Device tools

Table of Contents
1. Introduction to fakeroot
2. Fakeroot in Scratchbox
2.1. Known issues
3. Debugging
4. Building fakeroot
5. Implementation of network fakeroot
A. fakeroot 1.2.3 manual page

Chapter 1. Introduction to fakeroot

Fakeroot [1] is a utility that runs programs in an environment that looks as if they were run with super-user privileges. It is used primarily for setting file ownerships and modes before packaging them. You can for example create device nodes and store them in a tarball while logged in as a normal user. Of course, the programs run from a fakeroot session cannot really do privileged system calls; fakeroot keeps an in-memory database of file ownerships and such things.

Fakeroot was developed by the Debian Project [2] to help in building Debian packages. The Debian packaging system needs a root environment so that it would be as easy as possible to set up ownerships and permissions.

Fakeroot is released under the GNU General Public License version 2 [3].

Chapter 2. Fakeroot in Scratchbox

Scratchbox [4] introduces new requirements for fakeroot. During the development of Scratchbox an enhanced version of fakeroot was developed with the name fakeroot-net. It was later merged with the upstream project and nowadays Scratchbox uses the upstream codebase.

Differences between the default and the Scratchbox version are:

Scratchbox provides the fakeroot command, the fakeroot daemon (faked) and a host version of the fakeroot library (libfakeroot). They are sufficient for running host tools in fakeroot, but a target version of libfakeroot needs to be installed for each Scratchbox target in order to run target binaries in fakeroot. Installing Scratchbox [6] describes how to do that. Scratchbox's fakeroot is compatible with the libfakeroot provided by the Debian package, so you can use that aswell.


As described above, the fakeroot daemon provided by the Debian package does not use the same database format as Scratchbox's version. This should not be a problem though, since thanks to Scratchbox's binary redirection feature the host version of the fakeroot command is normally used even when the target version is installed.

Refer to fakeroot's manual page (Appendix A) for usage instructions.

2.1. Known issues

The fakeroot environment is imposed upon a process by using the C-library's LD_PRELOAD environment variable. libfakeroot is preloaded by the dynamic linker whenever it loads a binary. This means that fakeroot does not work with statically linked binaries.

There is also another side-effect. Since libfakeroot is loaded into the same process image with the "victim" program, they share the same file descriptor table. Some programs (such as the configure scripts) use hard-coded descriptor numbers. libfakeroot needs one file descriptor for its communication socket, and if the program starts to use the same file descriptor, there will be trouble. fakeroot tries to monitor the status of its descriptor so that it can open a new socket if the descriptor has been changed. If you start seeing messages about hijacked file descriptors, you can try to make fakeroot use some other file descriptor with the --fd-base option. Its default value is (descriptor_table_size - 100).

Chapter 3. Debugging

The fakeroot daemon can be launched with debug enabled and left running on the foreground:

  $ faked --debug --foreground
The first number is the TCP/IP port it listens to, and the second number is its process ID. Now, in another terminal, setup a fakeroot session manually that uses the daemon we started:

  $ export FAKEROOTKEY=33366
        $ export LD_PRELOAD=/scratchbox/tools/lib/libfakeroot-tcp.so.0
Now you can run programs in the hand-made fakeroot session and see the daemon's cryptic debug output in the other terminal. This way you can also use a debugger to debug a program within a fakeroot environment.

/scratchbox/tools/lib/libfakeroot-tcp.so.0 is the host version. If you are running target binaries, you should set LD_PRELOAD to /usr/lib/libfakeroot/libfakeroot-tcp.so.0.

When using a remote fakeroot session, the communication can be traced using the sbrsh daemon's debug log. See Scratchbox Remote Shell [5] for instructions.

Chapter 4. Building fakeroot

This chapter contains instructions for building fakeroot from source code using Scratchbox's configuration options. You shouldn't normally need to do that, since fakeroot is included in Scratchbox and all toolchains ship libfakeroot binaries for their target architectures. See Installing Scratchbox [6] and Scratchbox toolchains [7] for more information.

Fakeroot should be cross-compiled inside Scratchbox. The fakeroot source package is available in the /scratchbox/packages directory in the Scratchbox installation, but you can also download it from Debian [7].

Here fakeroot is compiled for a preconfigured Scratchbox target:

  1. Extract the fakeroot source package:

    [sbox-HOST: ~] > tar xfz /scratchbox/packages/fakeroot_1.2.3.tar.gz
  2. Select the target your wish to compile for:

    [sbox-HOST: ~] > sb-conf select ARM
  3. Go to the source directory:

    [sbox-ARM: ~] > cd fakeroot-1.2.3
  4. Configure fakeroot using the options used by Scratchbox:

    [sbox-ARM: ~/fakeroot-1.2.3] > ./configure \
    --prefix=/usr --mandir=/usr/share/man --libdir=/usr/lib/libfakeroot \
    --program-suffix=-tcp --with-ipc=tcp --with-dbformat=path

    Fakeroot uses the /usr/lib/libfakeroot directory for its real libraries. A fake library is installed to /usr/lib to work around a bug in an old version of the dynamic linker.

  5. Build the real libfakeroot-tcp and install it along with the fakeroot-tcp command on the target:

    [sbox-ARM: ~/fakeroot-1.2.3] > make
    [sbox-ARM: ~/fakeroot-1.2.3] > make install
  6. Clean the configuration and go to the fake directory:

    [sbox-ARM: ~/fakeroot-1.2.3] > make distclean
    [sbox-ARM: ~/fakeroot-1.2.3] > cd fake
  7. Configure the fake fakeroot:

    [sbox-ARM: ~/fakeroot-1.2.3/fake] > ../configure \
    --prefix=/usr --mandir=/usr/share/man \
  8. Build and install the fake libfakeroot-tcp:

    [sbox-ARM: ~/fakeroot-1.2.3/fake] > make
    [sbox-ARM: ~/fakeroot-1.2.3/fake] > make install
  9. We won't be building the non-TCP version so let's link it to the TCP version:

    [sbox-ARM: ~/fakeroot-1.2.3/fake] > ln -sf fakeroot-tcp /usr/bin/fakeroot
  10. If you are using the Debian devkit, you can also build a binary package for Debian:

    [sbox-ARM: ~/fakeroot-1.2.3/fake] > make distclean
    [sbox-ARM: ~/fakeroot-1.2.3/fake] > cd ..
    [sbox-ARM: ~/fakeroot-1.2.3] > dpkg-buildpackage -rfakeroot -b

    It is important to note that the Debian package uses the non-TCP version as the default fakeroot command. Also, neither version is configured with the --with-dbformat=path option. You can change the configure options by editing the debian/rules file. If you do that, you should also change the package name and/or the package version to reflect the incompatibility with the standard Debian package.


    The Debian package needs the "sharutils" package for running its tests. Scratchbox does not provide this package, so you might first need to install it on the target:

    [sbox-ARM: ~/fakeroot-1.2.3] > apt-get update
    [sbox-ARM: ~/fakeroot-1.2.3] > apt-get install sharutils

Chapter 5. Implementation of network fakeroot

faked maintains a list of entries based on their device and inode numbers of the files that have been modified during a fakeroot session. The entries contain a data structure that is essentially the same as the one used by the stat system call. The TCP version introduces an additional remote field in the entry, which works like a "namespace" for the devices and inodes. All files on the local filesystems belog to the default namespace (remote is not set).

When a remote command is run within a fakeroot session, sbrsh resolves the device numbers of the NFS filesystems that are listed in its config file for the used target. If they are not exported by the local host but some third host, it tries to find out if the NFS filesystems are mounted on the local host and use the device numbers of the mount points.

sbrshd receives the list of mount entries and finds out what their device numbers are on the target device. Then it creates a relay process that listens for connections from local fakeroot sessions. When it receives one, it makes a corresponding connection to the faked running on the Scratchbox host. It maintains as many connection pairs as there are processes running within the local fakeroot session. The relay copies messages from the local session to the remote daemon and responses from the daemon to the session, and translates the device numbers in the messages between the local and remote device number "spaces".

If the relay finds an unlisted device number in one of the incoming messages, it does not translate it but sets the value of the remote field to the IP address of the host it is running at. This way faked can serve unknown filesystems without the danger of device number/inode collisions.


[1] fakeroot.

[2] The Debian Project.

[3] GNU General Public License.

[4] Scratchbox website.

[5] Scratchbox Remote Shell, Timo Savola.

[6] Installing Scratchbox, Valtteri Rahkonen.

[7] Scratchbox toolchains, Ricardo Kekki.

[7] Debian - fakeroot.

Appendix A. fakeroot 1.2.3 manual page

fakeroot(1)                      Debian manual                     fakeroot(1)

       fakeroot  -  run a command in an environment faking root privileges for
       file manipulation

       fakeroot [-l|--lib library] [--faked faked-binary] [-i  load-file]  [-s
       save-file]   [-u|--unknown-is-real   ]  [-b|--fd-base  ]  [-h|--help  ]
       [-v|--version ] [--] [command]

       fakeroot runs a command in an environment wherein it  appears  to  have
       root  privileges  for  file  manipulation.  This is useful for allowing
       users to create archives (tar, ar, .deb etc.) with files in  them  with
       root  permissions/ownership.   Without  fakeroot one would need to have
       root privileges to create the constituent files of  the  archives  with
       the  correct  permissions  and ownership, and then pack them up, or one
       would have to  construct  the  archives  directly,  without  using  the

       fakeroot  works  by  replacing  the file manipulation library functions
       (chmod(2), stat(2) etc.) by ones that  simulate  the  effect  the  real
       library  functions would have had, had the user really been root. These
       wrapper functions are  in  a  shared  library  /usr/lib/libfakeroot.so*
       which is loaded through the LD_PRELOAD mechanism of the dynamic loader.
       (See ld.so(8))

       If you intend to build packages with fakeroot, please try building  the
       fakeroot  package first: the "debian/rules build" stage has a few tests
       (testing mostly for bugs in old fakeroot versions). If those tests fail
       (for  example  because you have certain libc5 programs on your system),
       other packages you build with fakeroot will quite likely fail too,  but
       possibly in much more subtle ways.

       Also,  note that it's best not to do the building of the binaries them-
       selves under fakeroot. Especially configure and friends don't  like  it
       when  the  system  suddenly  behaves differently from what they expect.
       (or, they randomly unset some  environment  variables,  some  of  which
       fakeroot needs).

       -l library, --lib library
              Specify an alternative wrapper library.

       --faked binary
              Specify an alternative binary to use as faked.

       [--] command
              Any  command  you want to be ran as fakeroot. Use '--' if in the
              command you have  other  options  that  may  confuse  fakeroot's
              option parsing.

       -s save-file
              Save  the  fakeroot  environment to save-file on exit. This file
              can be used to restore the environment later using -i.  However,
              this  file will leak and fakeroot will behave in odd ways unless
              you leave the files touched inside the fakeroot alone when  out-
              side the environment. Still, this can be useful. For example, it
              can be used with rsync(1) to back up and restore whole directory
              trees  complete  with user, group and device information without
              needing to be  root.  See  /usr/share/doc/fakeroot/README.saving
              for more details.

       -i load-file
              Load a fakeroot environment previously saved using -s from load-
              file.  Note that this does not implicitly save the file, use  -s
              as  well for that behaviour. Using the same file for both -i and
              -s in a single fakeroot invocation is safe.

       -u, --unknown-is-real
              Use the real ownership of files previously unknown  to  fakeroot
              instead of pretending they are owned by root:root.

       -b fd  Specify fd base (TCP mode only). fd is the minimum file descrip-
              tor number to use for TCP connections; this may be important  to
              avoid  conflicts  with the file descriptors used by the programs
              being run under fakeroot.

       -h     Display help.

       -v     Display version.

       Here is an example session with fakeroot.  Notice that inside the  fake
       root  environment  file manipulation that requires root privileges suc-
       ceeds, but is not really happening.

       $  whoami
       $ fakeroot /bin/bash
       #  whoami
       # mknod hda3 b 3 1
       # ls -ld hda3
       brw-r--r--   1 root     root       3,   1 Jul  2 22:58 hda3
       # chown joost:root hda3
       # ls -ld hda3
       brw-r--r--   1 joost    root       3,   1 Jul  2 22:58 hda3
       # ls -ld /
       drwxr-xr-x  20 root     root         1024 Jun 17 21:50 /
       # chown joost:users /
       # chmod a+w /
       # ls -ld /
       drwxrwxrwx  20 joost    users        1024 Jun 17 21:50 /
       # exit
       $ ls -ld /
       drwxr-xr-x  20 root     root         1024 Jun 17 21:50 //
       $ ls -ld hda3
       -rw-r--r--   1 joost    users           0 Jul  2 22:58 hda3

       Only the effects that user joost could do anyway happen for real.

       fakeroot was specifically written to  enable  users  to  create  Debian
       GNU/Linux  packages  (in  the  deb(5)  format) without giving them root
       privileges.  This  can  be  done  by  commands  like  dpkg-buildpackage
       -rfakeroot  or  debuild  -rfakeroot (actually, -rfakeroot is default in
       debuild nowadays, so you don't need that argument).

       fakeroot is a regular, non-setuid program. It does not enhance a user's
       privileges, or decrease the system's security.

       /usr/lib/libfakeroot/libfakeroot.so*  The shared library containing the
       wrapper functions.

              The key used to communicate with the fakeroot daemon.  Any  pro-
              gram  started  with  the right LD_PRELOAD and a FAKEROOTKEY of a
              running daemon will automatically connect to  that  daemon,  and
              have  the same "fake" view of the file system's permissions/own-
              erships.  (assuming  the  daemon  and  connecting  program  were
              started by the same user).

       Library versions
              Every command executed within fakeroot needs to be linked to the
              same version of the C library as fakeroot itself.

              fakeroot doesn't wrap open(), create(), etc. So, if  user  joost
              does either

              touch foo
              ls -al foo

              or the other way around,

              touch foo
              ls -al foo

              fakeroot has no way of knowing that in the first case, the owner
              of foo really should be joost while the second  case  it  should
              have  been root.  For the Debian packaging, defaulting to giving
              all "unknown" files uid=gid=0, is always OK. The real way around
              this  is  to  wrap  open()  and create(), but that creates other
              problems, as demonstrated by the libtricks package. This package
              wrapped  many  more  functions,  and tried to do a lot more than
              fakeroot .  It turned out that a minor upgrade of libc (from one
              where the stat() function didn't use open() to one with a stat()
              function that did (in some cases) use open()), would cause unex-
              plainable  segfaults  (that  is,  the  libc6  stat()  called the
              wrapped open(), which would then call the  libc6  stat(),  etc).
              Fixing  them wasn't all that easy, but once fixed, it was just a
              matter of time before another function started  to  use  open(),
              never  mind  trying  to port it to a different operating system.
              Thus I decided to keep the number of functions wrapped by  fake-
              root  as  small  as possible, to limit the likelihood of 'colli-

       GNU configure (and other such programs)
              fakeroot, in effect, is changing the  way  the  system  behaves.
              Programs  that  probe the system like GNU configure may get con-
              fused by this (or if they don't, they  may  stress  fakeroot  so
              much  that fakeroot itself becomes confused). So, it's advisable
              not to run "configure" from within fakeroot. As configure should
              be   called   in   the   "debian/rules  build"  target,  running
              "dpkg-buildpackage -rfakeroot" correctly takes care of this.

       It doesn't wrap open(). This isn't bad by itself, but if a program does
       open("file", O_WRONLY, 000), writes to file "file", closes it, and then
       again tries to open to read the file, then that open fails, as the mode
       of  the file will be 000. The bug is that if root does the same, open()
       will succeed, as the file permissions aren't checked at all for root. I
       choose not to wrap open(), as open() is used by many other functions in
       libc (also those that are already wrapped),  thus  creating  loops  (or
       possible  future  loops,  when the implementation of various libc func-
       tions slightly change).

       fakeroot is distributed under the GNU General Public License.  (GPL 2.0
       or greater).

       joost witteveen

       Clint Adams

       Timo Savola

       mostly  by  J.H.M.  Dassen <jdassen@debian.org> Rather a lot mods/addi-
       tions by joost and Clint.

       faked(1) dpkg-buildpackage(1), debuild(1) /usr/share/doc/fakeroot/DEBUG

Debian Project                   6 August 2004                     fakeroot(1)