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

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fakeroot(1) Debian manual fakeroot(1)

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

SYNOPSIS
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]

DESCRIPTION
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
archiver.

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).

OPTIONS
-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.

EXAMPLES
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
joost
$ fakeroot /bin/bash
# whoami
root
# 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).

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

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

ENVIRONMENT
FAKEROOTKEY
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).

LD_LIBRARY_PATH

LD_PRELOAD
Fakeroot is implemented by wrapping system calls. This is
accomplished by setting LD_LIBRARY_PATH=/usr/lib/fakeroot and
LD_PRELOAD=libfakeroot.so.0. That library is loaded before the
system's C library, and so most of the library functions are
intercepted by it. If you need to set either LD_LIBRARY_PATH or
LD_PRELOAD from within a fakeroot environment, it should be set
relative to the given paths, as in
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/foo/bar/

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

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

touch foo
fakeroot
ls -al foo

or the other way around,

fakeroot
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‐
sions’.

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.

BUGS
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).

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

AUTHORS
joost witteveen

Clint Adams

Timo Savola

MANUAL PAGE
mostly by J.H.M. Dassen Rather a lot mods/addi‐
tions by joost and Clint.

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

Debian Project 6 August 2004 fakeroot(1)