fakeroot
Section: Debian manual (1)
Updated: 5 October 2014
Page Index
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
or similar location on your platform.
The shared object 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
themselves 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 outside 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 descriptor
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 succeeds, 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-*.so
The shared library containing the wrapper functions.
ENVIRONMENT
- FAKEROOTKEY
-
The key used to communicate with the fakeroot daemon. Any program
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/ownerships.
(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 unexplainable 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 fakeroot as small as possible, to limit the likelihood
of 'collisions'.
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
confused 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 functions slightly change).
COPYING
fakeroot
is distributed under the GNU General Public License.
(GPL 2.0 or greater).
AUTHORS
- joost witteveen
-
<joostje@debian.org>
- Clint Adams
-
<clint@debian.org>
- Timo Savola
-
MANUAL PAGE
mostly by J.H.M. Dassen
<
jdassen@debian.org>
Rather a lot mods/additions by joost and Clint.
SEE ALSO
faked(1)
dpkg-buildpackage(1),
debuild(1)
/usr/share/doc/fakeroot/DEBUG