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Manpage of fakeroot

fakeroot

Section: Debian GNU/Linux manual (1)
Updated: 26 July 1997
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NAME

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

SYNOPSIS

fakeroot [-l|--lib library] [--faked faked-binary] [--] [command]  

DESCRIPTION

fakeroot runs a command in an environment were 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 have 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 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 environemnt variables, some of which fakeroot needs).

 

OPTIONS

--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.
 

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/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 filesystem's permissions/ownerships. (assuming the daemon and connecting program were started by the same user).
 

LIMITATIONS

Library versions
Every command executed within fakeroot needs to be linked to the same version of the C library as fakeroot itself. Because the potato version of debian now uses libc6 only (glibc2.1), this isn't that much of a problem any more.
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) 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 likelyhood 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 doens'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, the open will suceed, 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).  

AUTHOR

joost witteveen <joostje@debian.org>  

MANUAL PAGE

mostly by J.H.M. Dassen <jdassen@wi.LeidenUniv.nl> Rather a lot mods/additions by joost.  

SEE ALSO

faked(1) dpkg-buildpackage(1), build(1L) /usr/doc/fakeroot/DEBUG


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
EXAMPLES
SECURITY ASPECTS
FILES
ENVIRONMENT
LIMITATIONS
BUGS
COPYING
AUTHOR
MANUAL PAGE
SEE ALSO

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Time: 01:59:27 GMT, July 08, 2020