Ubuntu Manpage: pseudo - run a command in a virtual root environment

NAME

       pseudo - run a command in a virtual root environment

SYNOPSIS

       pseudo [-dflv] [ -x flags ] [ -P prefix ] [ -rR root ] [ -t timeout ] [command]

       pseudo -h

       pseudo [-dflv] [ -x flags ] [ -P prefix ] [-BC] -i path

       pseudo [-dflv] [ -x flags ] [ -P prefix ] [-BC] -m from -M to

       pseudo [-dflv] [ -x flags ] [ -P prefix ] -S

       pseudo [-dflv] [ -x flags ] [ -P prefix ] -V

DESCRIPTION

The pseudo utility provides a virtual root environment, hereafter referred to as the
pseudo environment, allowing the creation of file system images and packages by users
without root privileges. The pseudo environment is implemented by pushing a special
library (libpseudo.so) into the LD_PRELOAD environment variable. This library intercepts
a large number of common filesystem operations and some user-id related operations, and
returns values that look as though the operations had been performed by a root user. This
is in turn managed by a daemon program which keeps a list of virtualized file ownership
and permissions; this daemon program itself is pseudo.

The pseudo program itself can also be used as a program launcher. The launcher is used to
automatically configure a working environment, then execute processes within that
environment. Alternatively, you can bypass this by setting up certain environment
variables (see the ENVIRONMENT section below). The pseudo client library (libpseudo.so)
can then start the server automatically.

The pseudo command can be invoked in one of several possible modes:

-B The -B option causes pseudo to scan its database, as with the -C option, but
instead of reporting mismatches, pseudo attempts to repair them. Specifically,
device and inode number mismatches are corrected, and symlink or directory
mismatches result in deletion of database entries.

-C The -C option causes pseudo to scan its database, comparing against the
filesystem, and reporting likely errors. This may be unreliable when the server
is actively running.

-h The -h option causes pseudo to print a usage message and exit.

-i The -i option causes pseudo to attempt to correct device number mismatches by
checking inodes; if path has the same inode number as recorded in the database,
but a different device number, all instances of the device number recorded in the
database are updated to the device number in the live filesystem for path. This
is intended to handle the mismatches that can occur when remounting an NFS
filesystem. The -i option implies the -C option. You can also specify the -B
option to request that the database be rebuilt.

-m The -m and -M options cause pseudo to rename files, replacing the string from with
the string to. The -m option pair implies the -C option. You can also specify
the -B option to request that the database be rebuilt.

-V The -V option causes pseudo to print configuration information and exit
immediately.

-S The -S option causes pseudo to try to find an existing server, and if it finds
one, instructs that server to shut down as soon as all clients are detached from
it. Note that the server will not shut down while clients are connected to it; in
this case, pseudo will print a list of the remaining client PIDs.

-d The -d option causes pseudo to immediately detach and run in the background as a
daemon. This is rarely useful except for debugging.

Finally, invoked without any of these options, pseudo sets up an emulated root
environment, then invokes command if it was provided, otherwise a shell (using the SHELL
environment variable if it is set, or /bin/sh otherwise).

The following options modify the behavior of pseudo:

-d (daemonize)
Run as a daemon; pseudo detaches from the calling environment and runs as a
daemon. The command returns successfully if this appears to have succeeded,
otherwise it produces an error message and returns a failure status.

-f (foreground)
Run in the foreground; pseudo runs as a server, and does not try to start other
commands. This mode is useful for debugging.

-l (log)
Enable logging. The pseudo daemon will log every filesystem transaction in the
log database.

-r root

-R root Set the PSEUDO_CHROOT environment variable, running as though the program had
called chroot(2) on the specified path. With -r, this implies changing the
working directory to the specified directory; with -R, it does not.

-t timeout
Set the timeout of the pseudo daemon, in seconds. The default is currently 30
seconds. After this long with no attached clients, the pseudo daemon shuts down
automatically. The server never shuts down while it has attached clients. Note
that this does not prevent continued use; new clients can restart the daemon if
they need it.

-v (verbose)
Increase the verbosity of the pseudo daemon, and the client library for any
programs started by this invocation of pseudo. This is equivalent to the numeric
form of the PSEUDO_DEBUG environment variable; multiple -v options increase the
debugging level.

-x (debug)
Set specific deugging flags (the pseudo utility's help message lists them). This
is equivalent to the string form of the PSEUDO_DEBUG environment variable.

EXAMPLES

The two most common usages of pseudo are using it to run specific commands, and setting up
an environment manually for running various other commands.

For the first case, the usage is reasonably simple:

$ /path/to/pseudo
# commands which require root privileges

You may have to use the -Pprefix option to tell pseudo where to look for its database and
server. If you specify a full path, pseudo assumes that PSEUDO_PREFIX should be the path
to the directory containing the pseudo program, or to the /bin directory containing the
pseudo program.

The other way to use pseudo is by setting up an environment. This is suitable for use in
Makefiles or similar environments, where you want to run a series of commands in the
pseudo environment, but not to keep invoking the pseudo command. To do this, set up the
PSEUDO_PREFIX, LD_PRELOAD, and LD_LIBRARY_PATH environment variables, then run programs
normally. You do not need to separately invoke the pseudo daemon; the client library
starts it as needed.

If you have moved a directory which pseudo was tracking, you may be able to get the
database reattached using the -m option. A typical usage might be:

$ /path/to/pseudo -B -m oldpath -M newpath

This requests that pseudo replace the string oldpath with the string newpath at the
beginnings of filenames, then regenerate the database, correcting any device/inode
numbers.

DIAGNOSTICS

       Depending on invocation, diagnostic messages usually go either to standard error or to the
       file PSEUDO_PREFIX /var/pseudo/pseudo.log.  By default, pseudo daemon messages go into the
       log file, but messages generated by the client code go to standard error.   These  can  be
       changed  using  the PSEUDO_DEBUG_FILE environment variable, documented in ENVIRONMENT.  At
       the default logging level, only critical messages are displayed.  If you have  raised  the
       logging  level  (using the -v option or the PSEUDO_DEBUG environment variable), additional
       messages are displayed.  Levels higher than 2 are very unlikely to be  useful  outside  of
       pseudo development.

       Diagnostic  messages  seen  by  default  are those which are believed to indicate either a
       serious internal flaw in pseudo or a completely unexpected  failure  from  the  underlying
       operating system.  In normal use, you should see no diagnostic messages.

ENVIRONMENT

The most significant environment variables for pseudo are LD_PRELOAD and LD_LIBRARY_PATH.
However, these variables have no special meaning to pseudo; rather, they are used in the
standard way to manipulate the dynamic linker into loading the libpseudo library so that
it can intercept calls into the underlying C library.

The following environment variables are used directly by pseudo:

PSEUDO_BINDIR
This directory holds the path to the pseudo binary; by default, it is the bin
directory under PSEUDO_PREFIX.

PSEUDO_CHROOT
This variable holds the current emulated chroot(2) path. Paths that are relative
to this are treated as though they were instead relative to the filesystem root.

PSEUDO_DEBUG
This variable holds either a numeric "debug level" for pseudo to run at, or a set
of specific debugging flags, generally letters. Use pseudo -h to see the
available flags. In general, this is useful only for debugging pseudo itself.

PSEUDO_DEBUG_FILE
The name of a file to use for debugging messages from the pseudo client; the
default is to log to standard error. If the string contains a single %s, that
string is replaced with the short program name, and if it contains a single %d,
that string is replaced with the process ID. Other format specifiers (other than
'%%') are not allowed. By default, the pseudo server logs to the file pseudo.log
in the var/pseudo directory, while clients log to standard error.

PSEUDO_DISABLED
If this variable is set to a value that doesn't look like f, F, n, N, s, S, or a
numeric zero, the pseudo client library does not modify the behavior of called
functions, though it continues to intercept them and block signals while
processing them. This variable is reevaluated on every call to fork(2), clone(2)
or related functions. If the value starts with a lowercase or uppercase s , the
pseudo client disables all server spawning and communications, but still operates
locally. This means that no filesystem mode or permissions changes are actually
recorded or reported, but functions like chown() will still report success, even
though nothing happens. This function is intended for debugging of issues which
are complicated by the server's involvement.

PSEUDO_ALLOW_FSYNC
If this variable is set, pseudo will allow fsync() and related system calls, even
it was configured with the --enable-force-async option. Otherwise, that option
results in all such calls being discarded silently, even when PSEUDO_DISABLED is
set. The value specified doesn't matter.

PSEUDO_ENOSYS_ABORT
If this variable is set, the pseudo client library calls abort() rather than
setting errno to ENOSYS in the event of a call to a missing underlying function.
This variable has no function outside of debugging pseudo itself.

PSEUDO_LIBDIR
This directory holds the path to the pseudo shared libraries; by default, it is
the lib directory under PSEUDO_PREFIX. (On 64-bit hosts, lib64 is also used.)

PSEUDO_LOCALSTATEDIR
This directory holds the pseudo database files and log files; by default, it is
the var/pseudo directory under PSEUDO_PREFIX.

PSEUDO_NOSYMLINKEXP
By default, when chrooted, pseudo prepends the chroot directory to the paths used
for absolute symlinks; this behavior ensures that opening symlinks produces
expected results in most cases. In some cases you may want to suppress this. If
this variable is unset, or set to any value other than 0, pseudo expands symlink
paths like this. If this variable is set to 0, the behavior is disabled.

PSEUDO_OPTS
This variable holds options to be passed to any new pseudo servers started.
Typically, when pseudo is used as a launcher, this will be set automatically;
however, you can also use it to pass options when using LD_PRELOAD to manually run
things in the pseudo environment.

PSEUDO_PASSWD
This variable holds the path to a directory containing password and group files to
use for emulation of various password and group routines. It should be the path
to a directory containing the etc directory containing files named passwd and
group. When pseudo is emulating a chroot environment, the chroot directory is
used by preference. The parallelism between these cases is why this variable
points at the parent directory of etc rather than the directory containing the
files. If there is no chroot environment, and this variable is also unset, pseudo
falls back to a directory specified at configure time, with the default being the
root directory. This is controlled by the PSEUDO_PASSWD_FALLBACK definition.

PSEUDO_PREFIX
If set, the variable PSEUDO_PREFIX is used to determine the path to use to find
the pseudo server, in PSEUDO_PREFIX/bin, and the pseudo data files, in
PSEUDO_PREFIX/var/pseudo. This variable is automatically set by the pseudo
program when it is used as a launcher.

PSEUDO_PROFILE_PATH
If pseudo was configured with profiling enabled, specifies a path in which to
write client profiling information for use with the pseudo_profile utility (not
built by default).

PSEUDO_TAG
If this variable is set in a client's environment, its value is communicated to
the server at the beginning of each client session, and recorded in the log
database if any logging occurs related to a specific client. Note that different
clients may have different tags associated with them; the tag value is per-client,
not per-server.

PSEUDO_UIDS, PSEUDO_GIDS
These variables are used internally to pass information about the current emulated
user and group identity from one process to another.

PSEUDO_UNLOAD
This variable is reevaluated on every call to fork(2), exec(3) or related
functions. If the variable exists libpseudo.so will be removed from LD_PRELOAD
and PSEUDO_DISABLED behavior will also be triggered. For processes that simply
fork(2), the behavior will be the same as if PSEUDO_DISABLED was set. For new
processes, after a call to exec(3) or system(3) pseudo will not be loaded in the
new process.

SHELL If set, this will be used when pseudo is invoked without either a command or one
of the options which directs it to do something other than run a command.
Otherwise, pseudo defaults to /bin/sh .

BUGS

       The  pseudo database is not particularly robust in the face of whole directory trees being
       moved, or changes in the underlying device and inode numbers.  It has a reasonable  chance
       of  recovering  if  only  the  path  or  the  device  numbers  have changed, but it is not
       particularly designed to address this.  A future release  is  expected  to  have  improved
       resilience in these cases.

       The filesystem on which pseudo keeps its database and files must at a minimum support UNIX
       domain sockets and reasonable file locking semantics.  Note that pseudo relies on flock(2)
       locking  semantics;  a  lock  has  to  persist into a child process.  This should probably
       eventually be fixed.

       The pseudo client library is probably thread-safe, but has not been adequately  tested  or
       debugged in that context.

       Filesystem  performance  is  noticably  worse  under pseudo than it is otherwise.  This is
       probably because nearly every operation (other than reads and writes)  involves  at  least
       one  round-trip  network communication with the server, and probably some kind of database
       activity.