fakeroot-sysv

fakeroot-sysv [options] command [arguments...]

-l library
    Specifies a different shared library to load for interception. By default, fakeroot finds its own library automatically.

-s save-file
    Save the fakeroot environment (UID/GID/mode mappings) to save-file on exit. This allows continuation of operations across multiple fakeroot invocations, ensuring consistent faked permissions.

-i load-file
    Load a fakeroot environment from load-file at startup. This must be a file previously created with the -s option.

-u
    Run an un-faked command within the fakeroot environment. This is primarily for testing or debugging, allowing you to see actual permissions vs. faked ones.

-D
    Activate debug mode. Prints detailed debugging information to stderr, useful for troubleshooting.

-v
    Display version information of fakeroot-sysv and exit.

-h
    Display a brief help message explaining command usage and options, then exit.

The fakeroot-sysv command allows a user to run a command in an environment where it appears to have root privileges for file manipulation operations, while in reality, no actual root privileges are granted. This is particularly useful for building software packages (e.g., Debian .deb packages, RPMs) that require files to be owned by root or specific system users and groups during the build process, but the build itself must be performed by an unprivileged user. It works by intercepting library calls to functions like stat, chown, and chmod.

Instead of performing the actual operation, fakeroot-sysv maintains an internal database of file ownerships and permissions, making it appear as if the operations succeeded as root. The -sysv suffix indicates that this particular implementation uses System V IPC (Inter-Process Communication) mechanisms for its internal communication and data storage, which is generally robust and reliable.

fakeroot-sysv does not provide actual root privileges. It cannot, for example, perform operations requiring kernel-level access, load kernel modules, or access raw devices. It only simulates file system ownership and permission changes at the user-space library level.

Operations performed outside the fakeroot-sysv environment will not reflect the simulated root privileges. There might be a slight performance overhead due to the interception mechanism and IPC communication.

fakeroot-sysv operates by replacing key functions in the C library (like stat, chown, chmod, mknod) with its own versions. These replacement functions are loaded into the executing program's memory via the LD_PRELOAD environment variable. When a program running inside the fakeroot-sysv environment calls one of these intercepted functions, fakeroot-sysv does not pass the call directly to the kernel. Instead, it consults or updates an internal database (stored via System V IPC shared memory and semaphores) that maps original file UIDs/GIDs to their faked counterparts. This allows it to report back faked root privileges (e.g., UID 0, GID 0) even though the underlying filesystem still sees the unprivileged user's actual UID/GID.

While fakeroot-sysv is a common and robust implementation, other versions exist, such as fakeroot-tcp, which uses TCP sockets for inter-process communication instead of SysV IPC. The choice between implementations can sometimes depend on system configuration or specific needs, though fakeroot-sysv is often the default or preferred due to its local nature and robustness. Regardless of the implementation, the core functionality of simulating root privileges remains the same, providing a consistent facade for package building and other unprivileged operations requiring temporary root-like access.

The fakeroot utility was originally developed primarily to assist in the creation of Debian packages. During the package building process, it's often necessary to perform operations like changing file ownerships to root or specific system users (e.g., 'root:root', 'daemon:daemon'), or setting specific file permissions, which typically require root privileges. However, building packages as an actual root user is a security risk.

fakeroot solved this by providing a fake root environment. The -sysv implementation, utilizing System V IPC, was one of the early and robust methods for its internal communication, ensuring consistency and reliability of the fake filesystem state across multiple processes within the environment. It has been a cornerstone tool for secure and reproducible package building workflows on Linux distributions.

fakeroot(1), chroot(8), sudo(8), dpkg-buildpackage(1), debuild(1)