with-overlayfs(1) -- Execution wrapper for private volatile global path modifications

SYNOPSIS

with-overlayfs (options...) -- (command args...)

DESCRIPTION

The with-overlayfs program is designed to mimick the modification of global paths which otherwise cannot be modified by the current user. The original paths can be read-only paths. The purpose is to run the provided command with its arugments under a temporary environment in which the changes have seemingly taken place, but are not visible to any other process, and would vanish with the termination of the command.

OPTIONS

• -v Print version and exit

• --help,-h, or no args

   Short a command line syntax summery
  

• --discard-after-exec

   Discard the effects from the namespace immediately after execution (uses am LD_PRELOAD wrapper)
  

• --dir-start [path] (dir options...) --dir-end

   Describe operations under the provided path. `(dir options...)` can be:
  

  --replace [target] [source]

  Replace 'target' under 'path' with the filename provided as 'source'. The file will be copied, and the storage location is for the overlayfs mounts taking place is /tmp.

EXAMPLES

• Example A

We can experiment with a program that reads a file under /etc, without modifying /etc.

    with-overlayfs                                  \
        --dir-start /etc                            \
        --replace  myconf.conf myconf-modified.conf \
        --dir-end                                   \
        -- ./program my-arg

• Example B

One can imagine a scenario where a dynamically linked executable must run on a system that has conflicting shared libraries. If such executable might fork and invoke other executables, then LD_LIBRARY_PATH cannot be used.

Without resorting to solving the problem with VMs (or dockers), one can put a copy of the libaries on which the executable is dependant in a special directory, and instruct with-overlayfs to virtually copy the files under their expected resting place, /usr/lib64:

A wrapper script for the program can be made using with-overlayfs, e.g.:

    #!/bin/bash

    exec with-overlayfs                                                             \
        --discard-after-exec                                                        \
        --dir-start /usr/lib64                                                      \
        --replace  libc.so.6            /opt/program/special/libc.so.6              \
        --replace  libpthread.so.0      /opt/program/special/libpthread.so.0        \
        --replace  libm.so.6            /opt/program/special/libm.so.6              \
        --replace  libgcc_s.so.1        /opt/program/special/libgcc_s.so.1          \
        --replace  libdl.so.2           /opt/program/special/libdl.so.2             \
        --replace  librt.so.1           /opt/program/special/librt.so.1             \
        --replace  ld-linux-x86-64.so.2 /opt/program/special/ld-linux-x86-64.so.2   \
        --replace  libstdc++.so.6       /opt/program/special/libstdc++.so.6         \
        --dir-end                                                                   \
        --  /usr/bin/program "$@"

Building

Common Linux distributions

The required packages are make, g++, automake, autoconf, libtool, and the pandoc package. With all the dependencies, it can be built and installed as such:

    ./autogen.sh
    ./configure
    make
    make install

RPM under Fedora

First,

    yum install mock rpm-build

Then, the following will generate the RPM and invoke mock:

    ./packaging/build-srpm -o . -- -r default

The resulting outputs are in a subdirectory under the current directory.

    $ ls -1 with-overlayfs-0.1-2015.08.01.182506.git471b7f084d8d.fc21.src.rpm.mockresult
    build.log
    root.log
    state.log
    with-overlayfs-0.1-2015.08.01.182506.git471b7f084d8d.fc21.src.rpm
    with-overlayfs-0.1-2015.08.01.182506.git471b7f084d8d.fc21.x86_64.rpm
    with-overlayfs-debuginfo-0.1-2015.08.01.182506.git471b7f084d8d.fc21.x86_64.rpm