ld-preload-open / path-mapping.so

This library can trick a process into opening absolute paths from a different location, similar to bind mounts, but without root access. The main difference that an affected process can (apparently) see and access files inside the virtual mapped directory, but cannot see the virtual directory itself. Also, in contrast to mounts, every process on a system can have its own mapping (disregarding mount namespaces in current kernels, which basically also require root access).

WARNING: This library is kind of a hack.

It works quite well, mostly, but unforseen side effects or crashes are totally possible (see Potential problems below).

DO NOT USE for mission critical software!

Example 1

One example are "Environment Modules" which can be loaded with the command module load to activate some software in the current shell. If you have multiple versions of a software, which should be available side by side, you can use this library to trick them into loading version specific assets from a common absolute path.

Imagine you are an admin and would like to provide both version 2019 and version 2021 of someprogram to your users. However, someprogram loads a file from a hard-coded path /usr/share/someprogram/assets, but different versions of the program ship with different versions of the file /usr/share/someprogram/assets.

By creating a wrapper script which runs the original program with path-mapping.so, you can force each version to load its own version, without altering the executable binary:

#!/bin/bash
# in file /modules/someprogram/v2019/wrapper/someprogram
PATH_MAPPING="/usr/share/someprogram:/modules/someprogram/v2019/share/someprogram" \
  LD_PRELOAD=/path/to/path-mapping.so \
  /modules/someprogram/v2019/bin/someprogram

Then just add /modules/someprogram/v2019/wrapper to PATH in your module definition file for version 2019 of someprogram and your users will be able to load it with module load someprogram/2019 and run the wrapper script as someprogram.

Example 2

Another example use-case might be to run a program with hard-coded paths from your home folder. Imagine, someprogram tries to load files from /usr/share/someprogram, with no way to configure that path (apart from re-compiling, if you have the source code at all). If you can't put the files there (for whatever reason), you could place them in $HOME/.local/share/someprogram instead.

With the following command, when the program tries to open e.g. /usr/share/someprogram/icons.png (which does not exist), path-mapping.so would intercept that file access and rewrite the path to $HOME/.local/share/someprogram/icons.png, which does exist:

PATH_MAPPING="/usr/share/someprogram:$HOME/.local/share/someprogram" \
  LD_PRELOAD=/path/to/path-mapping.so \
  someprogram

How it works

The path mapping works by intercepting standard library functions which have a path argument, like open or chmod. If this argument matches the given prefix (the first part) of the mapping, the prefix is replaced by the destination (the second part) of the mapping. Then the original standard library function is called with this possibly modified path.

Most Linux libc functions that do something with files are supported (except those that I forgot). The actual list of functions is quite long and can be looked up in the code. However, even if all functions with a path argument are overloaded, there are some pitfalls. See below under Potential problems for more information.

Path mapping configuration

There are two ways to specify the path mappings. An arbitrary number of mappings can be used at once.

1. If the environment variable PATH_MAPPING exists, path-mapping.so will try to initialize the mappins from there. The first part of each pair is the prefix, and the second part is the destination, so the number of given paths must be even. All parts are separated by colons:

   export PATH_MAPPING="/usr/virtual1:/map/dest1:/usr/virtual2:/map/dest2"

2. If PATH_MAPPING is unset or empty, the mapping specified in the variable default_path_map will be used instead. You can modify it if you don't want to set PATH_MAPPING, for example like this:

   static const char *default_path_map[][2] = {
       { "/usr/virtual1", "/map/dest1" },
       { "/usr/virtual1", "/map/dest2" },
   };

Compiling and installation

Just run make all to compile the different versions of the library:

• path-mapping-quiet.so is compiled with #define QUIET. It will not print anything, except in case of a fatal configuration error, before stopping the process.
• path-mapping.so will print out a diagnostig string to stderr when a path is mapped to a new destination.
• path-mapping-debug.so is compiled with #define DEBUG. It will will additionally print out one line for each function call to any overridden function. This is very slow and noisy. Only use it to determine which paths need overriding.

Choose one of those files and place it anywhere convenient. Note its absolute path and provide it to the target program as LD_PRELOAD, for example:

cd $HOME/repos/
git clone https://github.com/fritzw/ld-preload-open.git
cd ld-preload-open
make all
make test
export PATH_MAPPING=/somewhere:/$HOME
LD_PRELOAD=$HOME/repos/ld-preload-open/path-mapping.so /bin/ls /somewhere
# This should print something like the following:
# PATH_MAPPING[0]: /somewhere => /home/you
# Mapped Path: __xstat('/somewhere') => '/home/you'
# Mapped Path: opendir('/somewhere') => '/home/you'
# ... followed by the contents of your home directory.

Compile time options

There some options which can be set during compile time by defining some preprocessor macros. For example, adding #define QUIET or compiling with -DQUIET will remove all informational printf commands.

Normally, the loaded path mapping will be printed to stderr at startup, and an info message will be printed to stderr each time a path mapping is applied.

• QUIET: Removes all info_fprintf commands. The resulting path-mapping.so will not print anything, except for initialization errors which will exit() the program immediately.
• DEBUG: Enables all debug_fprintf calls. This will print additional output to stderr each time an overloaded functions called, including the path argument(s).
• DISABLE_*: These options allow you to disable the overloading of some specific functions if you desire. See the code in path-mapping.c for a complete list.

Tests

Run make test to execute the included test suite. Most things should be tested, but multiple variants of the same function are usually not tested separately.

Potential problems

On first glance, this library might look like it can be used as a replacement for mount --bind. However, since this is quite a hacky solution that runs only in user space, there are some issues where things do not work quite as one would expect. Some of these could be fixed or worked around, but in some cases that would require significantly more work than just overloading a few functions.

1. Only absolute paths are currently mapped, relative paths are not. If a program does open("/usr/virtual1/file"), it will be mapped to /map/dest1/file, but chdir("/usr") followed by open("virtual1/file") will fail with ENOENT.

   The same problem applies to functions ending in at, like openat. These functions have a parameter int dirfd, relative to which the path argument is searched (if it is not an absolute path). This library makes no attempt to find out the path of the directory which dirfd represents.

2. Return values from standard library functions are not mapped. For example, getcwd() will return /map/dest1 after a calling chdir("/usr/virtual1") (from the example above).

   However, this is usually not be a problem, because the program can then internally use that existing path for all future accesses, which will succeed as expected. Even an interactive bash session can work (to a certain extent) inside virtual mapped directories.

3. Virtual mapped entries do not appear in directory listings. The example mapping for /usr/virtual1 will not show up in ls /usr or find /usr.

4. Symlinks that point into virtual directories will not work, because symlinks are evaluated by the kernel, not in user space. For example, the following will fail:

   export PATH_MAPPING=/tmp/virtual:/tmp/real
   export LD_PRELOAD=/path/to/path-mapping.so
   mkdir /tmp/real
   touch /tmp/real/file
   ln -s /tmp/virtual /tmp/link
   ls -l /tmp/virtual/file # works
   ls -l /tmp/link/file # fails because kernel can not see `/tmp/virtual`

5. Creating relative symlinks that cross a mapping boundary will not work as expected:

   export PATH_MAPPING=/tmp/1/virtual:/tmp/real
   export LD_PRELOAD=/path/to/path-mapping.so
   mkdir /tmp/real
   echo content >/tmp/realfile
   ln -s ../../realfile /tmp/1/virtual/virtuallink
   cat /tmp/1/virtual/virtuallink # fails because /realfile does not exist

   The created link would point to /tmp/realfile, if /tmp/1/virtual/ was a real directory. But since the symlink is evaluated relative to /tmp/real, it will actually point to /realfile, which does not exist.

6. If a programs manually loads a function like fopen from libc.so using ldopen and dlsym, then LD_PRELOAD can not intercept that. In this case, the path mapping will not work.

7. If a standard library function internally calls an overloaded function like stat or open, then LD_PRELOAD can not intercept that.

8. If internal workings of the libc change in the future, a program might just stop working.

9. Path mapping does not work if a program talks to the kernel directly using syscalls (which would be very bad practice) instead of using the libc functions to make the syscalls for it. Or if a program uses a different standard library, which does syscalls directly instead of falling back to the standard libc functions (not sure if something like that exists in practice).

License

This repository is released unter the MIT license, see the file LICENSE for details.