Brent Baccala <email@example.com>
This code takes advantage of the dynamic C library's ability to "preload" other libraries which can override its base functionality and provide various useful services. Currently, a user-level directory overlay facility and an HTTP file system are (partially) implemented. The code is known to work under RedHat Linux with a patched GLIBC.
Download from: http://www.freesoft.org/software/preload/preload-0.1.tgz
The dynamic loader has a feature, originally introduced by Sun, which allows other libraries to be "preloaded" when the C library is loaded. These preload libraries can override functions provided by the standard C library, allowing bizarre and/or useful redefinitions of things like open(). There are several caveats, though. First, the standard GNU C library uses function names prefixed by "__" for internal functions. For example, the C library function fopen() uses __open() internally. So, if you want to override the system call open(), and catch everything that calls it, you need to override open(), to get all the calls to open from the user program, as well as overriding __open(), to get all the calls from within the C library itself. Furthermore, the GNU standard C library doesn't export the "__" functions, so the library has to be patched to export these functions - which rather defeats the purpose of preload libraries in the first place (not having to patch the standard C library), but that's the way it is.
Once a shared preload library has been compiled, it can be loaded in one of two ways. First, by placing the name of the file in the LD_PRELOAD environment variable. This allows preload libraries to be loaded on a per-shell basis. Second, by placing the name of the file in the /etc/ld.so.preload file, the library can be loaded on a system-wide basis. However, since a user can always link a program statically, this system-wide feature can't be absolutely relied upon. Each preload library can define an _init function to be run when it loads.
The Freesoft collection of preload libraries currently includes several useful preload libs. "cwd.so" is a utility library that implements the concept of a Current Working Directory in user space. It translates all filenames in system calls into absolute filenames, making it easier for the other libraries to work with them. "unions.so" is an overlay library, which allows a directory tree to be overlaid on top of another, allowing source code and object code to be seperated, for example, while presenting the appearance that all the code resides in the same directory. "httpfs.so" implements an HTTP file system, which allows URLs to be treated as normal filenames. "trace.so" is a utility library which allows debugging of other preload libraries.
To try it out, add it to the environment variable LD_PRELOAD, i.e:
$ export LD_PRELOAD=/usr/local/lib/cwd.so:/usr/local/lib/httpfs.so
The exact location of the files is unimportant, but cwd.so must be preloaded before httpfs.so. Now, try something like:
$ cat http://www.freesoft.org/
Preload before libc (and after cwd.so) to perform "union" or "overlay" mounts, where one directory tree appears to become layered on top of another. Can be used to make CDs "writable", to conveniently seperate source and object files in build trees, to allow package management software to "install" files that actually end up in packages, etc.
Depends on cwd.so module, which implements CWD in user space, and translates all pathnames in system calls into absolute paths.
$ export LD_PRELOAD=/usr/local/lib/cwd.so:/usr/local/lib/unions.so $ export UNIONS=overlay@target(options):...
For example, export UNIONS=/tmp/cdrom@/mnt/cdrom(create,copyonwrite) causes all the files in /tmp/cdrom to appear overlayed on /mnt/cdrom. Any attempted changes to /mnt/cdrom will actually occur on /tmp/cdrom.
/etc/ld.so.preload should contain lines such as:
/usr/local/lib/cwd.so /usr/local/lib/httpfs.so /usr/local/lib/unions.so
/etc/unions should contain lines like:
Code walkthrough/cleanup Figure out how to handle deletes Decide if I should "rm -rf" this whole directory :-) Fixup httpfs module and add WebDAV support Define an API to provide streams-like functionality - affect a single FD Define an API to figure out what other modules are loaded (so unions.so can demand cwd.so) Create a userspace NFS and/or Coda module Create a module to auto-expand zip/tar files Create a module to provide Hurd translator-type functionality
So I typed "mv boot boot.changed" and then "mv boot.bak boot". Well, the first mv moved the directory in the source partition (it moved it into the binary, overlay partition, but that's another, unrelated, story). But I still had the directory called "boot" in the binary parition, which I had forgotten about. So the second mv moved "boot.bak" into the existing directory "boot" and it became a subdirectory.
So, maybe "mv", or more precisely link()/unlink(), should operate on all files/dirs of the given name. Or maybe that would create more problems in another way. I related this story to point out some of the problems you can get into using this code.
My advise? Use it for well-defined things like make's, but do any manual operations like edits and moves in the normal filestructure.