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- Intended audience
- =================
- This document is intended for everybody who wants to understand VFS
- code. Knowledge of programming is a must.
- Preface
- =======
- While VFS should be considered an excellent idea, which came ahead of
- its time, the implementation used in GNU Midnight Commander is now
- showing its age.
- The VFS code was left us without any decent documentation. Most
- functions don't have comments explaining what they do. Most comments
- describe quirks and implementation details, rather than the intended
- functionality of the code. This document is an attempt to reconstruct
- understanding of the VFS code and help its future developers.
- Being the part of GNU Midnight Commander most exposed to potential
- security threats, the VFS code needs to be kept is a good shape.
- Understanding the code is the key to making and keeping it secure.
- Basics of code organization
- ===========================
- VFS code it to a certain extent object oriented. The code dealing with
- a certain type of data (e.g. tar archives) can be thought
- of as a class in the terms of object oriented programming. They may
- reuse some code from their parent classes. For instance, tar and cpio
- archives have a common parent class direntry, which contains some common
- code for archives.
- Individual archives or connections can be considered as instances of
- those classes. They provide POSIX like interface to their structure,
- but don't expose that structure directly to the common VFS layer.
- Each VFS object has a directory tree associated with it. The tree
- consists of entries for files and directories. In some VFS classes, the
- entries have names and a are associated with nameless inodes, which
- contain information such as size, timestamps and other data normally
- contained in POSIX "struct stat".
- File vfs.c serves as a multiplexor. It exports functions similar to
- POSIX but with "mc_" prepended to them. For example, mc_open() will act
- like open(), but will treat VFS names in a special way.
- Common utility functions not intended to be used outside the VFS code
- should go to utilvfs.c and possibly to other files. Presently, there is
- a lot of such code in vfs.c.
- Hierarchy of classes
- ====================
- vfs ---- direntry ---- cpio } archives
- | | ---- tar }
- | |
- | | ---- fish } remote systems
- | | ---- ftpfs }
- |
- |---- extfs ---- extfs archives
- |---- localfs ---- sfs ---- sfs archives
- |---- undelfs
- Properties of classes
- =====================
- read only inode->entry local cache full tree
- mapping loaded
- cpio yes* yes* no yes
- tar yes* yes* no yes
- fish no yes yes no
- ftpfs no yes yes no
- extfs no no yes yes
- localfs no no N/A N/A
- sfs no yes yes N/A
- undelfs no yes no yes
- "*" means that this property should change during further development.
- Mapping from inode to entry prevents implementing hard links. It is
- permissible for directories, which cannot be hardlinked. Not loading
- the full tree speeds up access to large archives and conserves memory.
- Stamping
- ========
- Stamping is the VFS equivalent of garbage collection. It's purpose is
- to destroy unreferenced VFS objects, in other words close archives or
- connections once they are unused for some time. There is a tree of
- items representing VFS objects. The common layer doesn't know the
- structure of the pointers, but it knows the class that should handle the
- pointer. Every item has a timestamp. Once the timestamp becomes too
- old, the object is freed.
- There are ways to keep objects alive if they are used. Also, objects
- can have parent objects, which are freed together with there original
- object if they are otherwise unreferenced.
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