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137 lines
6.1 KiB
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137 lines
6.1 KiB
Text
This document describes how FAT and VFAT file system permissions work
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in Linux with a focus on configuring them for Wine.
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Introduction
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------------
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Linux is able to access DOS and Windows file systems using either the
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FAT (older 8.3 DOS filesystems) or VFAT (newer Windows 95 or later
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long filename filesystems) modules. Mounted FAT or VFAT filesystems
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provide the primary means for which existing applications and their
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data are accessed through Wine for dual boot (Linux + Windows)
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systems.
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Wine maps mounted FAT filesystems, such as "/c", to driver letters,
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such as "c:", as indicated by the wine.conf file. The following
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excerpt from a wine.conf file does this:
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[Drive C]
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Path=/c
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Type=hd
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Although VFAT filesystems are preferable to FAT filesystems for their
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long filename support the term "FAT" will be used throughout the
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remainder of this document to refer to FAT filesystems and their
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derivatives. Also, "/c" will be used as the FAT mount point in
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examples throughout this document.
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Most modern Linux distributions either detect or allow existing FAT
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file systems to be configured so that can be mounted, in a location
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such as /c, either persistently (on bootup) or on an as needed basis.
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In either case, by default, the permissions will probably be configured
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so that they look something like:
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~>cd /c
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/c>ls -l
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-rwxr-xr-x 1 root root 91 Oct 10 17:58 autoexec.bat
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-rwxr-xr-x 1 root root 245 Oct 10 17:58 config.sys
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drwxr-xr-x 41 root root 16384 Dec 30 1998 windows
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where all the files are owned by "root", are in the "root" group and
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are only writable by "root" (755 permissions). This is restrictive in
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that it requires that Wine be run as root in order for applications to
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be able to write to any part of the filesystem.
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There three major approaches to overcoming the restrictive permissions
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mentioned in the previous paragraph:
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1) Run Wine as root
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2) Mount the FAT filesystem with less restrictive permissions
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3) Shadow the FAT filesystem by completely or partially copying it
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Each approach will be discusses in the following "Running Wine as
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root", "Mounting FAT filesystems" and "Shadowing FAT filesystems"
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sections.
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Running Wine as root
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--------------------
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Running Wine as root is the easiest and most thorough way of giving
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applications that Wine runs unrestricted access to FAT files systems.
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Running wine as root also allows applications to do things unrelated
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to FAT filesystems, such as listening to ports that are less than
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1024. Running Wine as root is dangerous since there is no limit to
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what the application can do to the system.
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Mounting FAT filesystems
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------------------------
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The FAT filesystem can be mounted with permissions less restrictive
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than the default. This can be done by either changing the user that
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mounts the FAT filesystem or by explicitly changing the permissions
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that the FAT filesystem is mounted with. The permissions are
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inherited from the process that mounts the FAT filesystem. Since the
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process that mounts the FAT filesystem is usually a startup script
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running as root the FAT filesystem inherits root's permissions. This
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results in the files on the FAT filesystem having permissions similar
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to files created by root. For example:
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~>whoami
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root
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~>touch root_file
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~>ls -l root_file
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-rw-r--r-- 1 root root 0 Dec 10 00:20 root_file
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which matches the owner, group and permissions of files seen on the
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FAT filesystem except for the missing 'x's. The permissions on the
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FAT filesystem can be changed by changing root's umask (unset
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permissions bits). For example:
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~>umount /c
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~>umask
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022
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~>umask 073
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~>mount /c
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~>cd /c
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/c>ls -l
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-rwx---r-- 1 root root 91 Oct 10 17:58 autoexec.bat
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-rwx---r-- 1 root root 245 Oct 10 17:58 config.sys
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drwx---r-- 41 root root 16384 Dec 30 1998 windows
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Mounting the FAT filesystem with a umask of 000 gives all users
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complete control over the it.
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Explicitly specifying the permissions of the FAT filesystem when it is
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mounted provides additional control. There are three mount options
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that are relevant to FAT permissions: "uid", "gid" and "umask". They
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can each be specified when the filesystem is manually mounted. For
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example:
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~>umount /c
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~>mount -o uid=500 -o gid=500 -o umask=002 /c
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~>cd /c
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/c>ls -l
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-rwxrwxr-x 1 sle sle 91 Oct 10 17:58 autoexec.bat
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-rwxrwxr-x 1 sle sle 245 Oct 10 17:58 config.sys
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drwxrwxr-x 41 sle sle 16384 Dec 30 1998 windows
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which gives "sle" complete control over /c. The options listed above
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can be made permanent by adding them to the /etc/fstab file:
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~>grep /c /etc/fstab
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/dev/hda1 /c vfat uid=500,gid=500,umask=002,exec,dev,suid,rw 1 1
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Note that the umask of 002 is common in the user private group file
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permission scheme. On FAT file systems this umask assures that all
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files are fully accessible by all users in the specified group (gid).
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Shadowing FAT filesystems
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-------------------------
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Shadowing provides a finer granularity of control. Parts of the
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original FAT filesystem can be copied so that the application can
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safely work with those copied parts while the application continue to
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directly read the remaining parts. This is done with symbolic links.
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For example, consider a system where an application named "AnApp" must
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be able to read and write to the c:\windows and c:\AnApp directories
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as well as have read access to the entire FAT filesystem. On this
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system the FAT filesystem has default permissions which should not be
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changed for security reasons or can not be changed due to lack of root
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access. On this system a shadow directory might be set up in the
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following manner:
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~>cd /
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/>mkdir c_shadow
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/>cd c_shadow
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/c_shadow>ln -s /c_/* .
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/c_shadow>rm windows AnApp
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/c_shadow>cp -R /c_/{windows,AnApp} .
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/c_shadow>chmod -R 777 windows AnApp
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/c_shadow>perl -p -i -e 's|/c$|/c_shadow|g' /usr/local/etc/wine.conf
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The above gives everyone complete read and write access to the
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"windows" and "AnApp" directories while only root has write access to
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all other directories.
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---
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Steven Elliott (elliotsl@mindspring.com)
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