Class: HomeFS

Inherits:
Object
  • Object
show all
Defined in:
lib/homefs/homefs.rb,
lib/homefs/debug.rb

Overview

An instance of HomeFS; this implements all FuseFS methods, and ultimately handles reading, writing, and inspection of files.

We rely on the context provided to us by FUSE to determine the calling user’s UID and GID.

Currently, HomeFS does not support POSIX locking (which has little effect in practice), and special files (those created by mknod/FIFOs). We do supported extended filesystem attributes, but only if you have the ‘ffi-xattr’ gem.

Author:

  • Dylan Frese

Defined Under Namespace

Classes: Wrapper

Instance Method Summary collapse

Constructor Details

#initialize(path, options = Hash.new) ⇒ HomeFS

Creates a new instance of HomeFS. This instance should probably be passed to RFuse, as it does nothing on its own. The path is relative to the calling user’s home directory, that is, the user interacting with the filesystem through open(2), read(2), write(2), etc. the file system.

Parameters:

  • path (String) —

    the path, relative to $HOME, to use as the root of



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# File 'lib/homefs/homefs.rb', line 22

def initialize(path, options = Hash.new)
    @relpath = path
    read_passwd
    read_group
end

Instance Method Details

#access(context, path, mode) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Check access permissions

Parameters:

  • context (Context)
  • path (String)
  • mode (Integer) —

    the permissions to check

Raises:

  • (Errno::EACCESS) —

    if the requested permission isn’t available



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# File 'lib/homefs/homefs.rb', line 270

def access(context, path, mode)
    uid = context.uid
    unless mode_mask(homepath(uid, path), mode * 0111, uid)
        raise Errno::EACCES
    end
end

#check_listable(uid, relpath) ⇒ void

This method returns an undefined value.

This method raises an Errno::EACCES if the user given by uid cannot list the given directory, i.e., if the user does not have the execute and read permissions on the given directory. If the path given is not a directory, the dirname of the given path is tested. HomeFS (not the actual filesystem)

Parameters:

  • relpath (String) —

    the path to test, relative to the root of

  • uid (Integer) —

    the UID to check the privilage of

Raises:

  • (Errno::EACCES) —

    if the path is not readable by the given user



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# File 'lib/homefs/homefs.rb', line 234

def check_listable(uid, relpath)
    hpath = homepath(uid, relpath)
    unless File.directory?(hpath)
        hpath = File.dirname(hpath)
    end
    unless readable?(hpath) && executable?(hpath)
        fail Errno::EACCES, relpath
    end
end

#check_owner(uid, relpath) ⇒ void

This method returns an undefined value.

This method raises an Errno::EACCES if the user given by uid is not the owner of the file described by relpath HomeFS (not the actual filesystem) path

Parameters:

  • relpath (String) —

    the path to test, relative to the root of

  • uid (Integer) —

    the UID to check ownership of

Raises:

  • (Errno::EACCES) —

    if the path is not owned by the given user

  • (Errno::ENOENT) —

    if no file or directory exists at the given



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# File 'lib/homefs/homefs.rb', line 253

def check_owner(uid, relpath)
    hpath = File.dirname(homepath(uid, relpath))
    unless File.stat(hpath).uid == uid
        fail Errno::EACCES, relpath
    end
end

#check_readable(uid, relpath) ⇒ void

This method returns an undefined value.

This method raises an Errno::EACCES if the user given by uid cannot read from the given path. HomeFS (not the actual filesystem) path

Parameters:

  • relpath (String) —

    the path to test, relative to the root of

  • uid (Integer) —

    the UID to check the privilage of

Raises:

  • (Errno::EACCES) —

    if the path is not readable by the given user

  • (Errno::ENOENT) —

    if no file or directory exists at the given



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# File 'lib/homefs/homefs.rb', line 218

def check_readable(uid, relpath)
    hpath = homepath(uid, relpath)
    unless readable?(hpath, uid)
        fail Errno::EACCES, relpath
    end
end

#check_writable(uid, relpath) ⇒ void

This method returns an undefined value.

This method raises an Errno::EACCES if the user given by uid cannot write to the given path. The check is the same as in #writable?. HomeFS (not the actual filesystem)

Parameters:

  • relpath (String) —

    the path to test, relative to the root of

  • uid (Integer) —

    the UID to check the privilage of

Raises:

  • (Errno::EACCES) —

    if the path is not writable by the given user



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# File 'lib/homefs/homefs.rb', line 202

def check_writable(uid, relpath)
    hpath = homepath(uid, relpath)
    unless writable?(hpath, uid)
        fail Errno::EACCES, relpath
    end
end

#chmod(context, path, mode) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Change file permissions

Parameters:

  • context (Context)
  • path (String)
  • mode (Integer)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 287

def chmod(context, path, mode)
    hpath = homepath(context.uid, path)
    check_owner(context.uid, path)
    File.chmod(mode, hpath)
end

#chown(context, path, uid, gid) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Change file ownership

Parameters:

  • context (Context)
  • path (String)
  • uid (Integer) —

    new user id

  • gid (Integer) —

    new group id

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 304

def chown(context, path, uid, gid)
    raise Errno::EACCES, path if context.uid != 0
    hpath = homepath(context.uid, path)
    File.chown(uid, gid, hpath)
end

#create(context, path, mode, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Create and open a file If the file does not exist, first create it with the specified mode,

and then open it.

Parameters:

  • context (Context)
  • path (String)
  • mode (Integer) —

    the file permissions to create

  • ffi (Fileinfo) —

    • use the FileInfo#fh attribute to store a

    filehandle

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 323

def create(context, path, mode, ffi)
    check_writable(context.uid, path)
    hpath = homepath(context.uid, path)

    # It is important that we create the file and chown it before we
    # set its mode to prevent a possible privilage escalation
    # race-condition. To be more specific, if this filesystem is
    # running as root (which it usually is), the file will be created
    # owned by root. If the mode were set when the file is created,
    # then an attacker could create a setuid file writable by anyone,
    # and then, in the 'real' filesystem (not HomeFS), quickly write a
    # small program that just executes /bin/bash. If the write
    # completed before handle.chown was called, the attacker could have
    # a setuid shell.
    handle = File.new(hpath, File::CREAT | File::WRONLY, 0600)
    handle.chown(context.uid, context.gid)
    handle.chmod(mode)
    ffi.fh = handle
end

#executable?(path, uid = nil) ⇒ Boolean

Test whether the user described by uid can execute the given path. If no file exists at the given path, an Errno::ENOENT is raised. method returns whether the file is executable by anyone.

Parameters:

  • path (String) —

    the path to test

  • uid (Integer) (defaults to: nil) —

    the UID to check the privilage of. If nil, this

Returns:

  • (Boolean) —

    whether the given path is executable

Raises:

  • (Errno::ENOENT) —

    if no file exists at path



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# File 'lib/homefs/homefs.rb', line 191

def executable?(path, uid = nil)
    mode_mask(path, 0111, uid)
end

#fgetattr(context, path, ffi) ⇒ Stat

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

Get attributes of an open file

Parameters:

  • context (Context)
  • path (String)
  • ffi (Fileinfo)

Returns:

  • (Stat) —

    file attributes

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 353

def fgetattr(context, path, ffi)
    ffi.fh.lstat
end

#flush(context, path, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Possibly flush cached data BIG NOTE: This is not equivalent to fsync(). It’s not a request to sync dirty data. Flush is called on each close() of a file descriptor. So if a filesystem wants to return write errors in close() and the file has cached dirty data, this is a good place to write back data and return any errors. Since many applications ignore close() errors this is not always useful.

NOTE: The flush() method may be called more than once for each open(). This happens if more than one file descriptor refers to an opened file due to dup(), dup2() or fork() calls. It is not possible to determine if a flush is final, so each flush should be treated equally. Multiple write-flush sequences are relatively rare, so this shouldn’t be a problem.

Filesystems shouldn’t assume that flush will always be called after some writes, or that if will be called at all.

Parameters:

  • context (Context)
  • path (String)
  • ffi (FileInfo)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 383

def flush(context, path, ffi) # We don't do any caching
end

#fsync(context, path, datasync, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Synchronize file contents

Parameters:

  • context (Context)
  • path (String)
  • datasync (Integer) —

    if non-zero, then only user data should be flushed, not the metadata

  • ffi (FileInfo)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 398

def fsync(context, path, datasync, ffi)
    if datasync
        ffi.fh.fdatasync
    else
        ffi.fh.fsync
    end
end

#ftruncate(context, path, size, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Change the size of an open file

Parameters:

  • context (Context)
  • path (String)
  • size (Integer)
  • ffi (Fileinfo)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 417

def ftruncate(context, path, size, ffi)
    ffi.fh.truncate(size)
end

#getattr(context, path) ⇒ Stat

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

Get file attributes. Similar to stat(). The ‘st_dev’ and ‘st_blksize’ fields are ignored. The ‘st_ino’ field is ignored except if the ‘use_ino’ mount option is given.

Parameters:

  • context (Context)
  • path (String)

Returns:

  • (Stat) —

    or something that quacks like a stat, or nil if the path does not exist

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 433

def getattr(context, path)
    check_listable(context.uid, path)
    File.lstat(homepath(context.uid, path))
end

#homepath(uid, path = nil) ⇒ String

Return the path to the root of HomeFS relative to the underlying filesystem. If path is specified, it is taken to be relative to the root of HomeFS.

Parameters:

  • uid (Integer) —

    the UID whose home directory to use as a base

  • path (String) (defaults to: nil) —

    a relative path to a resource

Returns:

  • (String) —

    path to the root of the HomeFS relative to the underlying filesystem, or if path is specified, the path to that resource relative to the underlying filesystem.

Raises:

  • (Errno::ENOENT) 


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# File 'lib/homefs/homefs.rb', line 90

def homepath(uid, path = nil)
    basepath = @homedirs[uid]
    # basepath shouldn't ever be nil, but fail gracefully just
    # in case.
    raise Errno::ENOENT if basepath == nil
    if path
        File.join(basepath, @relpath, path)
    else
        File.join(basepath, @relpath)
    end
end

#init(info) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Called when filesystem is initialised

Parameters:

  • context (Context)
  • info (Struct) —

    connection information



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# File 'lib/homefs/homefs.rb', line 446

def init(context, info)
end

#link(context, from, to) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Create a hard link to file

Parameters:

  • context (Context)
  • from (String)
  • to (String)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 459

def link(context, from, to)
    hfrom = homepath(context.uid, from)
    check_writable(context.uid, from)
    File.link(hfrom, to)
    File.chown(context.uid, hfrom) if context.uid == 0
end

#mkdir(context, path, mode) ⇒ Object

Note:

This method should usually only be called by FUSE, and not called directly



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# File 'lib/homefs/homefs.rb', line 469

def mkdir(context, path, mode)
    check_writable(context.uid, path)
    hpath = homepath(context.uid, path)
    Dir.mkdir(hpath, mode)
    File.chown(context.uid, context.gid, hpath) if context.uid == 0
end

#mode_mask(file, mask, uid = nil) ⇒ Boolean

Check whether the given file has a mode that matches the given mask. If check_ids is true, the third-least significant digit of the mask is set to zero if the calling user is not the owner of the file, and the second-least significant digit of the mask is zeroed if the calling user’s group is not the group of the file. This has the effect of only checking the owner’s permissions if we’re the owner, and only checking the group permissions if we’re in that group.

This method checks if any of the set bits in the mask are set in the file’s mode. It does not check if the mask matches the file’s mode, nor does it check if all set bits in the mask are set in the file’s mode.

The first three least significant digits of the mode, in octal, (that is, the three rightmost digits) control the read (4), write (2), and execute (1) permissions of the file. The next digit controls setuid (4), setgid (2), and the sticky bit (1) of the file.

It is recommended to see

man chmod

to get a more detailed description about file modes.

Examples:

Test if we can write to a file

mode_mask("/example", 0222, true)
mode_mask("/example", 0222)

Test if a file is executable by anyone

mode_mask("/binary", 0111, false)

Test if a directory has the sticky bit set

mode_mask("/tmp", 01000, false)

Parameters:

  • file (String) —

    the path to the file to check

  • mask (Integer) —

    the mask against which to check the file’s mode. It is recommended you write this in octal (with a leading 0).

  • uid (Integer) (defaults to: nil) —

    if not nil, only check user and group permissions if the user it represents is, respectively, the owner of the file/in the file’s group.

Returns:

  • (Boolean) —

    whether the mode of file matches the given mask



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# File 'lib/homefs/homefs.rb', line 138

def mode_mask(file, mask, uid = nil)
    stat = File.stat(file)
    fmode = stat.mode
    if uid
        fuid, fgid = stat.uid, stat.gid
        # Zero out the third digit (in octal).
        # We could use a constant here, but this works
        # for a mask of any length
        if uid != fuid
            mask &= ~(mask & 0700)
        end
        if !@groups[fgid].include?(uid)
            mask &= ~(mask & 0070)
        end
    end
    fmode & mask != 0
end

#open(context, path, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

File open operation file open flags etc. The fh attribute may be used to store an arbitrary filehandle object which will be passed to all subsequent operations on this file

Parameters:

  • context (Context)
  • path (String)
  • ffi (FileInfo)

Raises:

  • (Errno::ENOPERM) —

    if user is not permitted to open the file

  • (Errno) —

    for other errors



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# File 'lib/homefs/homefs.rb', line 490

def open(context, path, ffi)
    if ffi.flags & File::RDWR != 0
        mask = 0666
    elsif ffi.flags & File::WRONLY != 0
        mask = 0222
    else
        mask = 0444
    end
    hpath = homepath(context.uid, path)
    unless mode_mask(hpath, mask, context.uid)
        fail Errno::EACCES, path
    end

    # We pass the flags straight into File.open because the constants
    # in RFuse::Fcntl and File::Constants have the same values, because
    # they both have the same values as the open syscall. If this were
    # not the case, we'd have the map the values of RFuse::Fcntl to
    # their equivalents in File::Constants
    ffi.fh = File.open(hpath, ffi.flags)
end

#opendir(context, path, name) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Open directory Unless the ‘default_permissions’ mount option is given, this method should check if opendir is permitted for this directory. Optionally opendir may also return an arbitrary filehandle in the fuse_file_info structure, which will be available to #readdir, #fsyncdir, #releasedir.

Parameters:

  • context (Context)
  • path (String)
  • ffi (FileInfo)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 526

def opendir(context, path, ffi)
    check_listable(context.uid, path)
    ffi.fh = Dir.new(homepath(context.uid, path))
end

#read(context, path, size, offset, ffi) ⇒ String

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

Read data from an open file

Parameters:

  • context (Context)
  • path (String)
  • size (Integer)
  • offset (Integer)
  • ffi (FileInfo)

Returns:

  • (String) —

    should be exactly the number of bytes requested, or empty string on EOF

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 544

def read(context, path, size, offset, ffi)
    if offset < 0
        ffi.fh.seek(offset, :END)
    else
        ffi.fh.seek(offset, :SET)
    end
    ffi.fh.read(size)
end

#read_group ⇒ void

This method returns an undefined value.

Read or re-read /etc/group to update the internal table of group membership.



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# File 'lib/homefs/homefs.rb', line 61

def read_group
    # Try to read from getent
    group = `getent group`
    if $? != 0
        group = File.read('/etc/group')
    end

    @groups = Hash.new

    group.each_line.map { |line|
        next if line.strip.empty?
        _name, _, gid, members = line.split(':')
        members = members.strip.split(',').map {|member| @usernames[member]}
        [Integer(gid), members]
    }.reject { |line|
        line.nil?
    }.each { |gid, members|
        @groups[gid] = members
    }
end

#read_passwd ⇒ void

This method returns an undefined value.

Read or re-read /etc/passwd to update the internal table of home directories for each UID. This method does not try to guess as to which user IDs are owned by actual users of the system, and which are utility accounts such as ‘lp’ or ‘pulse’.



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# File 'lib/homefs/homefs.rb', line 33

def read_passwd
    # Try to read from getent
    passwd = `getent passwd`
    if $? != 0
        passwd = File.read('/etc/passwd')
    end

    @homedirs = Hash.new
    @usernames = Hash.new

    # Here we map each line in passwd to an array,
    # which Hash interprets as a key, value pair.
    passwd.each_line.map { |line|
        next if line.strip.empty?
        values = line.split(':')
        # Username, UID, home directory
        [values[0], Integer(values[2]), values[5]]
    }.reject { |line|
        line.nil?
    }.each { |username, uid, home|
        @homedirs[uid] = home
        @usernames[username] = uid
    }
end

#readable?(path, uid = nil) ⇒ Boolean

Test whether the user described by uid can read from the given path. If no file exists at the given path, an Errno::ENOENT is raised. method returns whether the file is readable by anyone.

Parameters:

  • path (String) —

    the path to test

  • uid (Integer) (defaults to: nil) —

    the UID to check the privilage of. If nil, this

Returns:

  • (Boolean) —

    whether the given path is readable

Raises:

  • (Errno::ENOENT) —

    if no file exists at path



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# File 'lib/homefs/homefs.rb', line 179

def readable?(path, uid = nil)
    mode_mask(path, 0444, uid)
end

#readdir(context, path, filler, offset, ffi) ⇒ Object

Note:

This method should usually only be called by FUSE, and not called directly



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# File 'lib/homefs/homefs.rb', line 556

def readdir(context, path, filler, offset, ffi)
    filler.push(".", nil, 0)
    filler.push("..", nil, 0)
    ffi.fh.pos = offset
    ffi.fh.each do |filename|
        next if filename == '.' || filename == '..'
        filler.push(filename, nil, 0)
    end
end

#readlink(context, path, size) ⇒ String

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

Resolve target of symbolic link

Parameters:

  • context (Context)
  • path (String)
  • size (Integer) —

    if the resolved path is greater than this size it should be truncated

Returns:

  • (String) —

    the resolved link path

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 577

def readlink(context, path, size)
    # Is it okay that we return the 'real' path here?
    File.readlink(homepath(context.uid, path))[0 ... size]
end

#release(context, path, ffi) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Release an open file Release is called when there are no more references to an open file: all file descriptors are closed and all memory mappings are unmapped.

For every #open call there will be exactly one #release call with the same flags and file descriptor. It is possible to have a file opened more than once, in which case only the last release will mean, that no more reads/writes will happen on the file.

Parameters:

  • context (Context)
  • path (String)
  • ffi (FileInfo) 


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# File 'lib/homefs/homefs.rb', line 598

def release(context, path, ffi)
    ffi.fh.close
end

#rename(context, from, to) ⇒ Object

Note:

This method should usually only be called by FUSE, and not called directly



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# File 'lib/homefs/homefs.rb', line 605

def rename(context, from, to)
    hfrom = homepath(context.uid, from)
    hto = homepath(context.uid, to)
    check_readable(context.uid, from)
    check_writable(File.dirname(from), context.uid)
    check_writable(File.dirname(to), context.uid)
    FileUtils.mv(hfrom, hto, :force => true)
end

#symlink(context, to, from) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Create a symbolic link Create a symbolic link named “from” which, when evaluated, will lead

to "to".

Parameters:

  • context (Context)
  • to (String)
  • from (String)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 626

def symlink(context, to, from)
    check_writable(context.uid, from)
    hfrom = homepath(context.uid, from)
    File.symlink(to, hfrom)
    begin
        File.lchown(context.uid, context.gid, hfrom)
    rescue
    end
end

#truncate(context, path, offset) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Change the size of a file

Parameters:

  • context (Context)
  • path (String)
  • offset (Integer)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 646

def truncate(context, path, offset)
    check_writable(context.uid, path)
    File.truncate(homepath(context.uid, path), offset)
end

#unlink(context, path) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Remove a file

Parameters:

  • context (Context)
  • path (String)

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 660

def unlink(context, path)
    check_writable(context.uid, File.dirname(path))
    File.unlink(homepath(context.uid, path))
end

#utimens(context, path, actime, modtime) ⇒ void

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

This method returns an undefined value.

Change access/modification times of a file

Parameters:

  • context (Context)
  • path (String)
  • actime (Integer) —

    access time in nanoseconds

  • modtime (Integer) —

    modification time in nanoseconds

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 676

def utimens(context, path, actime, modtime)
    check_writable(context.uid, path)
    File.utime(actime, modtime, homepath(context.uid, path))
end

#writable?(path, uid = nil) ⇒ Boolean

Test whether we can write to the given path. If no file exists at path, we test if the parent directory is either writable or has the sticky bit set. method returns whether the file is writable by anyone.

Parameters:

  • path (String) —

    the path to test

  • uid (Integer) (defaults to: nil) —

    the UID to check the privilage of. If nil, this

Returns:

  • (Boolean) —

    whether the given path is writable



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# File 'lib/homefs/homefs.rb', line 163

def writable?(path, uid = nil)
    if !File.exist?(path)
        mode_mask(File.dirname(path), 01222, uid)
    else
        mode_mask(path, 0222, uid)
    end
end

#write(context, path, data, offset, ffi) ⇒ Integer

This method is abstract.
Note:

This method should usually only be called by FUSE, and not called directly

Write data to an open file

Parameters:

  • context (Context)
  • path (String)
  • data (String)
  • offset (Integer)
  • ffi (FileInfo)

Returns:

  • (Integer) —

    exactly the number of bytes requested except on error

Raises:

  • (Errno) 


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# File 'lib/homefs/homefs.rb', line 694

def write(context, path, data, offset, ffi)
    if offset < 0
        ffi.fh.seek(offset, :END)
    else
        ffi.fh.seek(offset, :SET)
    end
    ffi.fh.write(data)
end