• Eric W. Biederman's avatar
    sysfs: Implement sysfs tagged directory support. · 3ff195b0
    Eric W. Biederman authored
    
    
    The problem.  When implementing a network namespace I need to be able
    to have multiple network devices with the same name.  Currently this
    is a problem for /sys/class/net/*, /sys/devices/virtual/net/*, and
    potentially a few other directories of the form /sys/ ... /net/*.
    
    What this patch does is to add an additional tag field to the
    sysfs dirent structure.  For directories that should show different
    contents depending on the context such as /sys/class/net/, and
    /sys/devices/virtual/net/ this tag field is used to specify the
    context in which those directories should be visible.  Effectively
    this is the same as creating multiple distinct directories with
    the same name but internally to sysfs the result is nicer.
    
    I am calling the concept of a single directory that looks like multiple
    directories all at the same path in the filesystem tagged directories.
    
    For the networking namespace the set of directories whose contents I need
    to filter with tags can depend on the presence or absence of hotplug
    hardware or which modules are currently loaded.  Which means I need
    a simple race free way to setup those directories as tagged.
    
    To achieve a reace free design all tagged directories are created
    and managed by sysfs itself.
    
    Users of this interface:
    - define a type in the sysfs_tag_type enumeration.
    - call sysfs_register_ns_types with the type and it's operations
    - sysfs_exit_ns when an individual tag is no longer valid
    
    - Implement mount_ns() which returns the ns of the calling process
      so we can attach it to a sysfs superblock.
    - Implement ktype.namespace() which returns the ns of a syfs kobject.
    
    Everything else is left up to sysfs and the driver layer.
    
    For the network namespace mount_ns and namespace() are essentially
    one line functions, and look to remain that.
    
    Tags are currently represented a const void * pointers as that is
    both generic, prevides enough information for equality comparisons,
    and is trivial to create for current users, as it is just the
    existing namespace pointer.
    
    The work needed in sysfs is more extensive.  At each directory
    or symlink creating I need to check if the directory it is being
    created in is a tagged directory and if so generate the appropriate
    tag to place on the sysfs_dirent.  Likewise at each symlink or
    directory removal I need to check if the sysfs directory it is
    being removed from is a tagged directory and if so figure out
    which tag goes along with the name I am deleting.
    
    Currently only directories which hold kobjects, and
    symlinks are supported.  There is not enough information
    in the current file attribute interfaces to give us anything
    to discriminate on which makes it useless, and there are
    no potential users which makes it an uninteresting problem
    to solve.
    Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
    Signed-off-by: default avatarBenjamin Thery <benjamin.thery@bull.net>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
    3ff195b0