Re: [PATCH v2 01/10] vfs: move cap_convert_nscap() call into vfs_setxattr()

ebiederm@xmission.com (Eric W. Biederman) writes:

So there is the basic question do we want to read the raw bytes on disk
or do we want to return something meaningful to the reader.  As the
existing tools use the xattr interface to set/clear fscaps returning
data to user space rather than raw bytes seems the perfered interface.

My ideal semantics would be:

- If current_user_ns() == sb->s_user_ns return the raw data.

  I don't know how to implement this first scenario while permitting
  stacked filesystems.

    After a little more thought I do.

    In getxattr if the get_cap method is not implemented by the
    filesystem if current_user_ns() == sb->s_user_ns simply treat it as
    an ordinary xattr read/write.

    Otherwise call vfs_get_cap and translate the result as described
    below.
    
    The key point of this is it allows for seeing what is actually on
    disk (when it is not confusing).

- Calculate the cpu_vfs_cap_data as get_vfs_caps_from_disk does.
  That gives the meaning of the xattr.

- If "from_kuid(current_userns(), krootid) == 0" return a v2 cap.

- If "rootid_owns_currentns()" return a v2 cap.

- Else return an error.  Probably a permission error.

  The fscap simply can not make sense to the user if the rootid does not
  map.  Return a v2 cap would imply that the caps are present on the
  executable (in the current context) which they are not.

Eric