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

Eric W. Biederman ebiederm at xmission.com
Tue Jan 12 18:49:12 UTC 2021

ebiederm at 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
    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.


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