On 7/23/17 7:35 PM, Jamal Hadi Salim wrote:

In the most basic form, the user specifies the attribute policy as:
[ATTR_GOO] = { .type = NLA_BITFIELD_32, .validation_data = &myvalidflags },

where myvalidflags is the bit mask of the flags the kernel understands.

If the user _does not_ provide myvalidflags then the attribute will
also be rejected.

No other netlink attribute has this requirement. Users of the attributes
are the only ones that know if a value is valid or not (e.g, attribute
passing a device index) and those are always checked in line.
Furthermore, you are locking this attribute into a static meaning of
what is a valid value when flags can be valid or invalid based on other
attributes passed in the request.