[gentoo-hardened] Re: aufs3.0 fails to emerge on Gentoo hardened and kernel 3.0.4

On 3 Jan 2012 at 22:10, Andrea Zuccherelli wrote:

> The switch I was referring to is
> '-fplugin-arg-constify_plugin-no-constify' gcc option.
> This should disable the constify_plugin but it is not checked on gcc
> callbacks when a 'no_const' attribute is found.

it doesn't exactly disable the plugin, it disables actually constifying
ops structures, but it still lets gcc understand the special attributes
introduced for this constification effort.

> Using the kernel option will turn off the plugin system wide.

that's because this is the only supported/meaningful way of using the plugin
(that's why modversion now depends on the plugin too).

> Using the gcc flag option willl turn it off only for this case.

and this will never work ;). consider that you compile your kernel with
constification enabled but some external module without it. now this external
module will believe that it has free reign over ops structures whereas the
kernel will happily enforce the read-only property on at least its own static
instances and may cause the external module to oops at runtime.

now if some code needs writable ops structure variables it has 3 options under
the plugin approach:

1. add __no_const to the structure declaration

2. typedef a __no_const version of the constified structure type

3. use pax_open_kernel/pax_close_kernel to temporarily override
   the (runtime enforced) constness of a given variable

each approach has its own conditions to use, here's a quick summary:

1. when all instances of the given structure type are runtime allocated
   (i.e., there're no static instances)

2. when some instances of the given structure type are statically allocated
   it's best to let them be consitified by the plugin and use the typedef'd
   __no_const type for dynamically allocated ones

3. when some constified statically allocated variables do need to be modified
   at runtime

if you look at PaX carefully, you'll find use cases for each of the above,
it should be your guidance for patching aufs as well. although i didn't look
at its code, i think aufs is case #1 or #2.