[PATCH v3 00/11] Symbol Namespaces

Matthias Maennich maennich at google.com
Thu Aug 22 00:03:41 AEST 2019

On Wed, 21 Aug, 06:38, Greg Kroah-Hartman wrote:
>On Wed, Aug 21, 2019 at 03:11:40PM +0200, Peter Zijlstra wrote:
>> On Wed, Aug 21, 2019 at 12:49:15PM +0100, Matthias Maennich wrote:
>> > As of Linux 5.3-rc5, there are 31205 [1] exported symbols in the kernel.
>> > That is a growth of roughly 1000 symbols since 4.17 (30206 [2]).  There
>> > seems to be some consensus amongst kernel devs that the export surface
>> > is too large, and hard to reason about.
>> >
>> > Generally, these symbols fall in one of these categories:
>> > 1) Symbols actually meant for drivers
>> > 2) Symbols that are only exported because functionality is split over
>> >    multiple modules, yet they really shouldn't be used by modules outside
>> >    of their own subsystem
>> > 3) Symbols really only meant for in-tree use
>> >
>> > When module developers try to upstream their code, it regularly turns
>> > out that they are using exported symbols that they really shouldn't be
>> > using. This problem is even bigger for drivers that are currently
>> > out-of-tree, which may be using many symbols that they shouldn't be
>> > using, and that break when those symbols are removed or modified.
>> >
>> > This patch allows subsystem maintainers to partition their exported
>> > symbols into separate namespaces, and module authors to import such
>> > namespaces only when needed.
>> >
>> > This allows subsystem maintainers to more easily limit availability of
>> > these namespaced symbols to other parts of the kernel. It can also be
>> > used to partition the set of exported symbols for documentation
>> > purposes; for example, a set of symbols that is really only used for
>> > debugging could be in a "SUBSYSTEM_DEBUG" namespace.
>> I'm missing how one can prohibit these random out of tree modules from
>> doing MODULE_IMPORT_NS().
>Nothing, but then they are explicitly being "bad" :)

As a side effect of this implementation (namespace imports via modinfo
tags), imports are very visible for (out-of-tree) modules, e.g.

$ modinfo drivers/usb/storage/ums-usbat.ko
  filename:       drivers/usb/storage/ums-usbat.ko
  import_ns:      USB_STORAGE
  license:        GPL
  author:         ...

>> That is; suppose I stick all the preempt_notifier symbols in a KVM
>> namespace, how do I enforce no out-of-tree modules ever do
>> MODULE_IMPORT_NS(KVM) and gain access?

That is actually a feature worth following up: Restricting the
namespaces that can be imported by modules. I am afraid it is not part
of this series, but should not be too hard once agreed how such a list
will be defined.

>> (the above would basically break virtualbox, which I knows uses preempt
>> notifiers too, but I don't give a rats arse about that)
>It's a huge red flag for anyone reviewing the code that this module is
>doing something it probably really should not be doing at all.  It will
>make reviewing code easier, this isn't there to try to "prevent bad
>actors" at all, sorry.


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