Re: [usb-storage] Re: [PATCH v4 12/12] RFC: watchdog: export core symbols in WATCHDOG_CORE namespace

From: Matthias Maennich
Date: Thu Sep 05 2019 - 07:25:50 EST

On Thu, Sep 05, 2019 at 12:41:47PM +0200, Jessica Yu wrote:
+++ Matthew Dharm [04/09/19 09:16 -0700]:
On Wed, Sep 4, 2019 at 5:12 AM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:

Note that I don't object to the patch set in general. There may be symbols
which only need be exported in the context of a single subsystem or even
driver (if a driver consists of more than one module). For example, a mfd
driver may export symbols which should only be called by its client drivers.
In such a situation, it may well be beneficial to limit the use of exported

I can appreciate this benefit.

I am not sure what good that does in practice (if I understand correctly,
a driver only has to declare that it wants to use a restricted use symbol
if it wants to use it), but that is a different question.

I think this question implies that you are coming from the perspective
of "security" or wanting to restrict access to the underlying
functions, rather than wanting to clean-up the way symbols are handled
for manageability / maintainability purposes (which is the goal, as I
understand it).

The goal of this patch set is to introduce structure into the exported
surface that goes beyond naming conventions like 'usb_*'. So, it is
rather about maintainability then security. In particular, creating the
visibility of which parts of the kernel use which other parts, might
help to find cases where suboptimal choices were made. Maybe already
during development/review.

As Guenter correctly noted, a module is able to declare that it wants to
use a namespace. One idea that came up earlier was to maybe restrict the
namespaces that can actually be imported by modules. But I would see
anything in that direction as beyond the scope of this series.

A nice side effect of having to declare the usage is that it shows up in
modinfo and module users can reason about how the module interacts with
the rest of the kernel.

HOWEVER, I have one question: If these patches are included, and
someone wants to introduce a bit of code which needs to use two
symbols from different namespaces but with the same name, can that be
done? That is, if driver A has symbol 'foo' and driver B has symbol
'foo' (both in their respective namespaces), and driver C wants to use and, can that be supported?

As of now, we currently don't support this - modpost will warn if a
symbol is exported more than once (across modules + vmlinux), and the
module loader currently assumes exported symbol names are unique. Do
you have a concrete use case? If there is a strong need for this, I
don't think it'd be too hard to implement.

The implementation does not change the fact that symbol names need to be
unique. As Arnd just mentioned in the other thread: the linker will
already fail if two builtin symbols use the same name. It is rather a
tag attached to the symbol.