linux-kernel - Re: Module loading problem since 5.3

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Date:   Wed, 23 Oct 2019 11:49:40 +0100
From:   Matthias Maennich <maennich@...gle.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Jessica Yu <jeyu@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: Module loading problem since 5.3

On Fri, Oct 18, 2019 at 12:18:48PM +0000, Luis Chamberlain wrote:
>On Wed, Oct 16, 2019 at 02:37:10PM +0100, Matthias Maennich wrote:
>> On Wed, Oct 16, 2019 at 12:50:30PM +0000, Luis Chamberlain wrote:
>> > On Mon, Oct 14, 2019 at 03:44:40PM +0100, Matthias Maennich wrote:
>> > > Hi Luis!
>> > >
>> > > On Mon, Oct 14, 2019 at 08:52:35AM +0000, Luis Chamberlain wrote:
>> > > > On Fri, Oct 11, 2019 at 09:26:05PM +0200, Heiner Kallweit wrote:
>> > > > > On 10.10.2019 19:15, Luis Chamberlain wrote:
>> > > > > >
>> > > > > >
>> > > > > > On Thu, Oct 10, 2019, 6:50 PM Heiner Kallweit <hkallweit1@...il.com <mailto:hkallweit1@...il.com>> wrote:
>> > > > > >
>> > > > > >        MODULE_SOFTDEP("pre: realtek")
>> > > > > >
>> > > > > >     Are you aware of any current issues with module loading
>> > > > > >     that could cause this problem?
>> > > > > >
>> > > > > >
>> > > > > > Nope. But then again I was not aware of MODULE_SOFTDEP(). I'd encourage an extension to lib/kmod.c or something similar which stress tests this. One way that comes to mind to test this is to allow a new tests case which loads two drives which co depend on each other using this macro. That'll surely blow things up fast. That is, the current kmod tests uses request_module() or get_fs_type(), you'd want a new test case with this added using then two new dummy test drivers with the macro dependency.
>> > > > > >
>> > > > > > If you want to resolve this using a more tested path, you could have request_module() be used as that is currently tested. Perhaps a test patch for that can rule out if it's the macro magic which is the issue.
>> > > > > >
>> > > > > >   Luis
>> > > > >
>> > > > > Maybe issue is related to a bug in introduction of symbol namespaces, see here:
>> > > > > https://lkml.org/lkml/2019/10/11/659
>> > > >
>> > > > Can you have your user with issues either revert 8651ec01daed or apply the fixes
>> > > > mentioned by Matthias to see if that was the issue?
>> > > >
>> > > > Matthias what module did you run into which let you run into the issue
>> > > > with depmod? I ask as I think it would be wise for us to add a test case
>> > > > using lib/test_kmod.c and tools/testing/selftests/kmod/kmod.sh for the
>> > > > regression you detected.
>> > >
>> > > The depmod warning can be reproduced when using a symbol that is built
>> > > into vmlinux and used from a module. E.g. with CONFIG_USB_STORAGE=y and
>> > > CONFIG_USB_UAS=m, the symbol `usb_stor_adjust_quirks` is built in with
>> > > namespace USB_STORAGE and depmod stumbles upon this emitting the
>> > > following warning (e.g. during make modules_install).
>> > >
>> > >  depmod: WARNING: [...]/uas.ko needs unknown symbol usb_stor_adjust_quirks
>> > >
>> > > As there is another (less intrusive) way of implementing the namespace
>> > > feature, I posted a patch series [1] on last Thursday that should
>> > > mitigate the issue as the ksymtab entries depmod eventually relies on
>> > > are no longer carrying the namespace in their names.
>> > >
>> > > Cheers,
>> > > Matthias
>> > >
>> > > [1] https://lore.kernel.org/lkml/20191010151443.7399-1-maennich@google.com/
>> >
>> > Yes but kmalloc() is built-in, and used by *all* drivers compiled as
>> > modules, so why was that an issue?
>>
>> I believe you meant, "why was that *not* an issue?".
>
>Right.
>
>> In ksymtab, namespaced symbols had the format
>>
>>  __ksymtab_<NAMESPACE>.<symbol>
>>
>> while symbols without namespace would still use the old format
>>
>>  __ksymtab_<symbol>
>
>Ah, I didn't see the symbol namespace patches, good stuff!
>
>> These are also the names that are extracted into System.map (using
>> scripts/mksysmap). Depmod is reading the System.map and for symbols used
>> by modules that are in a namespace, it would not find a match as it does
>> not understand the namespace notation. Depmod would still not emit a
>> warning for symbols without namespace as their format did not change.
>
>Can we have a test case for this to ensure we don't regress on this
>again? Or put another way, what test cases were implemented for symbol
>namespaces?

While modpost and kernel/module.c are the tests at build and runtime
resp. to enforce proper use of symbol namespaces, I could imagine to
test for the proper layout in the ksymtab entries (note, as mentioned
earlier there are some fixes in flight to finalize the layout).
In addition, I could imagine adding a test that tries to load a module
that uses symbols from a namespace without importing it. The kernel
should deny loading or complain about it (depending on the
configuration). These are also some of the test cases I had when working
on that feature. I did not implement these as automated tests though. I
will put that on my list but help with that would be very welcome.

Cheers,
Matthias