Seeking advice on "monkey patching" a driver

Greg KH greg at
Thu Jul 1 13:59:13 EDT 2021

On Thu, Jul 01, 2021 at 11:25:49AM -0500, Ian Pilcher wrote:
> I maintain a couple of out-of-tree modules that enable "mainstream"
> distributions to be used on the Thecus N5550 NAS.
> The disk activity LEDs in this NAS are software controlled, so the
> n5550_ahci_leds module exists to "inject" a wrapper around libahci's
> qc_issue() function which triggers the correct LED (if any).
> It's certainly a hack, but it makes use of the modules much simpler, as
> there's no need to patch and rebuild libahci, override the distro-
> provided module, etc.  As long as modprobe is configured to load
> n5550_ahci_leds immediately after libahci and before any consumers of
> libahci are loaded, things "just work".
> The current version of n5550_ahci_leds attempts to ensure that no
> libahci consumers are loaded before it modifies libahci's
> ahci_ops.qc_issue.  It does this by:
>   * locking module_mutex
>   * getting a reference to the libahci module (with find_module())
>   * checking libahci's reference count
>   * grabbing a reference to *itself* to prevent itself from being
>     unloaded
>   * modifying ahci_ops.qc_issue
>   * unlocking module_mutex

Oh that's horrible, please no, do not do that :)

> (There similar logic in the n5550_ahci_leds_enabled_store function to
> reverse the modifications, if no other libahci consumers are loaded.)
> It's very possible that some or all of these precautions are
> unnecessary (or that they're inadequate).  I am most definitely not an
> expert at kernel development or the details of the kernel's module
> loading mechanism.  I and few others have, however, been successfully
> using these modules for a number of years.
> I've just discovered that neither module_mutex nor find_module() are
> available in recent kernels, and I'm unsure how to proceed.  The two
> options that I've been able to think of thus far are:
> * YOLO!  I can simply remove the checks from the module and rely on user
>   space to ensure that n5550_ahci_leds is loaded before any libahci
>   consumers load (i.e. before udevd starts).
> * kprobes - I have a feeling that this is the "correct" way to do this
>   (and it would have the benefit of working even if libahci or its
>   consumers aren't built as modules).  OTOH, it isn't clear how I would
>   go about accessing the arguments passed into the function without
>   JProbes, and I'm not thrilled with the idea of adding additional
>   overhead and/or locking to the disk I/O path.

How about a third option, the correct one:
	- submit your code changes upstream and they get merged into the
	  main kernel tree and no monkeypatching is ever needed at all!

Have you submitted your changes upstream to the existing drivers?  What
is preventing that from happening today?


greg k-h

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