kernel linker: Overriding a driver shipped with kernel via module?

[Slawa Olhovchenkov]

On Tue, Nov 18, 2014 at 06:16:21PM -0700, Ian Lepore wrote:

> On Tue, 2014-11-18 at 16:36 -0800, John-Mark Gurney wrote:
> > Slawa Olhovchenkov wrote this message on Wed, Nov 19, 2014 at 04:27 +0400:
> > > On Tue, Nov 18, 2014 at 04:15:10PM -0800, John-Mark Gurney wrote:
> > > 
> > > > Slawa Olhovchenkov wrote this message on Tue, Nov 18, 2014 at 16:45 +0400:
> > > > > On Mon, Nov 17, 2014 at 03:15:29PM -0800, Xin Li wrote:
> > > > > 
> > > > > > Right now one can declare version for a module by doing something like:
> > > > > > 
> > > > > > MODULE_VERSION(module_name, module_version);
> > > > > > 
> > > > > > Sometimes, it may be desirable for a vendor to release a new driver
> > > > > > that overrides the driver shipped with the kernel itself.  However, it
> > > > > > seems that the MODULE_VERSION facility would just refuse the module
> > > > > > when preloaded with kernel.
> > > > > > 
> > > > > > Looking at some other vendor drivers, they are using a slightly
> > > > > > different module name to overcome this limitation.  Is that the only
> > > > > > way to do it?
> > > > > 
> > > > > I think now time to move to modulated kernel and load all drivers
> > > > > currently present in GENERIC as modules (via loader.conf).
> > > > 
> > > > This becomes slightly more difficult for storage drivers which must
> > > > be loaded at boot time so the you can mount root from it...  But yes,
> > > > we are interested in methods to make it easier/more automatic for
> > > > modules to be loaded to support the hardware that is present in a
> > > > system...
> > > 
> > > When loader can load kernel -- loader can load driver module, this is
> > > not Linux (but yes, loader need plugable and stackable framework for
> > > access FS -- currenly booting from ZFS over gstripe not allowed).
> > 
> > That isn't the only issue.. another issue is identifing the correct
> > kernel module(s) to load at boot... iirc, you cannot unload a kernel
> > module loaded at boot time...
> > 
> 
> You can do a kldunload on a module that was loaded by loader(8) and it
> will be unlinked from the kernel in terms of symbol resolution, but the
> memory cannot be freed.  You can then load and unload the same module
> again as much as you want, it just won't re-occupy the original address,
> and the original memory is lost forever (but considering the size of
> most modules, that's not such a big deal).

Some modules can't be unloaded anyway: netgraph, for example.