[PATCH v2] nvmem: fix registration vs use race

[Russell King (Oracle)]

On Wed, Jan 04, 2023 at 12:20:15AM +0900, Hector Martin wrote:
> On 03/01/2023 23.42, Russell King (Oracle) wrote:
> > The i.MX6 CPU frequency driver sometimes fails to register at boot time
> > due to nvmem_cell_read_u32() sporadically returning -ENOENT.
> > 
> > This happens because there is a window where __nvmem_device_get() in
> > of_nvmem_cell_get() is able to return the nvmem device, but as cells
> > have been setup, nvmem_find_cell_entry_by_node() returns NULL.
> > 
> > The occurs because the nvmem core registration code violates one of the
> > fundamental principles of kernel programming: do not publish data
> > structures before their setup is complete.
> > 
> > Fix this by making nvmem core code conform with this principle.
> > 
> > Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
> > Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
> > ---
> > v2: add fixes tag
> > 
> >  drivers/nvmem/core.c | 18 ++++++++----------
> >  1 file changed, 8 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> > index 321d7d63e068..6b89fb6fa582 100644
> > --- a/drivers/nvmem/core.c
> > +++ b/drivers/nvmem/core.c
> > @@ -835,22 +835,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
> >  	nvmem->dev.groups = nvmem_dev_groups;
> >  #endif
> >  
> > -	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> > -
> > -	rval = device_register(&nvmem->dev);
> > -	if (rval)
> > -		goto err_put_device;
> > -
> >  	if (nvmem->nkeepout) {
> >  		rval = nvmem_validate_keepouts(nvmem);
> >  		if (rval)
> > -			goto err_device_del;
> > +			goto err_put_device;
> 
> You can't call put_device() on a device that hasn't gone through
> device_initialize() yet.

Right, which is what I just realised while writing the previous reply.
We need to use device_initialize() and device_add(), so we can call
put_device() on it.

Error paths are difficult to properly test. :(

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