Re: [PATCH] nvmem: fix unregistering device in nvmem_register() error path

Johan Hovold
2021-12-22T09:04:03Z

On Wed, Dec 22, 2021 at 09:56:29AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Dec 22, 2021 at 09:38:27AM +0100, Johan Hovold wrote:
> > On Wed, Dec 22, 2021 at 08:44:44AM +0100, Greg Kroah-Hartman wrote:
> > > On Tue, Dec 21, 2021 at 06:46:01PM +0100, Rafał Miłecki wrote:
> > > > On 21.12.2021 17:06, Greg Kroah-Hartman wrote:
> > > > > On Tue, Dec 21, 2021 at 04:45:50PM +0100, Rafał Miłecki wrote:
> > > > > > From: Rafał Miłecki <rafal@milecki.pl>
> > > > > >
> > > > > > 1. Drop incorrect put_device() calls
> > > > > >
> > > > > > If device_register() fails then underlaying device_add() takes care of
> > > > > > calling put_device() if needed. There is no need to do that in a driver.
> > > > >
> > > > > Did you read the documentation for device_register() that says:
> > > > >
> > > > > * NOTE: _Never_ directly free @dev after calling this function, even
> > > > > * if it returned an error! Always use put_device() to give up the
> > > > > * reference initialized in this function instead.
> > > >
> > > > I clearly tried to be too smart and ignored documentation.
> > > >
> > > > I'd say device_add() behaviour is rather uncommon and a bit unintuitive.
> > > > Most kernel functions are safe to assume to do nothing that requires
> > > > cleanup if they fail.
> > > >
> > > > E.g. if I call platform_device_register() and it fails I don't need to
> > > > call anything like platform_device_put(). I just free previously
> > > > allocated memory.
> > >
> > > And that is wrong.
> >
> > It seems Rafał is mistaken here too; you certainly need to call
> > platform_device_put() if platform_device_register() fail, even if many
> > current users do appear to get this wrong.
>
> A short search found almost everyone getting this wrong. Arguably
> platform_device_register() can clean up properly on its own if we want
> it to do so. Will take a lot of auditing of the current codebase first
> to see if it's safe...

Right, but I found at least a couple of callers getting it it right, so
changing the behaviour now risks introducing a double free (which is
worse than a memleak on registration failure). But yeah, a careful
review might suffice.

Johan