Re: [PATCH] extcon: fix extcon_get_extcon_dev() error handling

[Dan Carpenter <dan.carpenter@oracle.com>]

On Thu, Nov 18, 2021 at 06:22:58AM -0800, Guenter Roeck wrote:
> On 11/18/21 3:32 AM, Dan Carpenter wrote:
> > The extcon_get_extcon_dev() function returns error pointers on error
> > and NULL when it's a -EPROBE_DEFER defer situation.  There are eight
> > callers and only two of them handled this correctly.  In the other
> > callers an error pointer return would lead to a crash.
> > 
> > What prevents crashes is that errors can only happen in the case of
> > a bug in the caller or if CONFIG_EXTCON is disabled.  Six out of
> > eight callers use the Kconfig to either depend on or select
> > CONFIG_EXTCON.  Thus the real life impact of these bugs is tiny.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > ---
> > The two callers where the drivers can be built without CONFIG_EXTCON
> > are TYPEC_FUSB302 and CHARGER_MAX8997.
> > 
> [ ... ]
> > diff --git a/drivers/usb/typec/tcpm/fusb302.c b/drivers/usb/typec/tcpm/fusb302.c
> > index 7a2a17866a82..8594b59bd527 100644
> > --- a/drivers/usb/typec/tcpm/fusb302.c
> > +++ b/drivers/usb/typec/tcpm/fusb302.c
> > @@ -1706,8 +1706,8 @@ static int fusb302_probe(struct i2c_client *client,
> >   	 */
> >   	if (device_property_read_string(dev, "linux,extcon-name", &name) == 0) {
> >   		chip->extcon = extcon_get_extcon_dev(name);
> > -		if (!chip->extcon)
> > -			return -EPROBE_DEFER;
> > +		if (IS_ERR(chip->extcon))
> > +			return PTR_ERR(chip->extcon);
> 
> Why does the code not need to return -EPROBE_DEFER ? The description states
> that NULL is returned in that situation. Doesn't that mean that defer situations
> are no longer handled with this patch in place ?

I'm not sure I understand what you are saying here.  In the original
code, extcon_get_extcon_dev() would return NULL and relied on the
callers to change NULL into a -EPROBE_DEFER.  If extcon_get_extcon_dev()
returned ERR_PTR(-EINVAL) (which is impossible as mentioned) the it
would lead to a crash.

In the new code, the extcon_get_extcon_dev() function returns
-EPROBE_DEFER directly so the caller code is much simpler.

> 
> Also, with this patch in place, the code will no longer work if extcon is disabled,
> because extcon_get_extcon_dev() will return -ENODEV and the above code will bail out.
> The behavior of the code wasn't optimal in that case (it would wait until timeout
> in tcpm_get_current_limit() before returning), but at least it didn't fail.

Huh.  You are right.  Initialy I thought that tcpm_get_current_limit()
would crash.  This is one of the two drivers which I mentioned that can
be built without CONFIG_EXTCON.

I will modify the version of extcon_get_extcon_dev() where CONFIG_EXTCON
is disabled to return NULL.  That is the standard/correct way to write
these.  That will turn tcpm_get_current_limit() into a no-op.

A belt and suspenders approach might be to modify the Kconfig so this
driver selects CONFIG_EXTCON.

regards,
dan carpenter