linux-kernel - Re: [PATCH] extcon: fix extcon_get_extcon

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Message-ID: <a33fc3d7-a389-a805-0ef9-4b90de1d3d89@roeck-us.net>
Date:   Thu, 18 Nov 2021 06:22:58 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>
Cc:     Chanwoo Choi <cw00.choi@...sung.com>,
        Sebastian Reichel <sre@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-omap@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] extcon: fix extcon_get_extcon_dev() error handling

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@...cle.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 ?

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.

Thanks,
Guenter