Date:   Thu, 18 Nov 2021 17:51:35 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     MyungJoo Ham <myungjoo.ham@samsung.com>,
        Chanwoo Choi <cw00.choi@samsung.com>,
        Sebastian Reichel <sre@kernel.org>,
        Chen-Yu Tsai <wens@csie.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-usb@vger.kernel.org, linux-omap@vger.kernel.org,
        kernel-janitors@vger.kernel.org
Subject: Re: [PATCH] extcon: fix extcon_get_extcon_dev() error handling
Message-ID: <20211118145135.GO27562@kadam>
References: <20211118113026.GG1147@kili>
 <a33fc3d7-a389-a805-0ef9-4b90de1d3d89@roeck-us.net>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <a33fc3d7-a389-a805-0ef9-4b90de1d3d89@roeck-us.net>
User-Agent: Mutt/1.9.4 (2018-02-28)
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?lklTF8cO+GRi0OAxBsFM7EaToN4XO3bQ7IchOL/c4xKsOdxv9x0AvGDtNeV8?=
 =?us-ascii?Q?dvbDAXTmfZCOAQLQiC+vAwDeik4q0bMXOJ2NwILcdt/w4UBG/6dRIfd/yV8b?=
 =?us-ascii?Q?mAlt518xvY8H/FUKAe0nhTQmVe27PMoPq3W0q98imOEeIVo/cUasNQXSYJQE?=
 =?us-ascii?Q?kI9+YPULYGLR9UgiIxsj1qhxmA5YREIw1SYFrYKx15PU4dAapjpA7eGKHrcG?=
 =?us-ascii?Q?++Bz5YZRDZpm3qi6WN8Uo9nbPivDQfL/cfnjQO10rdGTluU6Cq0ElgVcHTwn?=
 =?us-ascii?Q?zKYrglPKUUiCivSkGNhEuItmOPEFLq+3ORnKKFyVMBnatJnuCyHciWA3AuTM?=
 =?us-ascii?Q?Q5vosnCseanhaji3L5/l9Vh/kCjdaGeijrbcImUQBhB9XmqGhfT1/0sRtclH?=
 =?us-ascii?Q?n9Unym1v6CeuaDZfukmtH+GzoBq++lyorqE87dCv8TzbJ9xxj62nures58a9?=
 =?us-ascii?Q?5+rOXpu75Fpgrh2s4EPYi8GAIdU/5N5GflTCyXYwXS5p5kbWPREMhBfGwo08?=
 =?us-ascii?Q?Zv8bkg7urk84A1pfY/zPxGmu9xNN9HtqduAheA6LWasbt4nLvFHzAXVR2ghB?=
 =?us-ascii?Q?TBwLj4Ad6k+DompJq3pkkWStzkQks9hP/c07TYUVKcFpPKdimakdZf6n6H9Y?=
 =?us-ascii?Q?MeyB9AD3Gqa/mjYsvgkTbquucGmAAAnA8CMqlBidQxKF/dhHvVqpoZxZK7Gh?=
 =?us-ascii?Q?PrJNoHgCeeUQYYZ5GqFtLFHE1ueDO1IXuTewWFKniGkcFvGcJrhDZ/7yVoKr?=
 =?us-ascii?Q?1jIwT+YMXMUD+RFYraWDZ/U3yEyDfUnWmcJVkMScRoyFpcrHNd6rnpwMRiVM?=
 =?us-ascii?Q?GZb/qbd7EDqkmK5EY9uDRbyjwPwyWZl2Q/a5YexpbwFi3/bLZ5HBGitJQVMQ?=
 =?us-ascii?Q?CedD7byWHLhUcZ2ts2lVGlOs2twUYbxEfS1HGIdaJfwDwoEJzrWJgftveoYf?=
 =?us-ascii?Q?LhOLLoqXN023a5vXSlQfTg5Ddp117eEtAyMdwCfrHIH2HTEPF3JgCkoZBu36?=
 =?us-ascii?Q?WqXE2lhjZw8REYqtob3mWipjyMKfyt8n6qhaoKSQhwf05wc21apkLr2SXtYg?=
 =?us-ascii?Q?7pLF2U3cf0+frxXmmfA9KxGbHMmVOeMxtPRsOkfvSqLIjIr3WTa2ndCACjyW?=
 =?us-ascii?Q?G5hXi0+DF3jtnCCm17zjOTuF3Akoo1nVnuQ7Buj+oHBKBF1JfIt8DFX0j0zL?=
 =?us-ascii?Q?jmUgt8ncgEsjKsFW7sPRQ5dxpeog1g7r2/dC+upXtHvrIC7pjqTm1hhyatO0?=
 =?us-ascii?Q?dFlIjZUXziwA47qKUKGjrj9DiBbPsoWiPuOy1v9PXtqXoTlL0wzfCGVFYaxW?=
 =?us-ascii?Q?x/zDJ25SUVOfgTfW97qK/AC115zdSrfgCVCzc8YZbzHfxN/Gu1X6XaNDRp1B?=
 =?us-ascii?Q?OqGlgFmu3ogvWn0RHpNxVm1lBS+gvzanZ8tzrn1N8nz+fC7yn1R9gIFGXk9T?=
 =?us-ascii?Q?VFiG8u3E3wR5nuxhhum077+zkAbIbZaxmymh4aQ1SNXxVXeF2tYrPCMDi6Gq?=
 =?us-ascii?Q?j757lsBlYoqmYdINXYVUX/zJ8Ls5kk4W0dULrwbX9grojRDpJxJBDbbp5tSf?=
 =?us-ascii?Q?cXM8Cb/kim3bCpPhi6fhZgBjOYTwChFvm82VFHoazzhLahZ24yArUUdz3HKJ?=
 =?us-ascii?Q?Qzx/elKNUt7YIxM8JDMqPa6Fuw5/vX14d5bgf/aiAi4DZmF3UtlYM4m/+1aO?=
 =?us-ascii?Q?MI21y6U6GLGjGiciAQy0QN0HprY=3D?=
X-OriginatorOrg: oracle.com
X-MS-Exchange-CrossTenant-Network-Message-Id: d8355295-eb6c-4a10-a2a6-08d9aaa2fdc7
X-MS-Exchange-CrossTenant-AuthSource: MWHPR1001MB2365.namprd10.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Nov 2021 14:52:07.5714
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: WL+wJTy73q0VVZmJ2zSp8Us2PIf62R3xbRvoXf/GsKXVMl4hrStJKU4asSZEpU0MsKu4A+LsQa/RWnzHAVsZ6+qUsnNBeVYi58FzeSfr7mI=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR1001MB2398
X-Proofpoint-Virus-Version: vendor=nai engine=6300 definitions=10171 signatures=668683
X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 phishscore=0 bulkscore=0
 adultscore=0 mlxscore=0 malwarescore=0 mlxlogscore=999 suspectscore=0
 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000
 definitions=main-2111180083
X-Proofpoint-ORIG-GUID: rJV_5CRcf4J6A7esdN5Da8iglD-idjDT
X-Proofpoint-GUID: rJV_5CRcf4J6A7esdN5Da8iglD-idjDT
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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