netdev - Re: [PATCH net v2 1/2] ptp: ocp: adjust sysfs entries to expose tty information

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Message-ID: <a38797d7-326d-4ca9-b764-61045ad17b50@linux.dev>
Date: Tue, 13 Aug 2024 19:24:38 +0100
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Vadim Fedorenko <vadfed@...a.com>
Cc: Jakub Kicinski <kuba@...nel.org>,
 Jonathan Lemon <jonathan.lemon@...il.com>, Jiri Slaby
 <jirislaby@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] ptp: ocp: adjust sysfs entries to expose tty
 information

On 13/08/2024 10:33, Greg Kroah-Hartman wrote:
> On Mon, Aug 05, 2024 at 03:04:59PM -0700, Vadim Fedorenko wrote:
>> Starting v6.8 the serial port subsystem changed the hierarchy of devices
>> and symlinks are not working anymore. Previous discussion made it clear
>> that the idea of symlinks for tty devices was wrong by design. Implement
>> additional attributes to expose the information. Fixes tag points to the
>> commit which introduced the change.
>>
>> Fixes: b286f4e87e32 ("serial: core: Move tty and serdev to be children of serial core port device")
>> Signed-off-by: Vadim Fedorenko <vadfed@...a.com>
>> ---
>>   drivers/ptp/ptp_ocp.c | 68 +++++++++++++++++++++++++++++++++----------
>>   1 file changed, 52 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/ptp/ptp_ocp.c b/drivers/ptp/ptp_ocp.c
>> index ee2ced88ab34..7a5026656452 100644
>> --- a/drivers/ptp/ptp_ocp.c
>> +++ b/drivers/ptp/ptp_ocp.c
>> @@ -3346,6 +3346,55 @@ static EXT_ATTR_RO(freq, frequency, 1);
>>   static EXT_ATTR_RO(freq, frequency, 2);
>>   static EXT_ATTR_RO(freq, frequency, 3);
>>   
>> +static ssize_t
>> +ptp_ocp_tty_show(struct device *dev, struct device_attribute *attr, char *buf)
>> +{
>> +	struct dev_ext_attribute *ea = to_ext_attr(attr);
>> +	struct ptp_ocp *bp = dev_get_drvdata(dev);
>> +	struct ptp_ocp_serial_port *port;
>> +
>> +	port = (void *)((uintptr_t)bp + (uintptr_t)ea->var);
> 
> That's insane pointer math, how do we know this is correct?
> 
>> +	return sysfs_emit(buf, "ttyS%d", port->line);
>> +}
>> +
>> +static umode_t
>> +ptp_ocp_timecard_tty_is_visible(struct kobject *kobj, struct attribute *attr, int n)
>> +{
>> +	struct ptp_ocp *bp = dev_get_drvdata(kobj_to_dev(kobj));
>> +	struct ptp_ocp_serial_port *port;
>> +	struct device_attribute *dattr;
>> +	struct dev_ext_attribute *ea;
>> +
>> +	if (strncmp(attr->name, "tty", 3))
>> +		return attr->mode;
>> +
>> +	dattr = container_of(attr, struct device_attribute, attr);
>> +	ea = container_of(dattr, struct dev_ext_attribute, attr);
>> +	port = (void *)((uintptr_t)bp + (uintptr_t)ea->var);
> 
> That's crazy pointer math, how are you ensured that it is correct?  Why
> isn't there a container_of() thing here instead?

Well, container_of cannot be used here because the attributes are static
while the function reads dynamic instance. The only values that are
populated into the attributes of the group are offsets.
But I can convert it to a helper which will check that the offset 
provided is the real offset of the structure we expect. And it could be 
reused in both "is_visible" and "show" functions.

> thanks,
> 
> greg k-h