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

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Message-ID: <dc9df0fd-6344-49ad-87c6-8e5c63857bd6@kernel.org>
Date: Wed, 14 Aug 2024 07:00:19 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Vadim Fedorenko <vadfed@...a.com>
Cc: Jakub Kicinski <kuba@...nel.org>,
 Jonathan Lemon <jonathan.lemon@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] ptp: ocp: adjust sysfs entries to expose tty
 information

On 13. 08. 24, 20:24, Vadim Fedorenko wrote:
> 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.

Strong NACK against this approach.

What about converting those 4 ports into an array and adding an enum { 
PORT_GNSS, POTR_GNSS2, PORT_MAC, PORT_NMEA }?

thanks,
-- 
js
suse labs