RE: [PATCH net-next v3 2/3] ptp: Add file permission checks on PHCs

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Feb 19 2025 at 09:45, Wojtek Wasko wrote:
> On Mon, Feb 17 2025 at 21:24, Thomas Gleixner wrote:
>> > One limitation
>> > remains: querying the adjusted frequency of a PTP device (using
>> > adjtime() with an empty modes field) is not supported for chardevs
>> > opened without WRITE permissions, as the POSIX layer mandates WRITE
>> > access for any adjtime operation.
>> 
>> That's a fixable problem, no?
>
> Absolutely, but to be honest I wasn't sure about how to properly change
> the access check in adjtime given it's a "generic" API. I ended up with
> something along the lines of:
>
>    if (tx->modes & ~(ADJ_NANO | ADJ_MICRO))
>      /* require WRITE */
>
> being that ADJ_NANO and ADJ_MICRO by themselves don't mean the clock will
> be modified. So the modes field is not really "empty" per se and the check
> becomes less self-explanatory.

ADJ_NANO and ADJ_MICRO modify the internal status. A read only operation
has to have tx->modes == 0 and the result will be served in the
NANO/MICRO representation which was set by the control application which
can write.

adjtimex(2) is clearly saying:

 "The modes field determines which parameters, if any, to set."

Consequently modes != 0 requires CAP_SYS_TIME, while modes == 0 is
unpriviledged. So requiring WRITE for the FD based posix clocks is not
asked too much.

Thanks,

        tglx