On Wed, Feb 19 2025 at 09:45, Wojtek Wasko wrote:

On Mon, Feb 17 2025 at 21:24, Thomas Gleixner wrote:

quoted

quoted

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