posix-clock: Store file pointer in struct posix_clock_context

File descriptor based pc_clock_*() operations of dynamic posix clocks
have access to the file pointer and implement permission checks in the
generic code before invoking the relevant dynamic clock callback.

Character device operations (open, read, poll, ioctl) do not implement a
generic permission control and the dynamic clock callbacks have no
access to the file pointer to implement them.

Extend struct posix_clock_context with a struct file pointer and
initialize it in posix_clock_open(), so that all dynamic clock callbacks
can access it.

Acked-by: Richard Cochran <[e-mail]>
Reviewed-by: Vadim Fedorenko <[e-mail]>
Reviewed-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Wojtek Wasko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/posix-clock.h

···
  * struct posix_clock_context - represents clock file operations context
  *
  * @clk:              Pointer to the clock
+ * @fp:               Pointer to the file used to open the clock
  * @private_clkdata:  Pointer to user data
  *
  * Drivers should use struct posix_clock_context during specific character
- * device file operation methods to access the posix clock.
+ * device file operation methods to access the posix clock. In particular,
+ * the file pointer can be used to verify correct access mode for ioctl()
+ * calls.
  *
  * Drivers can store a private data structure during the open operation
  * if they have specific information that is required in other file
···
  */
 struct posix_clock_context {
 	struct posix_clock *clk;
+	struct file *fp;
 	void *private_clkdata;
 };
 

kernel/time/posix-clock.c

···
 		goto out;
 	}
 	pccontext->clk = clk;
+	pccontext->fp = fp;
 	if (clk->ops.open) {
 		err = clk->ops.open(pccontext, fp->f_mode);
 		if (err) {