Re: [PATCH] inotify: Extend ioctl to allow to request id of new watch descriptor

[Matthew Wilcox <willy@infradead.org>]

On Thu, Feb 08, 2018 at 06:07:37PM +0300, Kirill Tkhai wrote:
> The new code is under CONFIG_CHECKPOINT_RESTORE #define, so small system
> may exclude it. The only change in generic inotify part is idr_alloc_cyclic()
> end argument. We had 0 there, and idr subsystem replaced it with INT_MAX
> in idr_get_free(). So, the max possible id was INT_MAX (see idr_get_free()
> again).
> 
> Since I need INOTIFY_IDR_END to check ioctl's third argument, it's better
> it's defined as positive number. But when not-zero value is passed
> to idr_get_free(), this function decrements it. Also, idr_alloc_cyclic()
> defined @end as int argument. So, it's impossible to pass positive @end
> argument to idr_alloc_cyclic() to get INT_MAX id. And after this patch
> inotify watch descriptors ids will take numbers [1, INT_MAX-1], INT_MAX
> will be unavailable.

Ummm.  Why not just do:

+#ifdef CONFIG_CHECKPOINT_RESTORE
+	case INOTIFY_IOC_SETNEXTWD:
+		ret = -EINVAL;
+		if (arg >= 1 && arg <= INT_MAX) {
+			spin_lock(&data->idr_lock);
+			idr_set_cursor(&data->idr, (unsigned int)arg);
+			spin_unlock(&data->idr_lock);
+			ret = 0;
+		}
+		break;
+#endif /* CONFIG_CHECKPOINT_RESTORE */