vmmon: compatibility with eventpoll switch to poll_mask()

Since commit 11c5ad0ec441 ("eventpoll: switch to ->poll_mask") in
v4.18-rc1, eventpoll switched from ->poll() to ->poll_mask(). Rather than
calling the callback directly (which would result in null pointer
dereference), use vfs_poll() wrapper. As this wrapper is only available
since 4.18-rc1 cycle as well, provide a copy to use when building against
older kernels.

vmmon-only/include/compat_poll.h

@@ -0,0 +1,30 @@
+#ifndef __COMPAT_POLL_H__
+#define __COMPAT_POLL_H__
+
+#include <linux/poll.h>
+
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(4, 18, 0))
+
+#ifndef __poll_t
+typedef unsigned int __poll_t;
+#endif
+
+static inline __poll_t compat_vfs_poll(struct file *file,
+				       struct poll_table_struct *pt)
+{
+	if (unlikely(!file->f_op->poll))
+		return DEFAULT_POLLMASK;
+	return file->f_op->poll(file, pt);
+}
+
+#else
+
+static inline __poll_t compat_vfs_poll(struct file *file,
+				       struct poll_table_struct *pt)
+{
+	return vfs_poll(file, pt);
+}
+
+#endif
+
+#endif /* __COMPAT_POLL_H__ */

vmmon-only/linux/hostif.c

@@ -74,6 +74,7 @@
 
 #include "pgtbl.h"
 #include "versioned_atomic.h"
+#include "compat_poll.h"
 
 #if !defined(CONFIG_HIGH_RES_TIMERS)
 #error CONFIG_HIGH_RES_TIMERS required for acceptable performance
@@ -2373,7 +2374,7 @@ HostIF_SemaphoreWait(VMDriver *vm,   // IN:
 
       poll_initwait(&table);
       current->state = TASK_INTERRUPTIBLE;
-      mask = file->f_op->poll(file, &table.pt);
+      mask = compat_vfs_poll(file, &table.pt);
       if (!(mask & (POLLIN | POLLERR | POLLHUP))) {
          vm->vmhost->vcpuSemaTask[vcpuid] = current;
          schedule_timeout(timeoutms * HZ / 1000);  // convert to Hz