This cast is invalid, arg being an unsigned long in this case. I'm
surprised the compiler doesn't warn about this.

For both functions, I'd suggest using something like:

  void __user *uarg = compat_ptr(arg);

in the prologue. This avoids the need for a cast when calling the actual
handler with uarg (void * is implicitly convertible to any pointer
type), which is not just shorter but also avoids this strange cast to a
pointer to a native struct, despite the pointer being actually to a
compat struct in this case.

I thought there was something fishy here! Do you know why the compiler would ignore this, could it have anything to do with how I've got the build configured (specifically the #ifdef CONFIG_COMPAT)?

Many thanks for the feedback as always,

Akram