[uaccess] Add userptr_sub() to find the difference between two user pointers

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/include/librm.h

 	return trivial_userptr_add ( userptr, offset );
 }
 
+static inline __always_inline off_t
+UACCESS_INLINE ( librm, userptr_sub ) ( userptr_t userptr,
+					userptr_t subtrahend ) {
+	return trivial_userptr_sub ( userptr, subtrahend );
+}
+
 static inline __always_inline void
 UACCESS_INLINE ( librm, memcpy_user ) ( userptr_t dest, off_t dest_off,
 					userptr_t src, off_t src_off,

src/include/ipxe/efi/efi_uaccess.h

 	return trivial_userptr_add ( userptr, offset );
 }
 
+static inline __always_inline off_t
+UACCESS_INLINE ( efi, userptr_sub ) ( userptr_t userptr,
+				      userptr_t subtrahend ) {
+	return trivial_userptr_sub ( userptr, subtrahend );
+}
+
 static inline __always_inline void
 UACCESS_INLINE ( efi, memcpy_user ) ( userptr_t dest, off_t dest_off,
 					userptr_t src, off_t src_off,

src/include/ipxe/linux/linux_uaccess.h

 	return trivial_userptr_add(userptr, offset);
 }
 
+static inline __always_inline off_t
+UACCESS_INLINE(linux, userptr_sub)(userptr_t userptr, userptr_t subtrahend)
+{
+	return trivial_userptr_sub ( userptr, subtrahend );
+}
+
 static inline __always_inline void
 UACCESS_INLINE(linux, memcpy_user)(userptr_t dest, off_t dest_off, userptr_t src, off_t src_off, size_t len)
 {

src/include/ipxe/uaccess.h

 	return ( userptr + offset );
 }
 
+/**
+ * Subtract user pointers
+ *
+ * @v userptr		User pointer
+ * @v subtrahend	User pointer to be subtracted
+ * @ret offset		Offset
+ */
+static inline __always_inline off_t
+trivial_userptr_sub ( userptr_t userptr, userptr_t subtrahend ) {
+	return ( userptr - subtrahend );
+}
+
 /**
  * Copy data between user buffers
  *
  */
 userptr_t userptr_add ( userptr_t userptr, off_t offset );
 
+/**
+ * Subtract user pointers
+ *
+ * @v userptr		User pointer
+ * @v subtrahend	User pointer to be subtracted
+ * @ret offset		Offset
+ */
+off_t userptr_sub ( userptr_t userptr, userptr_t subtrahend );
+
 /**
  * Convert virtual address to a physical address
  *