continue_range(uffd, msg->arg.pagefault.address, page_size);
stats->minor_faults++;
} else {
- /* Missing page faults */
+ /*
+ * Missing page faults.
+ *
+ * Here we force a write check for each of the missing mode
+ * faults. It's guaranteed because the only threads that
+ * will trigger uffd faults are the locking threads, and
+ * their first instruction to touch the missing page will
+ * always be pthread_mutex_lock().
+ *
+ * Note that here we relied on an NPTL glibc impl detail to
+ * always read the lock type at the entry of the lock op
+ * (pthread_mutex_t.__data.__type, offset 0x10) before
+ * doing any locking operations to guarantee that. It's
+ * actually not good to rely on this impl detail because
+ * logically a pthread-compatible lib can implement the
+ * locks without types and we can fail when linking with
+ * them. However since we used to find bugs with this
+ * strict check we still keep it around. Hopefully this
+ * could be a good hint when it fails again. If one day
+ * it'll break on some other impl of glibc we'll revisit.
+ */
if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
err("unexpected write fault");