+You will need to be able to obtain the current value of the user
+program's stack pointer. Within a system call or a page fault generated
+by a user program, you can retrieve it from @code{esp} member of the
+@struct{intr_frame} passed to @func{syscall_handler} or
+@func{page_fault}, respectively. If you verify user pointers before
+accessing them (@pxref{Accessing User Memory}), these are the only cases
+you need to handle. On the other hand, if you depend on page faults to
+detect invalid memory access, you will need to handle another case,
+where a page fault occurs in the kernel. Reading @code{esp} out of the
+@struct{intr_frame} passed to @func{page_fault} in that case will obtain
+the kernel stack pointer, not the user stack pointer. You will need to
+arrange another way, e.g.@: by saving @code{esp} into @struct{thread} on
+the initial transition from user to kernel mode.
+