The base of the bounds directory is set into mm_struct during
PR_MPX_REGISTER command execution. This member can be used to
check whether one application is mpx enabled.

Not really because by the time you ask the question another thread might
have decided to unregister it.

+int mpx_register(struct task_struct *tsk)
+{
+	struct mm_struct *mm = tsk->mm;
+
+	if (!cpu_has_mpx)
+		return -EINVAL;
+
+	/*
+	 * runtime in the userspace will be responsible for allocation of
+	 * the bounds directory. Then, it will save the base of the bounds
+	 * directory into XSAVE/XRSTOR Save Area and enable MPX through
+	 * XRSTOR instruction.
+	 *
+	 * fpu_xsave() is expected to be very expensive. In order to do
+	 * performance optimization, here we get the base of the bounds
+	 * directory and then save it into mm_struct to be used in future.
+	 */
+	mm->bd_addr = task_get_bounds_dir(tsk);
+	if (!mm->bd_addr)
+		return -EINVAL;

What stops two threads calling this in parallel ?

+
+	return 0;
+}
+
+int mpx_unregister(struct task_struct *tsk)
+{
+	struct mm_struct *mm = current->mm;
+
+	if (!cpu_has_mpx)
+		return -EINVAL;
+
+	mm->bd_addr = NULL;

or indeed calling this in parallel

What are the semantics across execve() ?

Alan

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