Mark Brown [off-list ref] writes:

+#ifdef CONFIG_ARM64_GCS
+static int gcs_restore_signal(void)
+{
+	u64 gcspr_el0, cap;
+	int ret;
+
+	if (!system_supports_gcs())
+		return 0;
+
+	if (!(current->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE))
+		return 0;
+
+	gcspr_el0 = read_sysreg_s(SYS_GCSPR_EL0);
+
+	/*
+	 * GCSPR_EL0 should be pointing at a capped GCS, read the cap...
+	 */
+	gcsb_dsync();
+	ret = copy_from_user(&cap, (__user void*)gcspr_el0, sizeof(cap));
+	if (ret)
+		return -EFAULT;
+
+	/*
+	 * ...then check that the cap is the actual GCS before
+	 * restoring it.
+	 */
+	if (!gcs_signal_cap_valid(gcspr_el0, cap))
+		return -EINVAL;
+
+	/* Invalidate the token to prevent reuse */
+	put_user_gcs(0, (__user void*)gcspr_el0, &ret);
+	if (ret != 0)
+		return -EFAULT;

You had mentioned that "ideally we'd be doing a compare and exchange
here to substitute in a zero". Is a compare and exchange not necessary
anymore, or is it just being left for later? In the latter case, a TODO
or FIXME comment mentioning it would be useful here.

+
+	current->thread.gcspr_el0 = gcspr_el0 + sizeof(cap);
+	write_sysreg_s(current->thread.gcspr_el0, SYS_GCSPR_EL0);
+
+	return 0;
+}

-- 
Thiago