livepatch: Add "stack_order" sysfs attribute

As previous discussion, maintainers think that patch-level sysfs interface is the only acceptable way to maintain the information of the order that klp_patch is applied to the system.

However, the previous patch introduce klp_ops into klp_func is a optimization methods of the patch introducing 'using' feature to klp_func.

But now, we don't support 'using' feature to klp_func and make 'klp_ops' patch not necessary.

Therefore, this new version is only introduce the sysfs feature of klp_patch 'stack_order'.

V1 -> V2:
1. According to the suggestion from Petr, to make the meaning more clear, rename 'order' to 'stack_order'.
2. According to the suggestion from Petr and Miroslav, this patch now move the calculating process to stack_order_show function. Adding klp_mutex lock protection.

V2 -> V3:
1. Squash 2 patches into 1. Update description of stack_order in ABI Document. (Suggested by Miroslav)
2. Update subject and commit log. (Suggested by Miroslav)
3. Update code format for some line of the patch. (Suggested by Miroslav)

V3 -> V4:
1. Rebase the patch of to branch linux-master.
2. Update the description of ABI Document Description. (Suggested by Petr & Josh)

Regards.
Wardenjohn.

Add "stack_order" sysfs attribute which holds the order in which a live patch module was loaded into the system. A user can then determine an active live patched version of a function.

cat /sys/kernel/livepatch/livepatch_1/stack_order -> 1

means that livepatch_1 is the first live patch applied

cat /sys/kernel/livepatch/livepatch_module/stack_order -> N

means that livepatch_module is the Nth live patch applied

---
 .../ABI/testing/sysfs-kernel-livepatch        |  9 +++++++
 kernel/livepatch/core.c                       | 24 +++++++++++++++++++
 2 files changed, 33 insertions(+)

index 3735d868013d..73e40d02345e 100644
@@ -55,6 +55,15 @@ Description:
 		An attribute which indicates whether the patch supports
 		atomic-replace.
 
+What:		/sys/kernel/livepatch/<patch>/stack_order
+Date:		Oct 2024
+KernelVersion:	6.13.0
+Description:
+		This attribute specifies the sequence in which live patch module
+		are applied to the system. If multiple live patches modify the same
+		function, the implementation with the biggest 'stack_order' number
+		is used, unless a transition is currently in progress.
+
 What:		/sys/kernel/livepatch/<patch>/<object>
 Date:		Nov 2014
 KernelVersion:	3.19.0

index 3c21c31796db..0cd39954d5a1 100644
@@ -347,6 +347,7 @@ int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
  * /sys/kernel/livepatch/<patch>/transition
  * /sys/kernel/livepatch/<patch>/force
  * /sys/kernel/livepatch/<patch>/replace
+ * /sys/kernel/livepatch/<patch>/stack_order
  * /sys/kernel/livepatch/<patch>/<object>
  * /sys/kernel/livepatch/<patch>/<object>/patched
  * /sys/kernel/livepatch/<patch>/<object>/<function,sympos>

@@ -452,15 +453,38 @@ static ssize_t replace_show(struct kobject *kobj,
 	return sysfs_emit(buf, "%d\n", patch->replace);
 }
 
+static ssize_t stack_order_show(struct kobject *kobj,
+				struct kobj_attribute *attr, char *buf)
+{
+	struct klp_patch *patch, *this_patch;
+	int stack_order = 0;
+
+	this_patch = container_of(kobj, struct klp_patch, kobj);
+
+	mutex_lock(&klp_mutex);
+
+	klp_for_each_patch(patch) {
+		stack_order++;
+		if (patch == this_patch)
+			break;
+	}
+
+	mutex_unlock(&klp_mutex);
+
+	return sysfs_emit(buf, "%d\n", stack_order);
+}
+
 static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled);
 static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition);
 static struct kobj_attribute force_kobj_attr = __ATTR_WO(force);
 static struct kobj_attribute replace_kobj_attr = __ATTR_RO(replace);
+static struct kobj_attribute stack_order_kobj_attr = __ATTR_RO(stack_order);
 static struct attribute *klp_patch_attrs[] = {
 	&enabled_kobj_attr.attr,
 	&transition_kobj_attr.attr,
 	&force_kobj_attr.attr,
 	&replace_kobj_attr.attr,
+	&stack_order_kobj_attr.attr,
 	NULL
 };
 ATTRIBUTE_GROUPS(klp_patch);

2.18.2