kmod/patch: export patched functions information via sysfs

This feature is implemented as:
```
[root@localhost kpatch]# insmod ./kpatch-meminfo.ko
[root@localhost kpatch]# ls /sys/kernel/kpatch/patches/kpatch_meminfo/functions/meminfo_proc_show/
new_addr  old_addr
[root@localhost kpatch]# cat /sys/kernel/kpatch/patches/kpatch_meminfo/functions/meminfo_proc_show/new_addr
0xffffffffa05211e0
[root@localhost kpatch]# cat /sys/kernel/kpatch/patches/kpatch_meminfo/functions/meminfo_proc_show/old_addr
0xffffffff8125d0e0
```

The patch module init function will allocate and init kpatch_func_obj with
customized kobj_type func_ktype. The attribute new_addr and old_addr of
kpatch_func_obj is attached to this func_ktype, so that these files could
be created by kobject_add automatically.

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>

kmod/patch/kpatch-patch-hook.c

@@ -22,6 +22,7 @@
 #include <linux/module.h>
 #include <linux/printk.h>
 #include <linux/slab.h>
+#include <linux/kallsyms.h>
 #include "kpatch.h"
 
 static bool replace;
@@ -33,6 +34,15 @@ extern char __kpatch_dynrelas, __kpatch_dynrelas_end;
 
 static struct kpatch_module kpmod;
 static struct kobject *patch_kobj;
+static struct kobject *functions_kobj;
+
+struct kpatch_func_obj {
+	struct kobject kobj;
+	struct kpatch_patch *patch;
+	char name[KSYM_NAME_LEN];
+};
+
+static struct kpatch_func_obj **funcs = NULL;
 
 static ssize_t patch_enabled_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
@@ -67,12 +77,83 @@ static ssize_t patch_enabled_store(struct kobject *kobj,
 
 	return count;
 }
+
 static struct kobj_attribute patch_enabled_attr =
 	__ATTR(enabled, 0644, patch_enabled_show, patch_enabled_store);
 
+static ssize_t func_old_addr_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	struct kpatch_func_obj *func =
+		container_of(kobj, struct kpatch_func_obj, kobj);
+
+	return sprintf(buf, "0x%lx\n", func->patch->old_addr);
+}
+
+static ssize_t func_new_addr_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	struct kpatch_func_obj *func =
+		container_of(kobj, struct kpatch_func_obj, kobj);
+
+	return sprintf(buf, "0x%lx\n", func->patch->new_addr);
+}
+
+static struct kobj_attribute old_addr_attr =
+	__ATTR(old_addr, S_IRUGO, func_old_addr_show, NULL);
+
+static struct kobj_attribute new_addr_attr =
+	__ATTR(new_addr, S_IRUGO, func_new_addr_show, NULL);
+
+static void func_kobj_free(struct kobject *kobj)
+{
+	struct kpatch_func_obj *func =
+		container_of(kobj, struct kpatch_func_obj, kobj);
+	kfree(func);
+}
+
+static struct attribute *func_kobj_attrs[] = {
+	&old_addr_attr.attr,
+	&new_addr_attr.attr,
+	NULL,
+};
+
+static ssize_t func_kobj_show(struct kobject *kobj,
+			      struct attribute *attr, char *buf)
+{
+	struct kobj_attribute *func_attr =
+		container_of(attr, struct kobj_attribute, attr);
+
+	return func_attr->show(kobj, func_attr, buf);
+}
+
+static const struct sysfs_ops func_sysfs_ops = {
+	.show	= func_kobj_show,
+};
+
+static struct kobj_type func_ktype = {
+	.release	= func_kobj_free,
+	.sysfs_ops	= &func_sysfs_ops,
+	.default_attrs	= func_kobj_attrs,
+};
+
+static struct kpatch_func_obj *func_kobj_alloc(void)
+{
+	struct kpatch_func_obj *func;
+	func = kzalloc(sizeof(*func), GFP_KERNEL);
+	if (!func)
+		return NULL;
+
+	kobject_init(&func->kobj, &func_ktype);
+
+	return func;
+}
+
 static int __init patch_init(void)
 {
 	int ret;
+	int i = 0;
+	struct kpatch_func_obj *func = NULL;
 
 	kpmod.mod = THIS_MODULE;
 	kpmod.patches = (struct kpatch_patch *)&__kpatch_patches;
@@ -82,6 +163,13 @@ static int __init patch_init(void)
 	kpmod.dynrelas_nr = (&__kpatch_dynrelas_end - &__kpatch_dynrelas) /
 			  sizeof(*kpmod.dynrelas);
 
+	funcs = kzalloc(kpmod.patches_nr * sizeof(struct kpatch_func_obj*),
+			GFP_KERNEL);
+	if (!funcs) {
+		ret = -ENOMEM;
+		goto err_ret;
+	}
+
 	patch_kobj = kobject_create_and_add(THIS_MODULE->name,
 					    kpatch_patches_kobj);
 	if (!patch_kobj) {
@@ -93,23 +181,62 @@ static int __init patch_init(void)
 	if (ret)
 		goto err_put;
 
+	functions_kobj = kobject_create_and_add("functions", patch_kobj);
+	if (!functions_kobj) {
+		ret = -ENOMEM;
+		goto err_sysfs;
+	}
+
+	for (i = 0; i < kpmod.patches_nr; i++) {
+		func = func_kobj_alloc();
+		if (!func) {
+			ret = -ENOMEM;
+			goto err_sysfs2;
+		}
+		funcs[i] = func;
+
+		sprint_symbol_no_offset(func->name, kpmod.patches[i].old_addr);
+
+		ret = kobject_add(&func->kobj, functions_kobj,
+				  "%s", func->name);
+		if (ret)
+			goto err_sysfs2;
+
+		func->patch = &kpmod.patches[i];
+	}
+
 	ret = kpatch_register(&kpmod, replace);
 	if (ret)
-		goto err_sysfs;
+		goto err_sysfs2;
 
 	return 0;
 
+err_sysfs2:
+	for (i = 0; i < kpmod.patches_nr; i++) {
+		if (funcs[i] != NULL)
+			kobject_put(&funcs[i]->kobj);
+	}
+	kobject_put(functions_kobj);
 err_sysfs:
 	sysfs_remove_file(patch_kobj, &patch_enabled_attr.attr);
 err_put:
 	kobject_put(patch_kobj);
 err_free:
+	kfree(funcs);
+err_ret:
 	return ret;
 }
 
 static void __exit patch_exit(void)
 {
+	int i;
 	WARN_ON(kpmod.enabled);
+
+	for (i = 0; i < kpmod.patches_nr; i++) {
+		kobject_put(&funcs[i]->kobj);
+	}
+	kfree(funcs);
+	kobject_put(functions_kobj);
 	sysfs_remove_file(patch_kobj, &patch_enabled_attr.attr);
 	kobject_put(patch_kobj);
 }