[PATCH 1/3] livepatch: introduce shadow variable API

[Joe Lawrence <joe.lawrence@redhat.com>]

Add three exported API for livepatch modules:

  void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data);
  void klp_shadow_detach(void *obj, char *var);
  void *klp_shadow_get(void *obj, char *var);

that implement "shadow" variables, which allow callers to associate new
shadow fields to existing data structures.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 include/linux/livepatch.h |   4 ++
 kernel/livepatch/Makefile |   2 +-
 kernel/livepatch/shadow.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 120 insertions(+), 1 deletion(-)
 create mode 100644 kernel/livepatch/shadow.c

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 194991ef9347..a0d068ecf7f8 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -164,6 +164,10 @@ static inline bool klp_have_reliable_stack(void)
 	       IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE);
 }
 
+void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data);
+void klp_shadow_detach(void *obj, char *var);
+void *klp_shadow_get(void *obj, char *var);
+
 #else /* !CONFIG_LIVEPATCH */
 
 static inline int klp_module_coming(struct module *mod) { return 0; }
diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile
index 2b8bdb1925da..b36ceda6488e 100644
--- a/kernel/livepatch/Makefile
+++ b/kernel/livepatch/Makefile
@@ -1,3 +1,3 @@
 obj-$(CONFIG_LIVEPATCH) += livepatch.o
 
-livepatch-objs := core.o patch.o transition.o
+livepatch-objs := core.o patch.o shadow.o transition.o
diff --git a/kernel/livepatch/shadow.c b/kernel/livepatch/shadow.c
new file mode 100644
index 000000000000..72d5e567dff9
--- /dev/null
+++ b/kernel/livepatch/shadow.c
@@ -0,0 +1,115 @@
+/*
+ * shadow.c - Shadow Variables
+ *
+ * Copyright (C) 2014 Josh Poimboeuf <jpoimboe@redhat.com>
+ * Copyright (C) 2014 Seth Jennings <sjenning@redhat.com>
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/hashtable.h>
+#include <linux/slab.h>
+#include <linux/livepatch.h>
+
+static DEFINE_HASHTABLE(klp_shadow_hash, 12);
+static DEFINE_SPINLOCK(klp_shadow_lock);
+
+struct klp_shadow {
+	struct hlist_node node;
+	struct rcu_head rcu_head;
+	void *obj;
+	char *var;
+	void *data;
+};
+
+void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data)
+{
+	unsigned long flags;
+	struct klp_shadow *shadow;
+
+	shadow = kmalloc(sizeof(*shadow), gfp);
+	if (!shadow)
+		return NULL;
+
+	shadow->obj = obj;
+
+	shadow->var = kstrdup(var, gfp);
+	if (!shadow->var) {
+		kfree(shadow);
+		return NULL;
+	}
+
+	shadow->data = data;
+
+	spin_lock_irqsave(&klp_shadow_lock, flags);
+	hash_add_rcu(klp_shadow_hash, &shadow->node, (unsigned long)obj);
+	spin_unlock_irqrestore(&klp_shadow_lock, flags);
+
+	return shadow->data;
+}
+EXPORT_SYMBOL_GPL(klp_shadow_attach);
+
+static void klp_shadow_rcu_free(struct rcu_head *head)
+{
+	struct klp_shadow *shadow;
+
+	shadow = container_of(head, struct klp_shadow, rcu_head);
+
+	kfree(shadow->var);
+	kfree(shadow);
+}
+
+void klp_shadow_detach(void *obj, char *var)
+{
+	unsigned long flags;
+	struct klp_shadow *shadow;
+
+	spin_lock_irqsave(&klp_shadow_lock, flags);
+
+	hash_for_each_possible(klp_shadow_hash, shadow, node,
+			       (unsigned long)obj) {
+		if (shadow->obj == obj && !strcmp(shadow->var, var)) {
+			hash_del_rcu(&shadow->node);
+			spin_unlock_irqrestore(&klp_shadow_lock, flags);
+			call_rcu(&shadow->rcu_head, klp_shadow_rcu_free);
+			return;
+		}
+	}
+
+	spin_unlock_irqrestore(&klp_shadow_lock, flags);
+}
+EXPORT_SYMBOL_GPL(klp_shadow_detach);
+
+void *klp_shadow_get(void *obj, char *var)
+{
+	struct klp_shadow *shadow;
+
+	rcu_read_lock();
+
+	hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
+				   (unsigned long)obj) {
+		if (shadow->obj == obj && !strcmp(shadow->var, var)) {
+			rcu_read_unlock();
+			return shadow->data;
+		}
+	}
+
+	rcu_read_unlock();
+
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(klp_shadow_get);
-- 
1.8.3.1