livepatch: introduce shadow variable API

Add exported API for livepatch modules:

  klp_shadow_get()
  klp_shadow_alloc()
  klp_shadow_get_or_alloc()
  klp_shadow_free()
  klp_shadow_free_all()

that implement "shadow" variables, which allow callers to associate new
shadow fields to existing data structures.  This is intended to be used
by livepatch modules seeking to emulate additions to data structure
definitions.

See Documentation/livepatch/shadow-vars.txt for a summary of the new
shadow variable API, including a few common use cases.

See samples/livepatch/livepatch-shadow-* for example modules that
demonstrate shadow variables.

[jkosina@suse.cz: fix __klp_shadow_get_or_alloc() comment as spotted by
 Josh]
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

2 files changed, 278 insertions, 1 deletions

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..67e4360521f3
--- /dev/null
+++ b/kernel/livepatch/shadow.c
@@ -0,0 +1,277 @@
+/*
+ * 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/>.
+ */
+
+/**
+ * DOC: Shadow variable API concurrency notes:
+ *
+ * The shadow variable API provides a simple relationship between an
+ * <obj, id> pair and a pointer value.  It is the responsibility of the
+ * caller to provide any mutual exclusion required of the shadow data.
+ *
+ * Once a shadow variable is attached to its parent object via the
+ * klp_shadow_*alloc() API calls, it is considered live: any subsequent
+ * call to klp_shadow_get() may then return the shadow variable's data
+ * pointer.  Callers of klp_shadow_*alloc() should prepare shadow data
+ * accordingly.
+ *
+ * The klp_shadow_*alloc() API calls may allocate memory for new shadow
+ * variable structures.  Their implementation does not call kmalloc
+ * inside any spinlocks, but API callers should pass GFP flags according
+ * to their specific needs.
+ *
+ * The klp_shadow_hash is an RCU-enabled hashtable and is safe against
+ * concurrent klp_shadow_free() and klp_shadow_get() operations.
+ */
+
+#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);
+
+/*
+ * klp_shadow_lock provides exclusive access to the klp_shadow_hash and
+ * the shadow variables it references.
+ */
+static DEFINE_SPINLOCK(klp_shadow_lock);
+
+/**
+ * struct klp_shadow - shadow variable structure
+ * @node:	klp_shadow_hash hash table node
+ * @rcu_head:	RCU is used to safely free this structure
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ * @data:	data area
+ */
+struct klp_shadow {
+	struct hlist_node node;
+	struct rcu_head rcu_head;
+	void *obj;
+	unsigned long id;
+	char data[];
+};
+
+/**
+ * klp_shadow_match() - verify a shadow variable matches given <obj, id>
+ * @shadow:	shadow variable to match
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ *
+ * Return: true if the shadow variable matches.
+ */
+static inline bool klp_shadow_match(struct klp_shadow *shadow, void *obj,
+				unsigned long id)
+{
+	return shadow->obj == obj && shadow->id == id;
+}
+
+/**
+ * klp_shadow_get() - retrieve a shadow variable data pointer
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ *
+ * Return: the shadow variable data element, NULL on failure.
+ */
+void *klp_shadow_get(void *obj, unsigned long id)
+{
+	struct klp_shadow *shadow;
+
+	rcu_read_lock();
+
+	hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
+				   (unsigned long)obj) {
+
+		if (klp_shadow_match(shadow, obj, id)) {
+			rcu_read_unlock();
+			return shadow->data;
+		}
+	}
+
+	rcu_read_unlock();
+
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(klp_shadow_get);
+
+void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
+		       size_t size, gfp_t gfp_flags, bool warn_on_exist)
+{
+	struct klp_shadow *new_shadow;
+	void *shadow_data;
+	unsigned long flags;
+
+	/* Check if the shadow variable already exists */
+	shadow_data = klp_shadow_get(obj, id);
+	if (shadow_data)
+		goto exists;
+
+	/* Allocate a new shadow variable for use inside the lock below */
+	new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
+	if (!new_shadow)
+		return NULL;
+
+	new_shadow->obj = obj;
+	new_shadow->id = id;
+
+	/* Initialize the shadow variable if data provided */
+	if (data)
+		memcpy(new_shadow->data, data, size);
+
+	/* Look for <obj, id> again under the lock */
+	spin_lock_irqsave(&klp_shadow_lock, flags);
+	shadow_data = klp_shadow_get(obj, id);
+	if (unlikely(shadow_data)) {
+		/*
+		 * Shadow variable was found, throw away speculative
+		 * allocation.
+		 */
+		spin_unlock_irqrestore(&klp_shadow_lock, flags);
+		kfree(new_shadow);
+		goto exists;
+	}
+
+	/* No <obj, id> found, so attach the newly allocated one */
+	hash_add_rcu(klp_shadow_hash, &new_shadow->node,
+		     (unsigned long)new_shadow->obj);
+	spin_unlock_irqrestore(&klp_shadow_lock, flags);
+
+	return new_shadow->data;
+
+exists:
+	if (warn_on_exist) {
+		WARN(1, "Duplicate shadow variable <%p, %lx>\n", obj, id);
+		return NULL;
+	}
+
+	return shadow_data;
+}
+
+/**
+ * klp_shadow_alloc() - allocate and add a new shadow variable
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ * @data:	pointer to data to attach to parent
+ * @size:	size of attached data
+ * @gfp_flags:	GFP mask for allocation
+ *
+ * Allocates @size bytes for new shadow variable data using @gfp_flags
+ * and copies @size bytes from @data into the new shadow variable's own
+ * data space.  If @data is NULL, @size bytes are still allocated, but
+ * no copy is performed.  The new shadow variable is then added to the
+ * global hashtable.
+ *
+ * If an existing <obj, id> shadow variable can be found, this routine
+ * will issue a WARN, exit early and return NULL.
+ *
+ * Return: the shadow variable data element, NULL on duplicate or
+ * failure.
+ */
+void *klp_shadow_alloc(void *obj, unsigned long id, void *data,
+		       size_t size, gfp_t gfp_flags)
+{
+	return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, true);
+}
+EXPORT_SYMBOL_GPL(klp_shadow_alloc);
+
+/**
+ * klp_shadow_get_or_alloc() - get existing or allocate a new shadow variable
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ * @data:	pointer to data to attach to parent
+ * @size:	size of attached data
+ * @gfp_flags:	GFP mask for allocation
+ *
+ * Returns a pointer to existing shadow data if an <obj, id> shadow
+ * variable is already present.  Otherwise, it creates a new shadow
+ * variable like klp_shadow_alloc().
+ *
+ * This function guarantees that only one shadow variable exists with
+ * the given @id for the given @obj.  It also guarantees that the shadow
+ * variable will be initialized by the given @data only when it did not
+ * exist before.
+ *
+ * Return: the shadow variable data element, NULL on failure.
+ */
+void *klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
+			       size_t size, gfp_t gfp_flags)
+{
+	return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, false);
+}
+EXPORT_SYMBOL_GPL(klp_shadow_get_or_alloc);
+
+/**
+ * klp_shadow_free() - detach and free a <obj, id> shadow variable
+ * @obj:	pointer to parent object
+ * @id:		data identifier
+ *
+ * This function releases the memory for this <obj, id> shadow variable
+ * instance, callers should stop referencing it accordingly.
+ */
+void klp_shadow_free(void *obj, unsigned long id)
+{
+	struct klp_shadow *shadow;
+	unsigned long flags;
+
+	spin_lock_irqsave(&klp_shadow_lock, flags);
+
+	/* Delete <obj, id> from hash */
+	hash_for_each_possible(klp_shadow_hash, shadow, node,
+			       (unsigned long)obj) {
+
+		if (klp_shadow_match(shadow, obj, id)) {
+			hash_del_rcu(&shadow->node);
+			kfree_rcu(shadow, rcu_head);
+			break;
+		}
+	}
+
+	spin_unlock_irqrestore(&klp_shadow_lock, flags);
+}
+EXPORT_SYMBOL_GPL(klp_shadow_free);
+
+/**
+ * klp_shadow_free_all() - detach and free all <*, id> shadow variables
+ * @id:		data identifier
+ *
+ * This function releases the memory for all <*, id> shadow variable
+ * instances, callers should stop referencing them accordingly.
+ */
+void klp_shadow_free_all(unsigned long id)
+{
+	struct klp_shadow *shadow;
+	unsigned long flags;
+	int i;
+
+	spin_lock_irqsave(&klp_shadow_lock, flags);
+
+	/* Delete all <*, id> from hash */
+	hash_for_each(klp_shadow_hash, i, shadow, node) {
+		if (klp_shadow_match(shadow, shadow->obj, id)) {
+			hash_del_rcu(&shadow->node);
+			kfree_rcu(shadow, rcu_head);
+		}
+	}
+
+	spin_unlock_irqrestore(&klp_shadow_lock, flags);
+}
+EXPORT_SYMBOL_GPL(klp_shadow_free_all);