livepatch: support for repatching a function

Add support for patching a function multiple times.  If multiple patches
affect a function, the function in the most recently enabled patch
"wins".  This enables a cumulative patch upgrade path, where each patch
is a superset of previous patches.

This requires restructuring the data a little bit.  With the current
design, where each klp_func struct has its own ftrace_ops, we'd have to
unregister the old ops and then register the new ops, because
FTRACE_OPS_FL_IPMODIFY prevents us from having two ops registered for
the same function at the same time.  That would leave a regression
window where the function isn't patched at all (not good for a patch
upgrade path).

This patch replaces the per-klp_func ftrace_ops with a global klp_ops
list, with one ftrace_ops per original function.  A single ftrace_ops is
shared between all klp_funcs which have the same old_addr.  This allows
the switch between function versions to happen instantaneously by
updating the klp_ops struct's func_stack list.  The winner is the
klp_func at the top of the func_stack (front of the list).

[ jkosina@suse.cz: turn WARN_ON() into WARN_ON_ONCE() in ftrace handler to
  avoid storm in pathological cases ]

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 950bc615842f..f14c6fb262b4 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -40,8 +40,8 @@ enum klp_state {
  * @old_addr:	a hint conveying at what address the old function
  *		can be found (optional, vmlinux patches only)
  * @kobj:	kobject for sysfs resources
- * @fops:	ftrace operations structure
  * @state:	tracks function-level patch application state
+ * @stack_node:	list node for klp_ops func_stack list
  */
 struct klp_func {
 	/* external */
@@ -59,8 +59,8 @@ struct klp_func {
 
 	/* internal */
 	struct kobject kobj;
-	struct ftrace_ops *fops;
 	enum klp_state state;
+	struct list_head stack_node;
 };
 
 /**