Re: [POC 09/23] livepatch: Handle race when livepatches are reloaded during a module load

klp_module_coming() might fail to load a livepatch module when
the related livepatch gets reloaded in the meantime.

Detect this situation by adding a timestamp into struct klp_patch.
local_clock is enough because klp_mutex must be released and taken
several times during this scenario.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
  include/linux/livepatch.h | 2 ++
  kernel/livepatch/core.c   | 9 +++++----
  2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index a4567c17a9f2..feb33f023f9f 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h

@@ -155,6 +155,7 @@ struct klp_state {
   * @obj_list:	dynamic list of the object entries
   * @enabled:	the patch is enabled (but operation may be incomplete)
   * @forced:	was involved in a forced transition
+ * ts:		timestamp when the livepatch has been loaded

   * @free_work:	patch cleanup from workqueue-context
   * @finish:	for waiting till it is safe to remove the patch module
   */

@@ -171,6 +172,7 @@ struct klp_patch {
  	struct list_head obj_list;
  	bool enabled;
  	bool forced;
+	u64 ts;
  	struct work_struct free_work;
  	struct completion finish;
  };

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 34e3ee2be7ef..8e693c58b736 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c

@@ -20,6 +20,7 @@
  #include <linux/moduleloader.h>
  #include <linux/completion.h>
  #include <linux/memory.h>
+#include <linux/sched/clock.h>
  #include <asm/cacheflush.h>
  #include "core.h"
  #include "patch.h"

@@ -854,6 +855,7 @@ static int klp_init_patch_early(struct klp_patch *patch)
  	kobject_init(&patch->kobj, &klp_ktype_patch);
  	patch->enabled = false;
  	patch->forced = false;
+	patch->ts = local_clock();
  	INIT_WORK(&patch->free_work, klp_free_patch_work_fn);
  	init_completion(&patch->finish);
  

@@ -1324,6 +1326,7 @@ int klp_module_coming(struct module *mod)
  {
  	char patch_name[MODULE_NAME_LEN];
  	struct klp_patch *patch;
+	u64 patch_ts;
  	int ret = 0;
  
  	if (WARN_ON(mod->state != MODULE_STATE_COMING))

@@ -1339,6 +1342,7 @@ int klp_module_coming(struct module *mod)
  			continue;
  
  		strncpy(patch_name, patch->obj->patch_name, sizeof(patch_name));
+		patch_ts = patch->ts;
  		mutex_unlock(&klp_mutex);
  
  		ret = klp_try_load_object(patch_name, mod->name);

@@ -1346,14 +1350,11 @@ int klp_module_coming(struct module *mod)
  		 * The load might have failed because the patch has
  		 * been removed in the meantime. In this case, the
  		 * error might be ignored.
-		 *
-		 * FIXME: It is not fully proof. The patch might have be
-		 * unloaded and loaded again in the mean time.
  		 */
  		mutex_lock(&klp_mutex);
  		if (ret) {
  			patch = klp_find_patch(patch_name);
-			if (patch)
+			if (patch && patch->ts == patch_ts)
  				goto err;