pstore: Do not leave timer disabled for next backend

The pstore.update_ms value was being disabled during pstore_unregister(),
which would cause any prior value to go unnoticed on the next
pstore_register(). Instead, just let del_timer() stop the timer, which
was always sufficient. This additionally refactors the timer reset code
and allows the timer to be enabled if the module parameter is changed
away from the default.

Link: https://lore.kernel.org/lkml/20200506152114.50375-10-keescook@chromium.org/Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -44,7 +44,7 @@ static int pstore_update_ms = -1;
 module_param_named(update_ms, pstore_update_ms, int, 0600);
 MODULE_PARM_DESC(update_ms, "milliseconds before pstore updates its content "
 		 "(default is -1, which means runtime updates are disabled; "
-		 "enabling this option is not safe, it may lead to further "
+		 "enabling this option may not be safe; it may lead to further "
 		 "corruption on Oopses)");
 
 /* Names should be in the same order as the enum pstore_type_id */
@@ -150,6 +150,14 @@ static const char *get_reason_str(enum kmsg_dump_reason reason)
 	}
 }
 
+static void pstore_timer_kick(void)
+{
+	if (pstore_update_ms < 0)
+		return;
+
+	mod_timer(&pstore_timer, jiffies + msecs_to_jiffies(pstore_update_ms));
+}
+
 /*
  * Should pstore_dump() wait for a concurrent pstore_dump()? If
  * not, the current pstore_dump() will report a failure to dump
@@ -460,8 +468,10 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 		}
 
 		ret = psinfo->write(&record);
-		if (ret == 0 && reason == KMSG_DUMP_OOPS)
+		if (ret == 0 && reason == KMSG_DUMP_OOPS) {
 			pstore_new_entry = 1;
+			pstore_timer_kick();
+		}
 
 		total += record.size;
 		part++;
@@ -604,11 +614,7 @@ int pstore_register(struct pstore_info *psi)
 		pstore_register_pmsg();
 
 	/* Start watching for new records, if desired. */
-	if (pstore_update_ms >= 0) {
-		pstore_timer.expires = jiffies +
-			msecs_to_jiffies(pstore_update_ms);
-		add_timer(&pstore_timer);
-	}
+	pstore_timer_kick();
 
 	/*
 	 * Update the module parameter backend, so it is visible
@@ -637,11 +643,7 @@ void pstore_unregister(struct pstore_info *psi)
 		return;
 	}
 
-	/* Stop timer and make sure all work has finished. */
-	pstore_update_ms = -1;
-	del_timer_sync(&pstore_timer);
-	flush_work(&pstore_work);
-
+	/* Unregister all callbacks. */
 	if (psi->flags & PSTORE_FLAGS_PMSG)
 		pstore_unregister_pmsg();
 	if (psi->flags & PSTORE_FLAGS_FTRACE)
@@ -651,6 +653,10 @@ void pstore_unregister(struct pstore_info *psi)
 	if (psi->flags & PSTORE_FLAGS_DMESG)
 		pstore_unregister_kmsg();
 
+	/* Stop timer and make sure all work has finished. */
+	del_timer_sync(&pstore_timer);
+	flush_work(&pstore_work);
+
 	free_buf_for_compression();
 
 	psinfo = NULL;
@@ -792,9 +798,7 @@ static void pstore_timefunc(struct timer_list *unused)
 		schedule_work(&pstore_work);
 	}
 
-	if (pstore_update_ms >= 0)
-		mod_timer(&pstore_timer,
-			  jiffies + msecs_to_jiffies(pstore_update_ms));
+	pstore_timer_kick();
 }
 
 static void __init pstore_choose_compression(void)