[PATCH v3] modules: wait do_free_init correctly

[Changbin Du]

The synchronization here is just to ensure the module init's been freed
before doing W+X checking. But the commit 1a7b7d922081 ("modules: Use
vmalloc special flag") moves do_free_init() into a global workqueue
instead of call_rcu(). So now rcu_barrier() can not ensure that do_free_init
has completed. We should wait it via flush_work().

Without this fix, we still could encounter false positive reports in
W+X checking, and the rcu synchronization is unnecessary which can
introduce significant delay.

Eric Chanudet reports that the rcu_barrier introduces ~0.1s delay on a
PREEMPT_RT kernel.
  [    0.291444] Freeing unused kernel memory: 5568K
  [    0.402442] Run /sbin/init as init process

With this fix, the above delay can be eliminated.

Fixes: 1a7b7d922081 ("modules: Use vmalloc special flag")
Signed-off-by: Changbin Du <changbin.du@xxxxxxxxxx>
Cc: Xiaoyi Su <suxiaoyi@xxxxxxxxxx>
Cc: Eric Chanudet <echanude@xxxxxxxxxx>

---
v3:
  - amend comment in do_init_module() and update commit msg.
v2:
  - fix compilation issue for no CONFIG_MODULES found by 0-DAY.
---
 include/linux/moduleloader.h | 8 ++++++++
 init/main.c                  | 5 +++--
 kernel/module/main.c         | 9 +++++++--
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 001b2ce83832..89b1e0ed9811 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -115,6 +115,14 @@ int module_finalize(const Elf_Ehdr *hdr,
 		    const Elf_Shdr *sechdrs,
 		    struct module *mod);
 
+#ifdef CONFIG_MODULES
+void flush_module_init_free_work(void);
+#else
+static inline void flush_module_init_free_work(void)
+{
+}
+#endif
+
 /* Any cleanup needed when module leaves. */
 void module_arch_cleanup(struct module *mod);
 
diff --git a/init/main.c b/init/main.c
index e24b0780fdff..f0b7e21ac67f 100644
--- a/init/main.c
+++ b/init/main.c
@@ -99,6 +99,7 @@
 #include <linux/init_syscalls.h>
 #include <linux/stackdepot.h>
 #include <linux/randomize_kstack.h>
+#include <linux/moduleloader.h>
 #include <net/net_namespace.h>
 
 #include <asm/io.h>
@@ -1402,11 +1403,11 @@ static void mark_readonly(void)
 	if (rodata_enabled) {
 		/*
 		 * load_module() results in W+X mappings, which are cleaned
-		 * up with call_rcu().  Let's make sure that queued work is
+		 * up with init_free_wq. Let's make sure that queued work is
 		 * flushed so that we don't hit false positives looking for
 		 * insecure pages which are W+X.
 		 */
-		rcu_barrier();
+		flush_module_init_free_work();
 		mark_rodata_ro();
 		rodata_test();
 	} else
diff --git a/kernel/module/main.c b/kernel/module/main.c
index 36681911c05a..b0b99348e1a8 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2489,6 +2489,11 @@ static void do_free_init(struct work_struct *w)
 	}
 }
 
+void flush_module_init_free_work(void)
+{
+	flush_work(&init_free_wq);
+}
+
 #undef MODULE_PARAM_PREFIX
 #define MODULE_PARAM_PREFIX "module."
 /* Default value for module->async_probe_requested */
@@ -2593,8 +2598,8 @@ static noinline int do_init_module(struct module *mod)
 	 * Note that module_alloc() on most architectures creates W+X page
 	 * mappings which won't be cleaned up until do_free_init() runs.  Any
 	 * code such as mark_rodata_ro() which depends on those mappings to
-	 * be cleaned up needs to sync with the queued work - ie
-	 * rcu_barrier()
+	 * be cleaned up needs to sync with the queued work by invoking
+	 * flush_module_init_free_work().
 	 */
 	if (llist_add(&freeinit->node, &init_free_list))
 		schedule_work(&init_free_wq);
-- 
2.25.1