[PATCH 2/6] module: add support for CONFIG_MODULE_DESTRUCTORS

[Johannes Berg]

From: Johannes Berg <johannes.berg@xxxxxxxxx>

At least in ARCH=um with CONFIG_GCOV (which writes all the
coverage data directly out from the userspace binary rather
than presenting it in debugfs) it's necessary to run all
the atexit handlers (dtors/fini_array) so that gcov actually
does write out the data.

Add a new config option CONFIG_MODULE_DESTRUCTORS that can
be selected via CONFIG_WANT_MODULE_DESTRUCTORS that the arch
selects (this indirection exists so the architecture doesn't
have to worry about whether or not CONFIG_MODULES is on).
Additionally, the architecture must then (when it exits and
no more module code can run) call run_all_module_destructors
to run the code for all modules that are still loaded. When
modules are unloaded, the handlers are called as well.

Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
---
 include/linux/module.h | 14 ++++++++++++++
 init/Kconfig           | 17 +++++++++++++++++
 kernel/module.c        | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 70 insertions(+)

diff --git a/include/linux/module.h b/include/linux/module.h
index 59f094fa6f74..8574d76a884d 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -517,6 +517,12 @@ struct module {
 	unsigned int num_ctors;
 #endif
 
+#ifdef CONFIG_MODULE_DESTRUCTORS
+	/* Destructor functions. */
+	ctor_fn_t *dtors;
+	unsigned int num_dtors;
+#endif
+
 #ifdef CONFIG_FUNCTION_ERROR_INJECTION
 	struct error_injection_entry *ei_funcs;
 	unsigned int num_ei_funcs;
@@ -853,4 +859,12 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 					     struct module *, unsigned long),
 				   void *data);
 
+#ifdef CONFIG_MODULE_DESTRUCTORS
+void run_all_module_destructors(void);
+#else
+static inline void run_all_module_destructors(void)
+{
+}
+#endif
+
 #endif /* _LINUX_MODULE_H */
diff --git a/init/Kconfig b/init/Kconfig
index 22946fe5ded9..b0f0f51f9d2c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2295,6 +2295,23 @@ config UNUSED_KSYMS_WHITELIST
 
 endif # MODULES
 
+config WANT_MODULE_DESTRUCTORS
+	bool
+	help
+	  Architectures may select this if they need atexit functions (such as
+	  generated by the compiler for -ftest-coverage/gcov) to run in modules.
+	  They're then responsible for calling run_all_module_destructors() at
+	  shutdown so that module destructors are called for all still loaded
+	  modules as well.
+
+	  Note that CONFIG_GCOV_KERNEL does *not* require this since it keeps
+	  all the coverage data in the kernel, notably CONFIG_GCOV in ARCH=um
+	  requires this.
+
+config MODULE_DESTRUCTORS
+	def_bool y
+	depends on WANT_MODULE_DESTRUCTORS && MODULES
+
 config MODULES_TREE_LOOKUP
 	def_bool y
 	depends on PERF_EVENTS || TRACING
diff --git a/kernel/module.c b/kernel/module.c
index 30479355ab85..3023b5f054d4 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -904,6 +904,27 @@ EXPORT_SYMBOL(module_refcount);
 /* This exists whether we can unload or not */
 static void free_module(struct module *mod);
 
+#ifdef CONFIG_MODULE_DESTRUCTORS
+static void do_mod_dtors(struct module *mod)
+{
+	unsigned long i;
+
+	for (i = 0; i < mod->num_dtors; i++)
+		mod->dtors[i]();
+}
+
+void run_all_module_destructors(void)
+{
+	struct module *mod;
+
+	/* we no longer need to care about locking at this point */
+	list_for_each_entry(mod, &modules, list)
+		do_mod_dtors(mod);
+}
+#else
+static inline void do_mod_dtors(struct module *mod) {}
+#endif
+
 SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 		unsigned int, flags)
 {
@@ -966,6 +987,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 				     MODULE_STATE_GOING, mod);
 	klp_module_going(mod);
 	ftrace_release_mod(mod);
+	do_mod_dtors(mod);
 
 	async_synchronize_full();
 
@@ -3263,6 +3285,23 @@ static int find_module_sections(struct module *mod, struct load_info *info)
 	}
 #endif
 
+#ifdef CONFIG_MODULE_DESTRUCTORS
+	mod->dtors = section_objs(info, ".dtors",
+				  sizeof(*mod->dtors), &mod->num_dtors);
+	if (!mod->dtors)
+		mod->dtors = section_objs(info, ".fini_array",
+				sizeof(*mod->dtors), &mod->num_dtors);
+	else if (find_sec(info, ".fini_array")) {
+		/*
+		 * This shouldn't happen with same compiler and binutils
+		 * building all parts of the module.
+		 */
+		pr_warn("%s: has both .dtors and .fini_array.\n",
+		       mod->name);
+		return -EINVAL;
+	}
+#endif
+
 	mod->noinstr_text_start = section_objs(info, ".noinstr.text", 1,
 						&mod->noinstr_text_size);
 
-- 
2.29.2