[RFC PATCH] modules: extend {kstrdup,kfree}_const() to handle per-module .rodata

[Dmitry Antipov <dmantipov@yandex.ru>]

Since kernel modules has their own .rodata sections, functions like
'kstrdup_const()' called from the module context are not required to
copy string constants from these sections. Likewise, 'kfree_const()'
in such a context becomes a no-op also because the whole module's
.rodata is freed at module unloading. OTOH this proof-of-concept
implementation introduces substantial overhead due to calls to
'__module_address()' from 'is_module_rodata()', which is much slower
than 'is_kernel_rodata()'. Anyway, comments are highly appreciated.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
---
 include/linux/module.h | 6 ++++++
 kernel/module/main.c   | 9 +++++++++
 mm/util.c              | 7 +++++--
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 14f391b186c6..7edaf2b730ce 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -623,6 +623,7 @@ bool is_module_address(unsigned long addr);
 bool __is_module_percpu_address(unsigned long addr, unsigned long *can_addr);
 bool is_module_percpu_address(unsigned long addr);
 bool is_module_text_address(unsigned long addr);
+bool is_module_rodata(unsigned long addr);
 
 static inline bool within_module_mem_type(unsigned long addr,
 					  const struct module *mod,
@@ -807,6 +808,11 @@ static inline bool is_module_text_address(unsigned long addr)
 	return false;
 }
 
+static inline bool is_module_rodata(unsigned long addr)
+{
+	return false;
+}
+
 static inline bool within_module_core(unsigned long addr,
 				      const struct module *mod)
 {
diff --git a/kernel/module/main.c b/kernel/module/main.c
index c3ce106c70af..5003cacd0786 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -3858,6 +3858,15 @@ bool is_module_text_address(unsigned long addr)
 	return __module_text_address(addr) != NULL;
 }
 
+bool is_module_rodata(unsigned long addr)
+{
+	struct module *mod;
+
+	guard(rcu)();
+	mod = __module_address(addr);
+	return mod && within_module_mem_type(addr, mod, MOD_RODATA);
+}
+
 void module_for_each_mod(int(*func)(struct module *mod, void *data), void *data)
 {
 	struct module *mod;
diff --git a/mm/util.c b/mm/util.c
index b05ab6f97e11..8dd1f1e95554 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -26,6 +26,7 @@
 #include <linux/compat.h>
 #include <linux/fsnotify.h>
 #include <linux/page_idle.h>
+#include <linux/module.h>
 
 #include <linux/uaccess.h>
 
@@ -42,7 +43,8 @@
  */
 void kfree_const(const void *x)
 {
-	if (!is_kernel_rodata((unsigned long)x))
+	if (!is_kernel_rodata((unsigned long)x) &&
+	    !is_module_rodata((unsigned long)x))
 		kfree(x);
 }
 EXPORT_SYMBOL(kfree_const);
@@ -98,7 +100,8 @@ EXPORT_SYMBOL(kstrdup);
  */
 const char *kstrdup_const(const char *s, gfp_t gfp)
 {
-	if (is_kernel_rodata((unsigned long)s))
+	if (is_kernel_rodata((unsigned long)s) ||
+	    is_module_rodata((unsigned long)s))
 		return s;
 
 	return kstrdup(s, gfp);
-- 
2.53.0