module: Use RCU in search_module_extables().

search_module_extables() returns an exception_table_entry belonging to a
module. The lookup via __module_address() can be performed with RCU
protection.
The returned exception_table_entry remains valid because the passed
address usually belongs to a module that is currently executed. So the
module can not be removed because "something else" holds a reference to
it, ensuring that it can not be removed.
Exceptions here are:
- kprobe, acquires a reference on the module beforehand
- MCE, invokes the function from within a timer and the RCU lifetime
  guarantees (of the timer) are sufficient.

Therefore it is safe to return the exception_table_entry outside the RCU
section which provided the module.

Use RCU for the lookup in search_module_extables() and update the
comment.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250108090457.512198-14-bigeasy@linutronix.de
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 2155814f35dd0e2bbd9e12814d0a90e7b84dfcc1..aebb17e1bfb976e6a6d889b844b7b883e90da313 100644 (file)
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -3703,28 +3703,23 @@ out:
 /* Given an address, look for it in the module exception tables. */
 const struct exception_table_entry *search_module_extables(unsigned long addr)
 {
-       const struct exception_table_entry *e = NULL;
        struct module *mod;
 
-       preempt_disable();
+       guard(rcu)();
        mod = __module_address(addr);
        if (!mod)
-               goto out;
+               return NULL;
 
        if (!mod->num_exentries)
-               goto out;
-
-       e = search_extable(mod->extable,
-                          mod->num_exentries,
-                          addr);
-out:
-       preempt_enable();
-
+               return NULL;
        /*
-        * Now, if we found one, we are running inside it now, hence
-        * we cannot unload the module, hence no refcnt needed.
+        * The address passed here belongs to a module that is currently
+        * invoked (we are running inside it). Therefore its module::refcnt
+        * needs already be >0 to ensure that it is not removed at this stage.
+        * All other user need to invoke this function within a RCU read
+        * section.
         */
-       return e;
+       return search_extable(mod->extable, mod->num_exentries, addr);
 }
 
 /**