Commit 2abf84f6 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Petr Pavlu
Browse files

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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250108090457.512198-14-bigeasy@linutronix.de


Signed-off-by: default avatarPetr Pavlu <petr.pavlu@suse.com>
parent 7d9dda6f

kernel/module/main.c

+9 −14
Original line number Diff line number Diff line
@@ -3703,28 +3703,23 @@ char *module_flags(struct module *mod, char *buf, bool show_state) 
/* 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);

}



/**