module: Use RCU in find_kallsyms_symbol().

The modules list and module::kallsyms can be accessed under RCU
assumption.

Use rcu_dereference() to reference the kallsyms pointer in
find_kallsyms_symbol().  Use a RCU section instead of preempt_disable in
callers of find_kallsyms_symbol(). Keep the preempt-disable in
module_address_lookup() due to __module_address().

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Petr Pavlu <[email protected]>

Author: Sebastian Andrzej Siewior

kernel/module/kallsyms.c

@@ -257,7 +257,7 @@ static const char *find_kallsyms_symbol(struct module *mod,
 {
 	unsigned int i, best = 0;
 	unsigned long nextval, bestval;
-	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
+	struct mod_kallsyms *kallsyms = rcu_dereference(mod->kallsyms);
 	struct module_memory *mod_mem;
 
 	/* At worse, next value is at end of module */
@@ -329,6 +329,7 @@ int module_address_lookup(unsigned long addr,
 	int ret = 0;
 	struct module *mod;
 
+	guard(rcu)();
 	preempt_disable();
 	mod = __module_address(addr);
 	if (mod) {
@@ -356,7 +357,7 @@ int lookup_module_symbol_name(unsigned long addr, char *symname)
 {
 	struct module *mod;
 
-	preempt_disable();
+	guard(rcu)();
 	list_for_each_entry_rcu(mod, &modules, list) {
 		if (mod->state == MODULE_STATE_UNFORMED)
 			continue;
@@ -368,12 +369,10 @@ int lookup_module_symbol_name(unsigned long addr, char *symname)
 				goto out;
 
 			strscpy(symname, sym, KSYM_NAME_LEN);
-			preempt_enable();
 			return 0;
 		}
 	}
 out:
-	preempt_enable();
 	return -ERANGE;
 }