Subject: [PATCH 10/28] module: Use RCU in find_symbol().
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Wed, 8 Jan 2025 10:04:39 +0100

module_assert_mutex_or_preempt() is not needed in find_symbol(). The
function checks for RCU-sched or the module_mutex to be acquired. The
list_for_each_entry_rcu() below does the same check.

Remove module_assert_mutex_or_preempt() from try_add_tainted_module().
Use RCU protection to invoke find_symbol() and update callers.

Acked-by: Peter Zijlstra (Intel) <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/module/main.c    |   30 ++++++++++++------------------
 kernel/module/version.c |   14 +++++++-------
 2 files changed, 19 insertions(+), 25 deletions(-)

--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -331,7 +331,7 @@ static bool find_exported_symbol_in_sect
 
 /*
  * Find an exported symbol and return it, along with, (optional) crc and
- * (optional) module which owns it.  Needs preempt disabled or module_mutex.
+ * (optional) module which owns it. Needs RCU or module_mutex.
  */
 bool find_symbol(struct find_symbol_arg *fsa)
 {
@@ -345,8 +345,6 @@ bool find_symbol(struct find_symbol_arg
 	struct module *mod;
 	unsigned int i;
 
-	module_assert_mutex_or_preempt();
-
 	for (i = 0; i < ARRAY_SIZE(arr); i++)
 		if (find_exported_symbol_in_section(&arr[i], NULL, fsa))
 			return true;
@@ -812,10 +810,9 @@ void __symbol_put(const char *symbol)
 		.gplok	= true,
 	};
 
-	preempt_disable();
+	guard(rcu)();
 	BUG_ON(!find_symbol(&fsa));
 	module_put(fsa.owner);
-	preempt_enable();
 }
 EXPORT_SYMBOL(__symbol_put);
 
@@ -1369,21 +1366,18 @@ void *__symbol_get(const char *symbol)
 		.warn	= true,
 	};
 
-	preempt_disable();
-	if (!find_symbol(&fsa))
-		goto fail;
-	if (fsa.license != GPL_ONLY) {
-		pr_warn("failing symbol_get of non-GPLONLY symbol %s.\n",
-			symbol);
-		goto fail;
+	scoped_guard(rcu) {
+		if (!find_symbol(&fsa))
+			return NULL;
+		if (fsa.license != GPL_ONLY) {
+			pr_warn("failing symbol_get of non-GPLONLY symbol %s.\n",
+				symbol);
+			return NULL;
+		}
+		if (strong_try_module_get(fsa.owner))
+			return NULL;
 	}
-	if (strong_try_module_get(fsa.owner))
-		goto fail;
-	preempt_enable();
 	return (void *)kernel_symbol_value(fsa.sym);
-fail:
-	preempt_enable();
-	return NULL;
 }
 EXPORT_SYMBOL_GPL(__symbol_get);
 
--- a/kernel/module/version.c
+++ b/kernel/module/version.c
@@ -79,17 +79,17 @@ int check_modstruct_version(const struct
 		.name	= "module_layout",
 		.gplok	= true,
 	};
+	bool have_symbol;
 
 	/*
 	 * Since this should be found in kernel (which can't be removed), no
-	 * locking is necessary -- use preempt_disable() to placate lockdep.
+	 * locking is necessary. Regardless use a RCU read section to keep
+	 * lockdep happy.
 	 */
-	preempt_disable();
-	if (!find_symbol(&fsa)) {
-		preempt_enable();
-		BUG();
-	}
-	preempt_enable();
+	scoped_guard(rcu)
+		have_symbol = find_symbol(&fsa);
+	BUG_ON(!have_symbol);
+
 	return check_version(info, "module_layout", mod, fsa.crc);
 }