[PATCH] ftrace: Use flexible array for hash buckets

[Rosen Penev]

Store ftrace hash buckets in the ftrace_hash allocation instead of
allocating the bucket array separately.

This keeps the bucket storage tied to the hash lifetime and simplifies
the allocation and cleanup paths.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
---
 kernel/trace/ftrace.c | 17 ++---------------
 kernel/trace/trace.h  |  2 +-
 2 files changed, 3 insertions(+), 16 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index b2611de3f594..25a9dca290dd 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1082,10 +1082,7 @@ struct ftrace_func_probe {
  * it all the time. These are in a read only section such that if
  * anyone does try to modify it, it will cause an exception.
  */
-static const struct hlist_head empty_buckets[1];
-static const struct ftrace_hash empty_hash = {
-	.buckets = (struct hlist_head *)empty_buckets,
-};
+static const struct ftrace_hash empty_hash = {};
 #define EMPTY_HASH	((struct ftrace_hash *)&empty_hash)
 
 struct ftrace_ops global_ops = {
@@ -1295,7 +1292,6 @@ void free_ftrace_hash(struct ftrace_hash *hash)
 	if (!hash || hash == EMPTY_HASH)
 		return;
 	ftrace_hash_clear(hash);
-	kfree(hash->buckets);
 	kfree(hash);
 }
 
@@ -1333,20 +1329,11 @@ EXPORT_SYMBOL_GPL(ftrace_free_filter);
 struct ftrace_hash *alloc_ftrace_hash(int size_bits)
 {
 	struct ftrace_hash *hash;
-	int size;
 
-	hash = kzalloc_obj(*hash);
+	hash = kzalloc_flex(*hash, buckets, BIT(size_bits));
 	if (!hash)
 		return NULL;
 
-	size = 1 << size_bits;
-	hash->buckets = kzalloc_objs(*hash->buckets, size);
-
-	if (!hash->buckets) {
-		kfree(hash);
-		return NULL;
-	}
-
 	hash->size_bits = size_bits;
 
 	return hash;
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 80fe152af1dd..5a3f81f17317 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1000,10 +1000,10 @@ enum {
 
 struct ftrace_hash {
 	unsigned long		size_bits;
-	struct hlist_head	*buckets;
 	unsigned long		count;
 	unsigned long		flags;
 	struct rcu_head		rcu;
+	struct hlist_head	buckets[];
 };
 
 struct ftrace_func_entry *
-- 
2.54.0