[PATCH net-next] net: skb: clean up dead code after skb_kfree_head() simplification

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Since commit 0f42e3f4fe2a ("net: skb: fix cross-cache free of
KFENCE-allocated skb head"), skb_kfree_head() always calls kfree()
and no longer uses end_offset to distinguish between skb_small_head_cache
and generic kmalloc caches.

Clean up the leftovers:

- Remove the unused end_offset parameter from skb_kfree_head() and
  update all callers.
- Remove the SKB_SMALL_HEAD_HEADROOM guard in __skb_unclone_keeptruesize()
  which was protecting the old skb_kfree_head() logic.
- Update the SKB_SMALL_HEAD_CACHE_SIZE comment to reflect that the
  non-power-of-2 sizing is no longer used for free-path disambiguation.

No functional change.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/core/skbuff.c | 33 +++++++++------------------------
 1 file changed, 9 insertions(+), 24 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e50f2d4867c1..c5441154795c 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -106,10 +106,9 @@ static struct kmem_cache *skbuff_ext_cache __ro_after_init;
 #define SKB_SMALL_HEAD_SIZE SKB_HEAD_ALIGN(max(MAX_TCP_HEADER, \
 					       GRO_MAX_HEAD_PAD))
 
-/* We want SKB_SMALL_HEAD_CACHE_SIZE to not be a power of two.
- * This should ensure that SKB_SMALL_HEAD_HEADROOM is a unique
- * size, and we can differentiate heads from skb_small_head_cache
- * vs system slabs by looking at their size (skb_end_offset()).
+/* SKB_SMALL_HEAD_CACHE_SIZE is the size used for the skbuff_small_head
+ * kmem_cache. The non-power-of-2 padding is kept for historical reasons and
+ * to avoid potential collisions with generic kmalloc bucket sizes.
  */
 #define SKB_SMALL_HEAD_CACHE_SIZE					\
 	(is_power_of_2(SKB_SMALL_HEAD_SIZE) ?			\
@@ -1071,7 +1070,7 @@ static int skb_pp_frag_ref(struct sk_buff *skb)
 	return 0;
 }
 
-static void skb_kfree_head(void *head, unsigned int end_offset)
+static void skb_kfree_head(void *head)
 {
 	kfree(head);
 }
@@ -1085,7 +1084,7 @@ static void skb_free_head(struct sk_buff *skb)
 			return;
 		skb_free_frag(head);
 	} else {
-		skb_kfree_head(head, skb_end_offset(skb));
+		skb_kfree_head(head);
 	}
 }
 
@@ -2361,7 +2360,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	return 0;
 
 nofrags:
-	skb_kfree_head(data, size);
+	skb_kfree_head(data);
 nodata:
 	return -ENOMEM;
 }
@@ -2407,20 +2406,6 @@ int __skb_unclone_keeptruesize(struct sk_buff *skb, gfp_t pri)
 	if (likely(skb_end_offset(skb) == saved_end_offset))
 		return 0;
 
-	/* We can not change skb->end if the original or new value
-	 * is SKB_SMALL_HEAD_HEADROOM, as it might break skb_kfree_head().
-	 */
-	if (saved_end_offset == SKB_SMALL_HEAD_HEADROOM ||
-	    skb_end_offset(skb) == SKB_SMALL_HEAD_HEADROOM) {
-		/* We think this path should not be taken.
-		 * Add a temporary trace to warn us just in case.
-		 */
-		pr_err_once("__skb_unclone_keeptruesize() skb_end_offset() %u -> %u\n",
-			    saved_end_offset, skb_end_offset(skb));
-		WARN_ON_ONCE(1);
-		return 0;
-	}
-
 	shinfo = skb_shinfo(skb);
 
 	/* We are about to change back skb->end,
@@ -6815,7 +6800,7 @@ static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
 	if (skb_cloned(skb)) {
 		/* drop the old head gracefully */
 		if (skb_orphan_frags(skb, gfp_mask)) {
-			skb_kfree_head(data, size);
+			skb_kfree_head(data);
 			return -ENOMEM;
 		}
 		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
@@ -6922,7 +6907,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
 	memcpy((struct skb_shared_info *)(data + size),
 	       skb_shinfo(skb), offsetof(struct skb_shared_info, frags[0]));
 	if (skb_orphan_frags(skb, gfp_mask)) {
-		skb_kfree_head(data, size);
+		skb_kfree_head(data);
 		return -ENOMEM;
 	}
 	shinfo = (struct skb_shared_info *)(data + size);
@@ -6958,7 +6943,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
 		/* skb_frag_unref() is not needed here as shinfo->nr_frags = 0. */
 		if (skb_has_frag_list(skb))
 			kfree_skb_list(skb_shinfo(skb)->frag_list);
-		skb_kfree_head(data, size);
+		skb_kfree_head(data);
 		return -ENOMEM;
 	}
 	skb_release_data(skb, SKB_CONSUMED);
-- 
2.43.0