bpf: Allow bpf_xdp_shrink_data to shrink a frag from head and tail

Move skb_frag_t adjustment into bpf_xdp_shrink_data() and extend its
functionality to be able to shrink an xdp fragment from both head and
tail. In a later patch, bpf_xdp_pull_data() will reuse it to shrink an
xdp fragment from head.

Additionally, in bpf_xdp_frags_shrink_tail(), breaking the loop when
bpf_xdp_shrink_data() returns false (i.e., not releasing the current
fragment) is not necessary as the loop condition, offset > 0, has the
same effect. Remove the else branch to simplify the code.

Signed-off-by: Amery Hung <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Reviewed-by: Maciej Fijalkowski <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: ameryhung

include/net/xdp_sock_drv.h

@@ -160,13 +160,23 @@ static inline struct xdp_buff *xsk_buff_get_frag(const struct xdp_buff *first)
 	return ret;
 }
 
-static inline void xsk_buff_del_tail(struct xdp_buff *tail)
+static inline void xsk_buff_del_frag(struct xdp_buff *xdp)
 {
-	struct xdp_buff_xsk *xskb = container_of(tail, struct xdp_buff_xsk, xdp);
+	struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
 
 	list_del(&xskb->list_node);
 }
 
+static inline struct xdp_buff *xsk_buff_get_head(struct xdp_buff *first)
+{
+	struct xdp_buff_xsk *xskb = container_of(first, struct xdp_buff_xsk, xdp);
+	struct xdp_buff_xsk *frag;
+
+	frag = list_first_entry(&xskb->pool->xskb_list, struct xdp_buff_xsk,
+				list_node);
+	return &frag->xdp;
+}
+
 static inline struct xdp_buff *xsk_buff_get_tail(struct xdp_buff *first)
 {
 	struct xdp_buff_xsk *xskb = container_of(first, struct xdp_buff_xsk, xdp);
@@ -389,8 +399,13 @@ static inline struct xdp_buff *xsk_buff_get_frag(const struct xdp_buff *first)
 	return NULL;
 }
 
-static inline void xsk_buff_del_tail(struct xdp_buff *tail)
+static inline void xsk_buff_del_frag(struct xdp_buff *xdp)
+{
+}
+
+static inline struct xdp_buff *xsk_buff_get_head(struct xdp_buff *first)
 {
+	return NULL;
 }
 
 static inline struct xdp_buff *xsk_buff_get_tail(struct xdp_buff *first)

net/core/filter.c

@@ -4153,34 +4153,45 @@ static int bpf_xdp_frags_increase_tail(struct xdp_buff *xdp, int offset)
 	return 0;
 }
 
-static void bpf_xdp_shrink_data_zc(struct xdp_buff *xdp, int shrink,
-				   enum xdp_mem_type mem_type, bool release)
+static struct xdp_buff *bpf_xdp_shrink_data_zc(struct xdp_buff *xdp, int shrink,
+					       bool tail, bool release)
 {
-	struct xdp_buff *zc_frag = xsk_buff_get_tail(xdp);
+	struct xdp_buff *zc_frag = tail ? xsk_buff_get_tail(xdp) :
+					  xsk_buff_get_head(xdp);
 
 	if (release) {
-		xsk_buff_del_tail(zc_frag);
-		__xdp_return(0, mem_type, false, zc_frag);
+		xsk_buff_del_frag(zc_frag);
 	} else {
-		zc_frag->data_end -= shrink;
+		if (tail)
+			zc_frag->data_end -= shrink;
+		else
+			zc_frag->data += shrink;
 	}
+
+	return zc_frag;
 }
 
 static bool bpf_xdp_shrink_data(struct xdp_buff *xdp, skb_frag_t *frag,
-				int shrink)
+				int shrink, bool tail)
 {
 	enum xdp_mem_type mem_type = xdp->rxq->mem.type;
 	bool release = skb_frag_size(frag) == shrink;
+	netmem_ref netmem = skb_frag_netmem(frag);
+	struct xdp_buff *zc_frag = NULL;
 
 	if (mem_type == MEM_TYPE_XSK_BUFF_POOL) {
-		bpf_xdp_shrink_data_zc(xdp, shrink, mem_type, release);
-		goto out;
+		netmem = 0;
+		zc_frag = bpf_xdp_shrink_data_zc(xdp, shrink, tail, release);
 	}
 
-	if (release)
-		__xdp_return(skb_frag_netmem(frag), mem_type, false, NULL);
+	if (release) {
+		__xdp_return(netmem, mem_type, false, zc_frag);
+	} else {
+		if (!tail)
+			skb_frag_off_add(frag, shrink);
+		skb_frag_size_sub(frag, shrink);
+	}
 
-out:
 	return release;
 }
 
@@ -4198,12 +4209,8 @@ static int bpf_xdp_frags_shrink_tail(struct xdp_buff *xdp, int offset)
 
 		len_free += shrink;
 		offset -= shrink;
-		if (bpf_xdp_shrink_data(xdp, frag, shrink)) {
+		if (bpf_xdp_shrink_data(xdp, frag, shrink, true))
 			n_frags_free++;
-		} else {
-			skb_frag_size_sub(frag, shrink);
-			break;
-		}
 	}
 	sinfo->nr_frags -= n_frags_free;
 	sinfo->xdp_frags_size -= len_free;