net: change sk_filter_trim_cap() to return a drop_reason by value

Current return value can be replaced with the drop_reason,
reducing kernel bloat:

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 1/11 up/down: 32/-603 (-571)
Function                                     old     new   delta
tcp_v6_rcv                                  3135    3167     +32
unix_dgram_sendmsg                          1731    1726      -5
netlink_unicast                              957     945     -12
netlink_dump                                1372    1359     -13
sk_filter_trim_cap                           882     858     -24
tcp_v4_rcv                                  3143    3111     -32
__pfx_tcp_filter                              32       -     -32
netlink_broadcast_filtered                  1633    1595     -38
sock_queue_rcv_skb_reason                    126      76     -50
tun_net_xmit                                1127    1074     -53
__sk_receive_skb                             690     632     -58
udpv6_queue_rcv_one_skb                      935     869     -66
udp_queue_rcv_one_skb                        919     853     -66
tcp_filter                                   154       -    -154
Total: Before=29722783, After=29722212, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409145625.2306224-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

5 files changed, 22 insertions, 23 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 5569d83b8be0..bf9c37b27646 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -121,20 +121,20 @@ EXPORT_SYMBOL_GPL(copy_bpf_fprog_from_user);
  *	@sk: sock associated with &sk_buff
  *	@skb: buffer to filter
  *	@cap: limit on how short the eBPF program may trim the packet
- *	@reason: record drop reason
  *
  * Run the eBPF program and then cut skb->data to correct size returned by
  * the program. If pkt_len is 0 we toss packet. If skb->len is smaller
  * than pkt_len we keep whole skb->data. This is the socket level
  * wrapper to bpf_prog_run. It returns 0 if the packet should
- * be accepted or -EPERM if the packet should be tossed.
+ * be accepted or a drop_reason if the packet should be tossed.
  *
  */
-int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb,
-		       unsigned int cap, enum skb_drop_reason *reason)
+enum skb_drop_reason
+sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap)
 {
-	int err;
+	enum skb_drop_reason drop_reason;
 	struct sk_filter *filter;
+	int err;
 
 	/*
 	 * If the skb was allocated from pfmemalloc reserves, only
@@ -143,21 +143,17 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb,
 	 */
 	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC)) {
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_PFMEMALLOCDROP);
-		*reason = SKB_DROP_REASON_PFMEMALLOC;
-		return -ENOMEM;
+		return SKB_DROP_REASON_PFMEMALLOC;
 	}
 	err = BPF_CGROUP_RUN_PROG_INET_INGRESS(sk, skb);
-	if (err) {
-		*reason = SKB_DROP_REASON_SOCKET_FILTER;
-		return err;
-	}
+	if (err)
+		return SKB_DROP_REASON_SOCKET_FILTER;
 
 	err = security_sock_rcv_skb(sk, skb);
-	if (err) {
-		*reason = SKB_DROP_REASON_SECURITY_HOOK;
-		return err;
-	}
+	if (err)
+		return SKB_DROP_REASON_SECURITY_HOOK;
 
+	drop_reason = 0;
 	rcu_read_lock();
 	filter = rcu_dereference(sk->sk_filter);
 	if (filter) {
@@ -168,11 +164,12 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb,
 		pkt_len = bpf_prog_run_save_cb(filter->prog, skb);
 		skb->sk = save_sk;
 		err = pkt_len ? pskb_trim(skb, max(cap, pkt_len)) : -EPERM;
+		if (err)
+			drop_reason = SKB_DROP_REASON_SOCKET_FILTER;
 	}
 	rcu_read_unlock();
 
-	*reason = err ? SKB_DROP_REASON_SOCKET_FILTER : 0;
-	return err;
+	return drop_reason;
 }
 EXPORT_SYMBOL(sk_filter_trim_cap);
 
diff --git a/net/core/sock.c b/net/core/sock.c
index 1ffcb15d0fc5..367fd7bad4ac 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -544,11 +544,12 @@ EXPORT_SYMBOL(sock_queue_rcv_skb_reason);
 int __sk_receive_skb(struct sock *sk, struct sk_buff *skb,
 		     const int nested, unsigned int trim_cap, bool refcounted)
 {
-	enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
+	enum skb_drop_reason reason;
 	int rc = NET_RX_SUCCESS;
 	int err;
 
-	if (sk_filter_trim_cap(sk, skb, trim_cap, &reason))
+	reason = sk_filter_trim_cap(sk, skb, trim_cap);
+	if (reason)
 		goto discard_and_relse;
 
 	skb->dev = NULL;
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index ab415de32443..2fddc7b6b717 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2392,7 +2392,8 @@ static int udp_queue_rcv_one_skb(struct sock *sk, struct sk_buff *skb)
 	    udp_lib_checksum_complete(skb))
 			goto csum_error;
 
-	if (sk_filter_trim_cap(sk, skb, sizeof(struct udphdr), &drop_reason))
+	drop_reason = sk_filter_trim_cap(sk, skb, sizeof(struct udphdr));
+	if (drop_reason)
 		goto drop;
 
 	udp_csum_pull_header(skb);
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index d7cf4c9508b2..3fac9cb47ae0 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -853,7 +853,8 @@ static int udpv6_queue_rcv_one_skb(struct sock *sk, struct sk_buff *skb)
 	    udp_lib_checksum_complete(skb))
 		goto csum_error;
 
-	if (sk_filter_trim_cap(sk, skb, sizeof(struct udphdr), &drop_reason))
+	drop_reason = sk_filter_trim_cap(sk, skb, sizeof(struct udphdr));
+	if (drop_reason)
 		goto drop;
 
 	udp_csum_pull_header(skb);
diff --git a/net/rose/rose_in.c b/net/rose/rose_in.c
index 0276b393f0e5..3aff3c2d45a9 100644
--- a/net/rose/rose_in.c
+++ b/net/rose/rose_in.c
@@ -101,7 +101,6 @@ static int rose_state2_machine(struct sock *sk, struct sk_buff *skb, int framety
  */
 static int rose_state3_machine(struct sock *sk, struct sk_buff *skb, int frametype, int ns, int nr, int q, int d, int m)
 {
-	enum skb_drop_reason dr; /* ignored */
 	struct rose_sock *rose = rose_sk(sk);
 	int queued = 0;
 
@@ -163,7 +162,7 @@ static int rose_state3_machine(struct sock *sk, struct sk_buff *skb, int framety
 		rose_frames_acked(sk, nr);
 		if (ns == rose->vr) {
 			rose_start_idletimer(sk);
-			if (!sk_filter_trim_cap(sk, skb, ROSE_MIN_LEN, &dr) &&
+			if (!sk_filter_trim_cap(sk, skb, ROSE_MIN_LEN) &&
 			    __sock_queue_rcv_skb(sk, skb) == 0) {
 				rose->vr = (rose->vr + 1) % ROSE_MODULUS;
 				queued = 1;