tcp: change bpf_skops_hdr_opt_len() signature

Some compilers do not inline bpf_skops_hdr_opt_len() from
tcp_established_options(), forcing an expensive stack canary
when CONFIG_STACKPROTECTOR_STRONG=y.

Change bpf_skops_hdr_opt_len() to return @remaining by value
to remove this stack canary from TCP fast path.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 1/1 up/down: 10/-59 (-49)
Function                                     old     new   delta
bpf_skops_hdr_opt_len                        297     307     +10
tcp_established_options                      574     515     -59
Total: Before=31456795, After=31456746, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260601093819.469626-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index ef0c10cd31c71ff585a937fde37f2b08b1214b5a..1b0a066bb6a31cc0d1847459bce081f183134361 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -466,22 +466,22 @@ static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
 }
 
 /* req, syn_skb and synack_type are used when writing synack */
-static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
-                                 struct request_sock *req,
-                                 struct sk_buff *syn_skb,
-                                 enum tcp_synack_type synack_type,
-                                 struct tcp_out_options *opts,
-                                 unsigned int *remaining)
+static u32 bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
+                                struct request_sock *req,
+                                struct sk_buff *syn_skb,
+                                enum tcp_synack_type synack_type,
+                                struct tcp_out_options *opts,
+                                u32 remaining)
 {
        struct bpf_sock_ops_kern sock_ops;
        int err;
 
        if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                           BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
-           !*remaining)
-               return;
+           !remaining)
+               return remaining;
 
-       /* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */
+       /* remaining has already been aligned to 4 bytes, so remaining >= 4 */
 
        /* init sock_ops */
        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
@@ -513,21 +513,21 @@ static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
        }
 
        sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
-       sock_ops.remaining_opt_len = *remaining;
+       sock_ops.remaining_opt_len = remaining;
        /* tcp_current_mss() does not pass a skb */
        if (skb)
                bpf_skops_init_skb(&sock_ops, skb, 0);
 
        err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
 
-       if (err || sock_ops.remaining_opt_len == *remaining)
-               return;
+       if (err || sock_ops.remaining_opt_len == remaining)
+               return remaining;
 
-       opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
+       opts->bpf_opt_len = remaining - sock_ops.remaining_opt_len;
        /* round up to 4 bytes */
        opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;
 
-       *remaining -= opts->bpf_opt_len;
+       return remaining - opts->bpf_opt_len;
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
@@ -575,13 +575,14 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
                       max_opt_len - nr_written);
 }
 #else
-static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
-                                 struct request_sock *req,
-                                 struct sk_buff *syn_skb,
-                                 enum tcp_synack_type synack_type,
-                                 struct tcp_out_options *opts,
-                                 unsigned int *remaining)
+static u32 bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
+                                struct request_sock *req,
+                                struct sk_buff *syn_skb,
+                                enum tcp_synack_type synack_type,
+                                struct tcp_out_options *opts,
+                                u32 remaining)
 {
+       return remaining;
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
@@ -1050,7 +1051,8 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                remaining -= tcp_options_fit_accecn(opts, 0, remaining);
        }
 
-       bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
+       remaining = bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts,
+                                         remaining);
 
        return MAX_TCP_OPTION_SPACE - remaining;
 }
@@ -1137,8 +1139,8 @@ static unsigned int tcp_synack_options(const struct sock *sk,
                remaining -= tcp_options_fit_accecn(opts, 0, remaining);
        }
 
-       bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
-                             synack_type, opts, &remaining);
+       remaining = bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
+                                         synack_type, opts, remaining);
 
        return MAX_TCP_OPTION_SPACE - remaining;
 }
@@ -1227,7 +1229,8 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
                                            BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
                unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
 
-               bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
+               remaining = bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts,
+                                                 remaining);
 
                size = MAX_TCP_OPTION_SPACE - remaining;
        }