BPF_SOCK_OPS_RWND_INIT lets a sockops BPF program pick the initial TCP
receive window, e.g. to advertise a larger window up front in environments
where that is known to be safe. Today it is only effective for the passive
(listener) side; on the active (connect) side the value is computed and
then silently discarded.

On the passive path tcp_openreq_init_rwin() inflates full_space when the
program returns a non-zero window, so tcp_select_initial_window() can offer
it:

	else if (full_space < (u64)rcv_wnd * mss)
		full_space = min_t(u64, (u64)rcv_wnd * mss, INT_MAX);

tcp_select_initial_window() only clamps the requested window *down* to the
available space, so without inflating the space first the BPF reply can
never raise the offered window above tcp_full_space(sk).

tcp_connect_init() calls tcp_rwnd_init_bpf() but never inflates full_space,
so on connect() the requested window is clamped back to tcp_full_space(sk)
(~64KB at the default rcvbuf) and the program's value is ignored.

Inflate full_space in tcp_connect_init() as well; tp->advmss is the mss the
listener path uses (both are tcp_mss_clamp(tp, dst_metric_advmss(dst))).
Read full_space after tcp_rwnd_init_bpf() so a program that also adjusts
SO_RCVBUF is still reflected. Compute the inflated value in u64 and clamp
to INT_MAX to avoid overflow (full_space is int, rcv_wnd is u32), and
apply the same overflow fix to the existing listener-side computation.

Fixes: 13d3b1ebe287 ("bpf: Support for setting initial receive window")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Tejas Birajdar <tejasbirajdar@meta.com>
---
v2:
  - Do the inflated full_space arithmetic in u64 and clamp to INT_MAX to
    avoid overflow, and apply the same fix to the existing listener path
    in tcp_openreq_init_rwin().
  - Read full_space after tcp_rwnd_init_bpf() so a program that also raises
    SO_RCVBUF via bpf_setsockopt() is reflected in the offered window.
  - Re-ran the sockops BPF/RWND functional test and the full in-tree
    packetdrill regression suite on the revised code (details below).
v1: https://lore.kernel.org/netdev/20260722170033.2763794-1-tejasbirajdar@meta.com/

Functional test: a cgroup sockops BPF program returning
skops->reply = N for BPF_SOCK_OPS_RWND_INIT was attached to the
connecting socket and driven with packetdrill on the active-open
(connect) path (advmss 1460, negotiated wscale 8). The offered window on
the first post-handshake ACK now tracks the requested value:

  req_segs   offered window (bytes)   = req_segs * advmss
  256        373760
  1024       1495040  (~1.43 MB)
  4096       5980160  (~5.7 MB)

Without an attached program the window stays at the default (~262 KB);
before this patch the requested value was discarded on connect() and the
default was advertised regardless.

Regression: the in-tree tools/testing/selftests/net/packetdrill suite
(471 test cases across ipv4/ipv6/ipv4-mapped-ipv6) was run under
virtme-ng on this commit and on its parent; both produce an identical
pass/fail set, so this patch introduces no newly failing tests.

 net/ipv4/tcp_minisocks.c | 4 ++--
 net/ipv4/tcp_output.c    | 6 +++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index ddc4b17a826b..f8c1123aba43 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -453,8 +453,8 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
 	if (rcv_wnd == 0)
 		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
-	else if (full_space < rcv_wnd * mss)
-		full_space = rcv_wnd * mss;
+	else if (full_space < (u64)rcv_wnd * mss)
+		full_space = min_t(u64, (u64)rcv_wnd * mss, INT_MAX);
 
 	/* tcp_full_space because it is guaranteed to be the first packet */
 	tcp_select_initial_window(sk_listener, full_space,
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d7c1444b5e30..ffa28c98c211 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4103,6 +4103,7 @@ static void tcp_connect_init(struct sock *sk)
 	const struct dst_entry *dst = __sk_dst_get(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	__u8 rcv_wscale;
+	int full_space;
 	u16 user_mss;
 	u32 rcv_wnd;
 
@@ -4137,10 +4138,13 @@ static void tcp_connect_init(struct sock *sk)
 		WRITE_ONCE(tp->window_clamp, tcp_full_space(sk));
 
 	rcv_wnd = tcp_rwnd_init_bpf(sk);
+	full_space = tcp_full_space(sk);
 	if (rcv_wnd == 0)
 		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
+	else if (full_space < (u64)rcv_wnd * tp->advmss)
+		full_space = min_t(u64, (u64)rcv_wnd * tp->advmss, INT_MAX);
 
-	tcp_select_initial_window(sk, tcp_full_space(sk),
+	tcp_select_initial_window(sk, full_space,
 				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
 				  &tp->rcv_wnd,
 				  &tp->window_clamp,
-- 
2.53.0-Meta