[PATCH AUTOSEL 6.6 21/62] tcp: fix initial tp->rcvq_space.space value for passive TS enabled flows

[Sasha Levin <sashal@kernel.org>]

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit cd171461b90a2d2cf230943df60d580174633718 ]

tcp_rcv_state_process() must tweak tp->advmss for TS enabled flows
before the call to tcp_init_transfer() / tcp_init_buffer_space().

Otherwise tp->rcvq_space.space is off by 120 bytes
(TCP_INIT_CWND * TCPOLEN_TSTAMP_ALIGNED).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Wei Wang <weiwan@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-7-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES** This commit should be backported to stable kernel trees. ##
Detailed Analysis ### Nature of the Fix This commit addresses a **subtle
but significant bug** in the TCP receive buffer space initialization for
passive (server-side) connections when TCP timestamps are enabled. The
fix is a simple 2-line reordering that moves the `advmss` adjustment for
timestamp options **before** the call to `tcp_init_transfer()`. ### The
Problem 1. **Root Cause**: In `tcp_rcv_state_process()` for
`TCP_SYN_RECV` state, the `tp->advmss` reduction for timestamp options
(`TCPOLEN_TSTAMP_ALIGNED = 12 bytes`) was happening **after**
`tcp_init_transfer()` was called. 2. **Impact**: Since
`tcp_init_transfer()` calls `tcp_init_buffer_space()`, which initializes
`tp->rcvq_space.space` using the formula: ```c tp->rcvq_space.space =
min3(tp->rcv_ssthresh, tp->rcv_wnd, (u32)TCP_INIT_CWND linux
tp->advmss); ``` The calculation was using an **unadjusted `advmss`
value**, leading to a 120-byte overestimate: - `TCP_INIT_CWND (10) ×
TCPOLEN_TSTAMP_ALIGNED (12) = 120 bytes` 3. **Consequence**: The
`rcvq_space.space` field is critical for TCP receive buffer auto-tuning
in `tcp_rcv_space_adjust()`, and this miscalculation could lead to
suboptimal buffer management and performance issues. ### Why This Should
Be Backported #### ✅ **Bug Fix Criteria Met**: 1. **Clear Bug**: This
fixes a real initialization ordering bug that affects TCP performance 2.
**User Impact**: Affects all passive TCP connections with timestamp
options enabled (very common) 3. **Minimal Risk**: The fix is a simple
2-line reordering with no functional changes 4. **Contained Scope**:
Only affects the initialization path in `tcp_rcv_state_process()` #### ✅
**Follows Stable Tree Rules**: 1. **Important**: TCP receive buffer
tuning affects network performance for most connections 2. **Small &
Contained**: The change moves just 2 lines of existing code 3. **No
Regression Risk**: The fix corrects an obvious ordering error without
introducing new logic 4. **No Architectural Changes**: No new features
or major changes to TCP stack #### ✅ **Comparison with Similar
Backported Commits**: The provided reference commits show a pattern of
TCP receive buffer and `rcvq_space` related fixes being consistently
backported: - Integer overflow fixes in `tcp_rcv_space_adjust()` ✅
**Backported** - Data race fixes for `sysctl_tcp_moderate_rcvbuf` ✅
**Backported** - MPTCP `rcv_space_init` fixes ✅ **Backported** -
Timestamp handling in `tcp_rcv_space_adjust()` ✅ **Backported** This fix
follows the exact same pattern: a small, surgical fix to TCP receive
buffer initialization logic. ### Technical Verification Looking at the
code change in `/home/sasha/linux/net/ipv4/tcp_input.c:6872-6873`, the
fix simply moves: ```c if (tp->rx_opt.tstamp_ok) tp->advmss -=
TCPOLEN_TSTAMP_ALIGNED; ``` from **after** `tcp_init_transfer()` to
**before** it. This ensures that when `tcp_init_buffer_space()` is
called within `tcp_init_transfer()`, it uses the correct timestamp-
adjusted `advmss` value. The change is **safe, targeted, and addresses a
clear functional bug** that affects TCP performance for a large class of
connections. It meets all criteria for stable tree backporting.

 net/ipv4/tcp_input.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 1f06ffa5f8a1b..7617d4920ed59 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6691,6 +6691,9 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		if (!tp->srtt_us)
 			tcp_synack_rtt_meas(sk, req);
 
+		if (tp->rx_opt.tstamp_ok)
+			tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
+
 		if (req) {
 			tcp_rcv_synrecv_state_fastopen(sk);
 		} else {
@@ -6715,9 +6718,6 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
 		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
 
-		if (tp->rx_opt.tstamp_ok)
-			tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
-
 		if (!inet_csk(sk)->icsk_ca_ops->cong_control)
 			tcp_update_pacing_rate(sk);
 
-- 
2.39.5