This is a follow-up to Sashiko's report[1].

Several BPF socket helpers acquire a socket reference only when
sk_is_refcounted() == true, and release it, independently, by
re-evaluating sk_is_refcounted() again at the time the release runs. TCP
connect(AF_UNSPEC)+listen() sets SOCK_RCU_FREE on an established socket.
If that happens while a reference is outstanding, the release side sees
sk_is_refcounted() == false and skips the put; the socket is leaked.

unreferenced object 0xffff88811617ce00 (size 3200):
  comm "softirq", pid 0, jiffies 4294848512
  hex dump (first 32 bytes):
    7f 00 00 01 7f 00 00 01 4d 43 02 f6 00 00 00 00  ........MC......
    02 00 07 41 00 00 00 00 00 00 00 00 00 00 00 00  ...A............
  backtrace (crc fb5bd4c8):
    kmem_cache_alloc_noprof+0x53e/0x640
    sk_prot_alloc+0x69/0x240
    sk_clone+0x79/0x1230
    inet_csk_clone_lock+0x30/0x760
    tcp_create_openreq_child+0x34/0x2750
    tcp_v4_syn_recv_sock+0x12e/0x1080
    tcp_check_req+0x447/0x2310
    tcp_v4_rcv+0x1026/0x3c90
    ip_protocol_deliver_rcu+0x93/0x340
    ip_local_deliver_finish+0x356/0x5c0
    ip_local_deliver+0x184/0x4a0
    ip_rcv+0x4f4/0x5b0
    __netif_receive_skb_one_core+0x153/0x1b0
    process_backlog+0x28d/0x1190
    __napi_poll+0xab/0x520
    net_rx_action+0x3f0/0xca0

The approach taken is to make acquire and release unconditional and
symmetric.

Note: TC bpf_sk_assign() has the same issue; it takes a reference only when
sk_is_refcounted() is true at assign time, but sock_pfree() (the skb
destructor it installs) re-checks sk_is_refcounted() independently at
release time. The same connect(AF_UNSPEC)+listen() transition leaks the
socket here too. I'd welcome suggestions on the right way to handle this.

[1]: https://lore.kernel.org/bpf/20260701235552.2B0AA1F00A3F@smtp.kernel.org/

Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
Michal Luczaj (3):
      bpf, sockmap: Use sock_hold() instead of refcount_inc_not_zero() in lookup
      bpf: Extract shared reqsk-to-listener upgrade
      bpf: Unconditionally take socket references in lookup helpers

 net/core/filter.c   | 80 ++++++++++++++++++++++++++---------------------------
 net/core/sock_map.c | 12 +++-----
 2 files changed, 44 insertions(+), 48 deletions(-)
---
base-commit: 94515f3a7d4256a5062176b7d6ed0471938cd51a
change-id: 20260628-sockmap-lookup-tcp-leak-bdaba3e083c5

Best regards,
--  
Michal Luczaj <mhal@rbox.co>