[RFC PATCH v2] Bluetooth: l2cap: prefer compatible local LE intervals

[Kyle Pepper <cochcoder@proton.me>]

Use a valid intersection of a peer interval range and local preferences
when they overlap. Otherwise retain the peer request instead of treating
an initial preference as a hard supported bound.

Validate the original wire request first. Only narrow its interval range;
leave latency, timeout and disjoint-range acceptance unchanged.

Assisted-by: OpenCode:gpt-6.1-sol
Signed-off-by: Kyle Pepper <cochcoder@proton.me>
---
Changes from v1:
- Treat local ranges as preferences, not supported bounds.
- Prefer a valid intersection; otherwise keep the original valid peer request.
- Never repair an invalid original request by clipping.
- Preserve latency, supervision timeout and disjoint-range acceptance.

Four native controls check both the accepted L2CAP response and resulting HCI
update range: overlap, fast HID fallback, slow idle fallback and exact match.
Baseline passes 3/4 (no narrower overlap); the strict-bound negative control
passes 2/4 (rejects HID/idle); this revision passes 4/4. Its full reference-guest
L2CAP suite passes 115/115 without skips or captured KASAN/lockdep alerts.
The source-extracted sanitizer fixture passes 397,723 parameter cases plus two
framing/role cases. These are finite tests, not proof for every controller.

The independent BlueZ interval-test companion is not yet sent. Its overlap test
intentionally fails without this Linux change. ASHA completed-interval startup
and stream gating are separate profile work, not a new kernel ABI or hard bound.

A combined Arch 7.2.9 trial with this change and revised BlueZ was built in a
clean chroot. I report that stack working on hardware; the running test kernel
and BlueZ package versions were verified. This is combined-stack evidence,
not isolated attribution or complete N7/paired-device/negative-path coverage.
It is not hardware testing of the bluetooth-next kernel itself. Remote CI for
this revision has not run. Target master still matches the tested source base.

 net/bluetooth/l2cap_core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 4dd56e007..afde04a19 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5031,6 +5031,19 @@ static inline int l2cap_conn_param_update_req(struct l2cap_conn *conn,
 	memset(&rsp, 0, sizeof(rsp));
 
 	err = hci_check_conn_params(min, max, latency, to_multiplier);
+	if (!err) {
+		u16 pref_min = max_t(u16, min, hcon->le_conn_min_interval);
+		u16 pref_max = min_t(u16, max, hcon->le_conn_max_interval);
+
+		/* Local preferences must not reject a valid peer request. */
+		if (pref_min <= pref_max &&
+		    !hci_check_conn_params(pref_min, pref_max, latency,
+					  to_multiplier)) {
+			min = pref_min;
+			max = pref_max;
+		}
+	}
+
 	if (err)
 		rsp.result = cpu_to_le16(L2CAP_CONN_PARAM_REJECTED);
 	else

base-commit: 397464709a204da9b8dc3b01d6ea42539d951cd2