[PATCH] Bluetooth: L2CAP: Increase default socket buffer to hold large SDUs

[Amir Abudubai]

The L2CAP send path requires a full SDU to be assembled in the socket
buffer before transmission can begin. Therefore, the socket buffer must
be large enough to hold a worst-case SDU.

Derive the required buffer space from the worst-case parameters:
Maximum SDU plus length field: 65535 + 2 = 65537 bytes
At MPS 24:                    2730 full PDUs + 17-byte tail
With 4-byte L2CAP headers:    28-byte full PDUs, 21-byte tail
At HCI MTU 27:                2730 * 2 + 1 = 5461 fragments

MPS 24 and MTU 27 are chosen because they produce the most fragmentation.
Apply a 4x safety factor because SKB_TRUESIZE can underestimate actual
usage because of class-size rounding and minimum allocations.

Signed-off-by: Amir Abudubai <amirabudubai@xxxxxxxxx>
---
The L2CAP send path deadlocks waiting for buffer space if it cannot fit
a full SDU in the send buffer. Using the system default typically causes
that to happen before the max allowed SDU. I didn't address the deadlock.
I just scaled the buffer so it is unlikely to happen unless the socket
gets resized after initialization.

 include/net/bluetooth/l2cap.h | 4 ++++
 net/bluetooth/l2cap_sock.c    | 1 +
 2 files changed, 5 insertions(+)

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 54eddcdd341f..0cf1ca4ec3cd 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -26,6 +26,7 @@
 
 #include <linux/unaligned.h>
 #include <linux/atomic.h>
+#include <linux/skbuff.h>
 
 /* L2CAP defaults */
 #define L2CAP_DEFAULT_MTU		672
@@ -41,6 +42,9 @@
 #define L2CAP_DEFAULT_MAX_PDU_SIZE	1492    /* Sized for AMP packet */
 #define L2CAP_DEFAULT_ACK_TO		200
 #define L2CAP_DEFAULT_MAX_SDU_SIZE	0xFFFF
+#define L2CAP_MAX_SDU_FRAGS		5461    /* Sized for worst-case CoC fragmentation */
+#define L2CAP_DEFAULT_SNDBUF \
+	(4 * L2CAP_MAX_SDU_FRAGS * SKB_TRUESIZE(SKB_DATA_ALIGN(HCI_MIN_LE_MTU + BT_SKB_RESERVE)))
 #define L2CAP_DEFAULT_SDU_ITIME		0xFFFFFFFF
 #define L2CAP_DEFAULT_ACC_LAT		0xFFFFFFFF
 #define L2CAP_BREDR_MAX_PAYLOAD		1019    /* 3-DH5 packet */
diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 7a2551ef7f50..5f4fde8b4a6d 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -2046,6 +2046,7 @@ static struct sock *l2cap_sock_alloc(struct net *net, struct socket *sock,
 
 	sk->sk_destruct = l2cap_sock_destruct;
 	sk->sk_sndtimeo = L2CAP_CONN_TIMEOUT;
+	sk->sk_sndbuf   = L2CAP_DEFAULT_SNDBUF;
 
 	INIT_LIST_HEAD(&l2cap_pi(sk)->rx_busy);
 
-- 
2.43.0