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From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
To: Marcel Holtmann <marcel@holtmann.org>
Cc: "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
Subject: Re: [RFC] Bluetooth: L2CAP: Fix to handling fragmented header
Date: Tue, 28 Jul 2020 09:39:37 -0700
Message-ID: <CABBYNZKJLcyJsCe9yZB6RAdtiCkpi-oMYPsJzWV_r19CJVT_Ww@mail.gmail.com> (raw)
In-Reply-To: <80073DFD-564E-4B4E-9F23-02ED4075321D@holtmann.org>

Hi Marcel,

On Tue, Jul 28, 2020 at 12:22 AM Marcel Holtmann <marcel@holtmann.org> wrote:
>
> Hi Luiz,
>
> > Bluetooth Core Specification v5.2, Vol. 3, Part A, section 1.4, table
> > 1.1:
> >
> > 'Start Fragments always either begin with the first octet of the Basic
> >  L2CAP header of a PDU or they have a length of zero (see [Vol 2] Part
> >  B, Section 6.6.2).'
> >
> > This text has been changed recently as it previously stated:
> >
> > 'Start Fragments always begin with the Basic L2CAP header of a PDU.'
> >
> > Apparently this was changed by the following errata:
> >
> > https://www.bluetooth.org/tse/errata_view.cfm?errata_id=10216
> >
> > In past this has not been a problem but it seems new controllers are
> > apparently doing it as it has been reported in Zephyr:
> >
> > https://github.com/zephyrproject-rtos/zephyr/issues/26900
> >
> > Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
> > ---
> > net/bluetooth/l2cap_core.c | 104 +++++++++++++++++++++++++++++--------
> > 1 file changed, 83 insertions(+), 21 deletions(-)
> >
> > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > index ade83e224567..193bea314222 100644
> > --- a/net/bluetooth/l2cap_core.c
> > +++ b/net/bluetooth/l2cap_core.c
> > @@ -8269,6 +8269,63 @@ static void l2cap_security_cfm(struct hci_conn *hcon, u8 status, u8 encrypt)
> >       mutex_unlock(&conn->chan_lock);
> > }
> >
> > +/* Append fragment into frame respecting the maximum len of rx_skb */
> > +static int l2cap_recv_frag(struct l2cap_conn *conn, struct sk_buff *skb,
> > +                        u16 len)
> > +{
> > +     if (!conn->rx_skb) {
> > +             /* Allocate skb for the complete frame (with header) */
> > +             conn->rx_skb = bt_skb_alloc(len, GFP_KERNEL);
> > +             if (!conn->rx_skb)
> > +                     return -ENOMEM;
> > +             /* Init rx_len */
> > +             conn->rx_len = len;
> > +     }
> > +
> > +     /* Copy as much as the rx_skb can hold */
> > +     len = min_t(u16, len, skb->len);
> > +     skb_copy_from_linear_data(skb, skb_put(conn->rx_skb, len), len);
> > +     skb_pull(skb, len);
> > +     conn->rx_len -= len;
> > +
> > +     return len;
> > +}
> > +
> > +static int l2cap_recv_header(struct l2cap_conn *conn, struct sk_buff *skb)
> > +{
> > +     struct l2cap_hdr *hdr;
> > +     struct sk_buff *rx_skb;
> > +     int len;
> > +
> > +     /* Append just enough to complete the header */
> > +     len = l2cap_recv_frag(conn, skb, L2CAP_HDR_SIZE - conn->rx_skb->len);
> > +
> > +     /* If header could not be read just continue */
> > +     if (len < 0 || conn->rx_skb->len < L2CAP_HDR_SIZE)
> > +             return len;
> > +
> > +     rx_skb = conn->rx_skb;
> > +     conn->rx_skb = NULL;
> > +
> > +     hdr = (struct l2cap_hdr *) rx_skb->data;
>
> so I think it is pointless to insist on getting the complete header. We really just need the first 2 octets.
>
> struct l2cap_hdr {
>         __le16     len;
>         __le16     cid;
> } __packed;

Indeed, I've totally forgotten about the cid so I will change this to
not use L2CAP_HDR_SIZE but 2 instead.

> Once we have received at least 2 octets, we can get_unaligned_le16(rx_skb->data) and then just continue.

Sure, I was trying to figure out if there is any way to grow the the
rx_skb since I will be just allocating 2 bytes for it if the header is
not available, we could perhaps take a different approach and always
allocate based on the conn->mtu that way we don't have to wait the
length to received to allocate a second skb and copy over the length
into it, obviously that would only be done if length was fragmented.

> Regards
>
> Marcel
>


-- 
Luiz Augusto von Dentz

      reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-28  7:04 Luiz Augusto von Dentz
2020-07-28  7:22 ` Marcel Holtmann
2020-07-28 16:39   ` Luiz Augusto von Dentz [this message]

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