Re: [PATCH v3] usb: gadget: ncm: Avoid dropping datagrams of properly parsed NTBs

[Maciej Żenczykowski]

On Sun, Feb 4, 2024 at 11:47 PM Krishna Kurapati
<quic_kriskura@xxxxxxxxxxx> wrote:
>
> It is observed sometimes when tethering is used over NCM with Windows 11
> as host, at some instances, the gadget_giveback has one byte appended at
> the end of a proper NTB. When the NTB is parsed, unwrap call looks for
> any leftover bytes in SKB provided by u_ether and if there are any pending
> bytes, it treats them as a separate NTB and parses it. But in case the
> second NTB (as per unwrap call) is faulty/corrupt, all the datagrams that
> were parsed properly in the first NTB and saved in rx_list are dropped.
>
> Adding a few custom traces showed the following:
> [002] d..1  7828.532866: dwc3_gadget_giveback: ep1out:
> req 000000003868811a length 1025/16384 zsI ==> 0
> [002] d..1  7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb toprocess: 1025
> [002] d..1  7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342
> [002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb seq: 0xce67
> [002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x400
> [002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb ndp_len: 0x10
> [002] d..1  7828.532869: ncm_unwrap_ntb: K: Parsed NTB with 1 frames
>
> In this case, the giveback is of 1025 bytes and block length is 1024.
> The rest 1 byte (which is 0x00) won't be parsed resulting in drop of
> all datagrams in rx_list.
>
> Same is case with packets of size 2048:
> [002] d..1  7828.557948: dwc3_gadget_giveback: ep1out:
> req 0000000011dfd96e length 2049/16384 zsI ==> 0
> [002] d..1  7828.557949: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342
> [002] d..1  7828.557950: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x800
>
> Lecroy shows one byte coming in extra confirming that the byte is coming
> in from PC:
>
> Transfer 2959 - Bytes Transferred(1025)  Timestamp((18.524 843 590)
> - Transaction 8391 - Data(1025 bytes) Timestamp(18.524 843 590)
> --- Packet 4063861
>       Data(1024 bytes)
>       Duration(2.117us) Idle(14.700ns) Timestamp(18.524 843 590)
> --- Packet 4063863
>       Data(1 byte)
>       Duration(66.160ns) Time(282.000ns) Timestamp(18.524 845 722)
>
> According to Windows driver, no ZLP is needed if wBlockLength is non-zero,
> because the non-zero wBlockLength has already told the function side the
> size of transfer to be expected. However, there are in-market NCM devices
> that rely on ZLP as long as the wBlockLength is multiple of wMaxPacketSize.
> To deal with such devices, it pads an extra 0 at end so the transfer is no
> longer multiple of wMaxPacketSize.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: 9f6ce4240a2b ("usb: gadget: f_ncm.c added")
> Signed-off-by: Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>
> ---
> Link to v2:
> https://lore.kernel.org/all/20240131150332.1326523-1-quic_kriskura@xxxxxxxxxxx/
>
> Changes in v2:
> Added check to see if the padded byte is 0x00.
>
> Changes in v3:
> Removed wMaxPacketSize check from v2.
>
>  drivers/usb/gadget/function/f_ncm.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
> index ca5d5f564998..e2a059cfda2c 100644
> --- a/drivers/usb/gadget/function/f_ncm.c
> +++ b/drivers/usb/gadget/function/f_ncm.c
> @@ -1338,7 +1338,15 @@ static int ncm_unwrap_ntb(struct gether *port,
>              "Parsed NTB with %d frames\n", dgram_counter);
>
>         to_process -= block_len;
> -       if (to_process != 0) {
> +
> +       /*
> +        * Windows NCM driver avoids USB ZLPs by adding a 1-byte
> +        * zero pad as needed.
> +        */
> +       if (to_process == 1 &&
> +           (*(unsigned char *)(ntb_ptr + block_len) == 0x00)) {
> +               to_process--;
> +       } else if (to_process > 0) {
>                 ntb_ptr = (unsigned char *)(ntb_ptr + block_len);
>                 goto parse_ntb;
>         }
> --
> 2.34.1
>

Reviewed-by: Maciej Żenczykowski <maze@xxxxxxxxxx>

Let's get this fix out.

Greg, there's further code cleanups (here and elsewhere) I'll send
once this is merged.
I don't want to annoy Krishna further ;-)