URB data not available through binary usbmon interface when URB_DMA_SG_COMBINED set

[Nicholas Chin <nic.c3.14@gmail.com>]

Hello,

Recently I've been working on reverse engineering the USB protocol of
the Digilent Analog Discovery 2, which uses an FT232H in FIFO mode. I
noticed that tools such as Wireshark and usbdump, which rely on the
binary usbmon interface, were showing many URBs with no data, even
though I would expect a payload to be present. Looking at the fields in
the URB, it seems that the len_urb field of struct mon_bin_hdr was a
non-zero value, but the len_cap field was set to zero.

With the legacy text interface, I am able to see data in the same URBs
that the binary interface was not showing.

I did some digging and logging of drivers/usb/mon/mon_bin.c and found
that the len_cap field was being zeroed in mon_bin_event() due to a
positive return value of the mon_bin_get_data() call.
Looking into that function, it seems that it returns without copying any
data if urb->num_sgs is non zero and the URB_DMA_SG_COMBINED flag is set
in the urb->transfer flags. The comment around that code indicates that
"If IOMMU coalescing occured, we cannot trust sg_page" which I assume is
supposed to be the reason for this behavior.

The analogous function in the text interface, mon_text_get_data(), does
not check URB_DMA_SG_COMBINED, and instead just reads the first 32 bytes
of the first sg buffer.

Could someone please explain why/if this behavior is required if
URB_DMA_SG_COMBINED flag is set? Is there a way that usbmon could return
data in this situation?

I would assume that if the full data is able to make its way to
userspace then it should be possible for usbmon to also access that data.

Regards,

Nicholas