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21 * pcap-usb-linux-common.c - common code for everything that needs to
22 * deal with Linux USB captures.
25 #include "pcap/pcap.h"
28 #include "pcap-usb-linux-common.h"
31 * Compute, from the data provided by the Linux USB memory-mapped capture
32 * mechanism, the amount of packet data that would have been provided
33 * had the capture mechanism not chopped off any data at the end, if, in
36 * Set the "unsliced length" field of the packet header to that value.
39 fix_linux_usb_mmapped_length(struct pcap_pkthdr *pkth, const u_char *bp)
41 const pcap_usb_header_mmapped *hdr;
45 * All callers of this routine must ensure that pkth->caplen is
46 * >= sizeof (pcap_usb_header_mmapped).
48 bytes_left = pkth->caplen;
49 bytes_left -= sizeof (pcap_usb_header_mmapped);
51 hdr = (const pcap_usb_header_mmapped *) bp;
52 if (!hdr->data_flag && hdr->transfer_type == URB_ISOCHRONOUS &&
53 hdr->event_type == URB_COMPLETE &&
54 (hdr->endpoint_number & URB_TRANSFER_IN) &&
55 pkth->len == sizeof(pcap_usb_header_mmapped) +
56 (hdr->ndesc * sizeof (usb_isodesc)) + hdr->urb_len) {
58 u_int pre_truncation_data_len, pre_truncation_len;
60 descs = (usb_isodesc *) (bp + sizeof(pcap_usb_header_mmapped));
63 * We have data (yes, data_flag is 0 if we *do* have data),
64 * and this is a "this is complete" incoming isochronous
65 * transfer event, and the length was calculated based
68 * That's not correct, because the data isn't contiguous,
69 * and the isochronous descriptos show how it's scattered.
71 * Find the end of the last chunk of data in the buffer
72 * referred to by the isochronous descriptors; that indicates
73 * how far into the buffer the data would have gone.
75 * Make sure we don't run past the end of the captured data
76 * while processing the isochronous descriptors.
78 pre_truncation_data_len = 0;
79 for (uint32_t desc = 0;
80 desc < hdr->ndesc && bytes_left >= sizeof (usb_isodesc);
81 desc++, bytes_left -= sizeof (usb_isodesc)) {
84 if (descs[desc].len != 0) {
85 desc_end = descs[desc].offset + descs[desc].len;
86 if (desc_end > pre_truncation_data_len)
87 pre_truncation_data_len = desc_end;
92 * Now calculate the total length based on that data
95 pre_truncation_len = sizeof(pcap_usb_header_mmapped) +
96 (hdr->ndesc * sizeof (usb_isodesc)) +
97 pre_truncation_data_len;
100 * If that's greater than or equal to the captured length,
101 * use that as the length.
103 if (pre_truncation_len >= pkth->caplen)
104 pkth->len = pre_truncation_len;
107 * If the captured length is greater than the length,
108 * use the captured length.
110 * For completion events for incoming isochronous transfers,
111 * it's based on data_len, which is calculated the same way
112 * we calculated pre_truncation_data_len above, except that
113 * it has access to all the isochronous descriptors, not
114 * just the ones that the kernel were able to provide us or,
115 * for a capture file, that weren't sliced off by a snapshot
118 * However, it might have been reduced by the USB capture
119 * mechanism arbitrarily limiting the amount of data it
120 * provides to userland, or by the libpcap capture code
121 * limiting it to being no more than the snapshot, so
122 * we don't want to just use it all the time; we only
123 * do so to try to get a better estimate of the actual
124 * length - and to make sure the on-the-network length
125 * is always >= the captured length.
127 if (pkth->caplen > pkth->len)
128 pkth->len = pkth->caplen;