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FCOE: Autodetect Ethernet FCS by examining EOF 

In the T11 version of FCOE, the length field was removed. If the last
four reported bytes don't look like the EOF plus padding, but the four
bytes before that do, then the Ethernet FCS is almost surely present so
treat it that way. Closes the other case of #4594
This commit is contained in:
John Thacker 2020-09-01 23:43:10 -04:00 committed by AndersBroman
parent f68719bd8a
commit 645716fb6d
1 changed files with 57 additions and 10 deletions
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67
epan/dissectors/packet-fcoe.c
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@ -91,6 +91,32 @@ static expert_field ei_fcoe_crc = EI_INIT;
static dissector_handle_t fc_handle;
/* Looks for the EOF at a given offset. Returns NULL if the EOF is not
* present, is not one of the known values, or if the next three bytes, if
* present, are not padding. Otherwise returns the entry from the EOF
* value_string. Intended for use with the newer T11 version, where the frame
* length is not explicitly set (and padding is used). */
static const gchar *
fcoe_get_eof(tvbuff_t *tvb, gint eof_offset)
{
guint8 eof = 0;
const gchar *eof_str;
gint padding_remaining;
if (!tvb_bytes_exist(tvb, eof_offset, 1)) {
return NULL;
}
padding_remaining = MIN(tvb_captured_length_remaining(tvb, eof_offset+1),3);
if (tvb_memeql(tvb, eof_offset+1, "\x00\x00\x00", padding_remaining)) {
return NULL;
}
eof = tvb_get_guint8(tvb, eof_offset);
eof_str = try_val_to_str(eof, fcoe_eof_vals);
return eof_str;
}
static int
dissect_fcoe(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
@ -130,10 +156,15 @@ dissect_fcoe(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_
ver = "pre-T11 ";
if (version != 0)
ver = wmem_strdup_printf(wmem_packet_scope(), ver, "pre-T11 ver %d ", version);
eof_offset = header_len + frame_len + 4;
eof_str = "none";
if (tvb_bytes_exist(tvb, eof_offset, 1)) {
eof = tvb_get_guint8(tvb, eof_offset);
eof_str = val_to_str(eof, fcoe_eof_vals, "0x%x");
}
/* Old format has a length field, so we can help the Ethernet dissector
* guess about the FCS; note this format does not pad after the EOF,
* so the trailer is only 5 octets, not 8. */
set_actual_length(tvb, header_len+frame_len+5);
* guess about the FCS; note this format does not pad after the EOF */
set_actual_length(tvb, eof_offset+1);
} else {
frame_len = tvb_reported_length_remaining(tvb, 0) -
FCOE_HEADER_LEN - FCOE_TRAILER_LEN;
@ -147,21 +178,37 @@ dissect_fcoe(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_
version = tvb_get_guint8(tvb, 0) >> 4;
if (version != 0)
ver = wmem_strdup_printf(wmem_packet_scope(), ver, "ver %d ", version);
eof_offset = header_len + frame_len + 4;
if (NULL == (eof_str = fcoe_get_eof(tvb, eof_offset))) {
/* We didn't find the EOF, look 4 bytes earlier */
if (NULL != (eof_str = fcoe_get_eof(tvb, eof_offset-4))) {
/* Found it, so it seems there's an Ethernet FCS. */
frame_len -= 4;
set_actual_length(tvb, eof_offset);
eof_offset -= 4;
} else {
if (tvb_bytes_exist(tvb, eof_offset, 1)) {
/* Hmm, we have enough bytes to look for the EOF
* but it's an unexpected value. */
eof = tvb_get_guint8(tvb, eof_offset);
eof_str = wmem_strdup_printf(wmem_packet_scope(), "0x%x", eof);
} else {
/* We just didn't capture enough to get the EOF */
eof_str = "none";
}
}
}
}
col_set_str(pinfo->cinfo, COL_PROTOCOL, "FCoE");
crc_offset = header_len + frame_len;
eof_offset = crc_offset + 4;
bytes_remaining = tvb_captured_length_remaining(tvb, header_len);
if (bytes_remaining > frame_len)
bytes_remaining = frame_len; /* backing length */
next_tvb = tvb_new_subset_length_caplen(tvb, header_len, bytes_remaining, frame_len);
eof_str = "none";
if (tvb_bytes_exist(tvb, eof_offset, 1)) {
eof = tvb_get_guint8(tvb, eof_offset);
eof_str = val_to_str(eof, fcoe_eof_vals, "0x%x");
}
/*
* Check the CRC.
*/