In pcap_read_bpf(), ep is set based on the return value of read(), but read()
from a BPF device doesn't necessarily return a value that's a multiple of the
alignment value for BPF_WORDALIGN(). However, whenever we increment bp, we
round up the increment value by a value rounded up by BPF_WORDALIGN(), so we
could increment bp past ep after processing the last packet in the buffer.
This can be reproduced by running a program that opens a capture device with a
timeout of 0, in a loop, calls pcap_dispatch() with a cnt argument of 1, and
reports when it returns a value of 0. The timeout of 0 means that the read()
that libpcap does shouldn't return until there's packet data, so a timeout
won't cause pcap_dispatch() to return 0. Do a large amount of network data
transfer, to fill up the BPF bucket; notice that, on occasion, the program will
report that pcap_dispatch() returns 0.
See the attached patch, which also fixes a case where, if you break out of the
packet read loop due to a pcap_breakloop() call, p->bp isn't advanced and p->cc
isn't reduced.
--- pcap-bpf.c.orig 2010-09-01 01:37:34.000000000 -0700
+++ pcap-bpf.c 2010-09-01 01:38:48.000000000 -0700
@@ -846,14 +846,28 @@
* processed so far.
*/
if (p->break_loop) {
+ p->bp = bp;
+ p->cc = ep - bp;
+ /*
+ * ep is set based on the return value of read(),
+ * but read() from a BPF device doesn't necessarily
+ * return a value that's a multiple of the alignment
+ * value for BPF_WORDALIGN(). However, whenever we
+ * increment bp, we round up the increment value by
+ * a value rounded up by BPF_WORDALIGN(), so we
+ * could increment bp past ep after processing the
+ * last packet in the buffer.
+ *
+ * We treat ep < bp as an indication that this
+ * happened, and just set p->cc to 0.
+ */
+ if (p->cc < 0)
+ p->cc = 0;
if (n == 0) {
p->break_loop = 0;
return (PCAP_ERROR_BREAK);
- } else {
- p->bp = bp;
- p->cc = ep - bp;
+ } else
return (n);
- }
}
caplen = bhp->bh_caplen;
@@ -905,6 +919,11 @@
if (++n >= cnt && cnt > 0) {
p->bp = bp;
p->cc = ep - bp;
+ /*
+ * See comment above about p->cc < 0.
+ */
+ if (p->cc < 0)
+ p->cc = 0;
return (n);
}
} else {
