p->cc can go negative in libpcap

[Guy Harris]

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 {