>Number: 150176
>Category: misc
>Synopsis: p->cc can go negative in libpcap
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Wed Sep 01 08:00:10 UTC 2010
>Closed-Date:
>Last-Modified:
>Originator: Guy Harris
>Release: Top of CVS tree
>Organization:
>Environment:
>Description:
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.
>How-To-Repeat:
Run 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.
>Fix:
Patch attached with submission follows:
diff --git a/pcap-bpf.c b/pcap-bpf.c
index 666acf9..bcbfbef 100644
--- a/pcap-bpf.c
+++ b/pcap-bpf.c
@@ -955,14 +955,28 @@ pcap_read_bpf(pcap_t *p, int cnt, pcap_handler callback,
u_char *user)
* 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;
@@ -1014,6 +1028,11 @@ pcap_read_bpf(pcap_t *p, int cnt, pcap_handler callback,
u_char *user)
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 {
>Release-Note:
>Audit-Trail:
>Unformatted:
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