Bug #1816

p->cc can go negative in libpcap

Added by guy about 6 years ago. Updated about 6 years ago.

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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.

patch.txt View (1.18 KB) guy, 09/01/2010 08:42 AM

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