[ovs-dev] mask_set_prefix_bits may causes discontiguous bit mask

[Peng He]

Hi, 
    I installed some rulesets generated by ClassBench (a ruleset benchmark for generating ACL and FW rules) in Open vSwitch, and I find many megaflow's mask has 
discontiguous bits. Like mask = 0xfc in network byte order. After some code investigation I find the problem is in the function *mask_set_prefix_bits*: 




staticvoidmask_set_prefix_bits(struct flow_wildcards *wc, uint8_t be32ofs,
                     unsignedint n_bits)
{
    ovs_be32 *mask = &((ovs_be32 *)&wc->masks)[be32ofs];
    unsignedint i;

    for (i = 0; i < n_bits / 32; i++) {
        mask[i] = OVS_BE32_MAX;
    }if (n_bits % 32) {
        mask[i] |= htonl(~0u << (32 - n_bits % 32));
    }}
For example, if the input n_bits = 6, the mask should be 0x2f, as there is 6 contiguous bits. However, the current code will generate 0xfc:
(0xFFFFFFFF << (32 - 6) ) = 0xFC000000
and htonl (0xFC000000) = 0xFC. 
I am writing this to make sure that I am correct. The OVS code I use is 2.4 release version. 
Thanks.