On Thu, 19 Jul 2007 11:20:43 -0700 (PDT) andrei radulescu-banu <[EMAIL PROTECTED]> wrote:
> > [Ben] If tcpdump and/or bridging needs to disable the hw-accel, then it can > explicitly do so by some API. That is better than overloading > the promisc flag in my opinion. > > I guess I could be persuaded in the end. But let me still play devil > advocate. The semantics of 'promiscuous', in my opinion, mean 'receive > everything', including vlan. > > > [Ben] This is especially true since promisc > is not easily readable by user-space and things like tcpdump > cannot have full control of promisc (if a mac-vlan has the NIC in > promisc mode, for instance, then tcpdump can never disable it.) > > I agree with all the above. For example when you run 'ifconfig' during > 'tcpdump', the interface does not have the promiscuous flag set!! In kernel it is a nice atomic counter, no problem. > > This confused me for a while, until I realized that tcpdump's packet socket > was using an obscure packet_dev_mc() API (af_packet.c) to get the interface > in promiscuous mode. The reason for this is that packet_mc_add() implements a > reference counted mechanism for promiscuous. So that: > - starting tcpdump instance 1 sets promiscuous mode > - starting tcpdump instance 2 bumps the ref count in packet_mc_add() > - killing tcpdump instance 1 bumps down the ref count, the interface stays > promiscuous > - killing tcpdump instance 2 truly clear promiscuous mode. > > The trick here is that when you kill tcpdump, the kernel clears the packet > socket, and in process bumps down the ref count. Had tcpdump manually > set/cleared the promisc flag, the interface would have stayed promisc after > tcpdump was killed. > > (The mac-vlan driver must have this corner problem as well. If a mac-vlan > interface is disabled while tcpdump runs, it may yank promiscuousness from > under tcpdump.) The kernel has no such problem > So if you want to create an ethtool API to set vlan-promiscuous mode, one > problem to grapple is that we need a similar mechanism to the above, so you > can run two concurrent tcpdump's (or tcpdump while bridging vlans) and the > vlan-promiscuous mode gets set correctly each time. For tcpdump at least, > the new ethtool API needs to be called from packet_mc_add(). - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
