On Mon, Sep 11, 2017 at 03:19:55PM +0800, Hangbin Liu wrote: > On Fri, Sep 08, 2017 at 04:51:13PM +0200, Phil Sutter wrote: > > Regarding Michal's concern about reentrancy, maybe we should go into a > > different direction and make rtnl_recvmsg() return a newly allocated > > buffer which the caller has to free. > > Hmm... But we could not free the buf in __rtnl_talk(). Because in > __rtnl_talk() we assign the answer with the buf address and return to caller. > > for (h = (struct nlmsghdr *)buf; status >= sizeof(*h); ) { > [...] > if (answer) { > *answer= h; > return 0; > } > } > > And the caller will keep use it in later code. Since there are plenty of > functions called rtnl_talk. I think it would be much more complex to free > the buffer every time. > > > Hi Michal, > > Would you like to tell me more about your concern with reentrancy? It's looks > arpd doesn't call rtnl_talk() or rtnl_dump_filter_l().
I checked again and arpd indeed isn't a problem. It doesn't seem to call any of the two functions (directly or indirectly) and while it's linked with "-lpthread", it's not really multithreaded. But my concern was rather about other potential users of libnetlink (i.e. those which are not part of iproute2). I must admit, though, that I'm not sure if libnetlink code is reentrant as of now. (And people are discouraged from using it in its own manual page.) That being said, I still like Phil's idea for a different reason. While investigating the issue with "ip link show dev eth ..." which led me to commit 6599162b958e ("iplink: check for message truncation in iplink_get()"), I quickly peeked at some other callers of rtnl_talk() and I'm afraid there may be others which wouldn't handle truncated message correctly. I assume the maxlen argument was always chosen to be sufficient for any expected messages but as the example of iplink_get() shows, messages returned by kernel my grow over time. That's why I like the idea of __rtnl_talk() returning a pointer to newly allocated buffer (of sufficient size) rather than copying the response into a buffer provided by caller and potentially truncating it. Michal Kubecek