Re: [PATCH 3/3] virarptable: End parsing earlier in case of NLMSG_DONE

On 8/16/24 8:45 AM, Martin Kletzander wrote: > Check for the last multipart message right as the first thing. The > presumption probably was that the last message might still contain a > payload we want to parse. However that cannot be true since that would > have to be a type RTM_NEWNEIGH. > > This was not caught because older > kernels were note sending NLMSG_DONE and probably relied on the fact > that the parsing just stops after all the messages are walked through, > which the NLMSG_OK macro successfully did. > > Resolves: https://issues.redhat.com/browse/RHEL-52449 Also: Resolves: https://bugzilla.redhat.com/2302245 Fixes: a176d67cdfaf5b8237a7e3a80d8be0e6bdf2d8fd (Hmm, and BTW I had suggested that patch 2/3 should have that Fixes: line, but I guess it can't be *totally* fixed until this patch, since prior to that we're checking the values of a struct ndmsg before checking the message type (i.e. it might not be an RTM_NEWNEIGH)) > Signed-off-by: Martin Kletzander <mkletzan(a)redhat.com&gt; > --- > So technically this still has some issues, maybe. I'm pretty sure there are other issues with our netlink code, but the effect of those issues is unknown, and unrelated to the current bug which your code fixes. So, IMO, "this" doesn't have some issues :-) > I could not find if our usage of libnl3 makes it easier for us so that we do not > have to check for NLMSG_{ERROR,OVERRUN,NOOP} or whether these checks should be > here as well Well, virArpTableGet() calls virNetlinkGetNeighbor(), which allows responses where the *first* message in the response is NLMSG_NEWNEIGH or NLMSG_ERROR, and that calls virNetlinkTalk() which fails on any NLMSG_ERROR that isn't 0 (an NLMSG_ERRO with error==0 is just an ack packet). virNetlinkTalk() calls virNetLinkCommand(), which just calls nl_recv(), and as far as I can tell from the libnl API docs, there's no special handling there for any type of message. > If yes, then we should add them. And we have (some of) these > checks elsewhere in the code, so "maybe". Have you ever tried to assimilate the libnl documentation? (https://www.infradead.org/~tgr/libnl/doc/api/) I'll refer you to my comment further down marked with "[*]".

> > Another thing is that we could avoid such errors by using nl_socket_set_cb(), > calling nl_recvmsgs_default() and then parsing only the valid messages in a > callback. This somehow reminds me of code that we had in netcf that I think needed to get a list of interfaces, and we used the nl_cache_* functions for this, which in the end (I thought) made the code even more complicated than if we'd just used lower level functions. There is some very useful stuff in libnl, but I think some times it complicates in the name of "simplifying". I guess I'm trying to say "Maybe, but don't get your hopes up".

> On top of that we could have an abstraction on top this to utilise in > all the netlink dumps we do, ditching our current abstraction which was a bit > hard for me to go through, to be honest. Everything about netlink from the very beginning has been "hard to go through", especially when mixed in with libnl. I think originally there were a couple different usages of netlink/libnl in libvirt authored by a couple different people, and then another usage or two came along with each new user trying to refactor existing code to work for them too, but without ever any underlying plan. Or something like that. The result is that (IMO) trying to understand some of our netlink code might do you more harm than good :-/. Not that I'm cynical or anything.

(P.S. Ditching the current abstraction(s) and making something new that works better for everyone would be wonderful to see. As long as someone else did it. The dreadful side of that is that I may soon need to add yet another usage of netlink to the code, and that could mean that either I will a) go through the pain and time consuming frustration of doing said ditching/rewriting so that my new code can be nicer, or b) live with the embarrasment of adding yet another hacky house of netlink cards to the code :-/)

> > And of course there might be other places in our codebase that expect the same > behaviour as this code did and we should fix 'em all. One other problem I noticed during my short dip into our netlink code last week is that in all cases except getting VFINFO (for SRIOV devices), we only read a single response packet after we make a request, when any response could have multiple packets.

> After all the debugging > for this piece I did not even check for those, maybe if this gets in I'll have a > long think about it. [*]Good luck with that. Be sure the beer cooler next to your desk is well stocked.

> src/util/virarptable.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/src/util/virarptable.c b/src/util/virarptable.c > index 8e805fb35332..604019c62a37 100644 > --- a/src/util/virarptable.c > +++ b/src/util/virarptable.c > @@ -84,6 +84,9 @@ virArpTableGet(void) > int len = nh->nlmsg_len; > void *addr; > > + if (nh->nlmsg_type == NLMSG_DONE) > + return table; I would prefer "break;" here rather than "return table;" just in case someone in the future adds something that requires some sort of cleanup code at the end of the function. This is fine too though. Reviewed-by: Laine Stump <laine(a)redhat.com&gt; > + > if (len < NLMSG_SPACE(sizeof(*r))) { > virReportError(VIR_ERR_INTERNAL_ERROR, "%s", > _("wrong nlmsg len")); > @@ -98,9 +101,6 @@ virArpTableGet(void) > (!(r->ndm_state == NUD_STALE || r->ndm_state == NUD_REACHABLE))) > continue; > > - if (nh->nlmsg_type == NLMSG_DONE) > - return table; > - > VIR_WARNINGS_NO_CAST_ALIGN > parse_rtattr(tb, NDA_MAX, NDA_RTA(r), NLMSG_PAYLOAD(nh, sizeof(*r))); > VIR_WARNINGS_RESET