Date: Tue, 05 Mar 2013 15:03:52 +0100 From: Andre Oppermann <andre@freebsd.org> To: Lawrence Stewart <lstewart@freebsd.org> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: Bug in sbsndptr() Message-ID: <5135FB48.1000809@freebsd.org> In-Reply-To: <513564AD.7000006@freebsd.org> References: <512CBADB.3050004@freebsd.org> <5134CD5D.6090107@freebsd.org> <513564AD.7000006@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On 05.03.2013 04:21, Lawrence Stewart wrote: > On 03/05/13 03:35, Andre Oppermann wrote: >> On 26.02.2013 14:38, Lawrence Stewart wrote: >>> Hi Andre, >> >> Hi Lawrence, :-) >> >>> A colleague and I spent a very frustrating day tracing an accounting bug >>> in the multipath TCP patch we're working on at CAIA to a bug in >>> sbsndptr(). I haven't tested it with regular TCP yet, but I believe the >>> following patch fixes the bug (proposed commit log message is at the top >>> of the patch): >>> >>> http://people.freebsd.org/~lstewart/patches/misctcp/sbsndptr_mnext_10.x.r247314.diff >>> >>> >>> The patch should have no tangible effect to operation other than to >>> ensure the function delivers on the promise to return the closest mbuf >>> in the chain for the given offset. >> >> I agree that the description of sbsndptr() can be misleading as it refers >> to the point in time when the pointer was updated last. Relative to now >> the real offset may be at the beginning of the next mbuf. > > Right, and we ran into the issue because we made an assumption based on > the use of the present tense in the comment: > > "Return closest mbuf in chain for current offset." I apologize for the incorrect and misleading description. :-) >> As you note in the proposed commit message by the time the send pointer >> is calculated we may have reached the end of the chain and must avoid >> storing a NULL pointer. The mbuf copy routines simply skips over the >> additional mbuf in the chain using the returned offset. >> >> I wonder how this has caused trouble with your multipath patch. You'd >> have to copy the sockbuf contents as well and unless you're using custom >> sockbuf and mbuf chain functions this shouldn't be a problem. Using >> custom functions on a socket buffer is a delicate approach. For a sockbuf >> consumer being able to handle valid offsets into an mbuf chain is a core >> feature and must-have part of the functionality. > > No custom sockbuf or mbuf routines are in use. We've implemented a > mapping shim between subflows and the socket buffer. When a subflow asks > the multipath layer for some data to send, the multipath layer returns a > mapping onto the socket buffer, which will remain valid until such time > as the subflow has marked the mapped data as acknowledged. > > Part of the map accounting is tracking the pointer of the first mbuf in > the sockbuf where the map's data begins. Our accounting assumed the mbuf > + the offset returned by sbsndptr had data available, which is how we > triggered the problem. We could have accounted for the issue in our new > map accounting code, but that would add additional complexity to some > already complex code and the better solution is to make sbsndptr DTRT. So effectively you run a separate sbsndptr for each subflow using the real sbsndptr to track the head of the queue? /me fears the day a mptcp import comes up. tcp-complexity^^3. :-o >>> I would appreciate a review and any thoughts. >> >> I think you have found a valid (micro-)optimization. However you're >> still making a dangerous assumption in that the next mbuf is indeed >> the one you want. This may not be true in subtle ways when the chain >> contains m_len=0 mbufs in it. I'm not aware of it actually happening >> but it can't be ruled out either if custom sockbuf manipulation functions >> are in use. > > True, though I'm struggling to think why there would be m_len=0 mbufs > interspersed with m_len > 0 mbufs in a socket send buffer mbuf chain. sbcompress() doesn't allow for m_len=0 mbufs. This holds true as long as the sbappend functions are used. If not, we may get anything there. As long as nobody is using custom sockbuf appends we're safe. Because I first assumed from your description some custom sockbuf munging the guarantee wouldn't haven been there anymore. >> I'd recommend the following: >> have you custom sockbuf function handle forward seeking like the other >> m_copy() functions; and/or apply a patch along the (untested) example >> below. > > If you believe it is both correct and possible for m_len=0 mbufs to > exist in a socket buffer chain, then I agree that we should amend my > proposed patch to loop and skip over m_len=0 mbufs as you've suggested. No. So far it is neither possible or correct. > However, I'm more inclined to suspect it is undesirable and potentially > buggy behaviour to end up with m_len=0 mbufs in a socket buffer chain on > which sbsndptr is being used, and would instead suggest a > "KASSERT(ret->m_len > 0, (...));" be added to the end of my proposed if > block. Agreed. -- Andre
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?5135FB48.1000809>