A problem encountered due to a bug in libpcap was reported to the caller as: An error occurred, but the cause is unknown This was because the error had been logged in the DHCPSnoop thread. The worker thread handling the API call to start a domain spins up the DHCPSnoop thread which watches for dhcp packets with libpcap, then uses virCondSignal() to notify the worker thread (which has been waiting with virCondWait()). The worker thread knows that there was an error (because threadStatus != THREAD_STATUS_OK), but the error info had been stored in thread-specific storage for the other thread, so the worker thread can only report that there was a failure, but it doesn't know why. The solution is to save the error that was logged (with virErrorPreserveLast() into the object the is used to share info between the threads, then we can set the error in the worker thread using virErrorRestore(). In the case of the error I was looking at, this changed the "unknown" message into: internal error: pcap_setfilter: can't remove kernel filter: Bad file descriptor Signed-off-by: Laine Stump <laine@laine.org> --- src/nwfilter/nwfilter_dhcpsnoop.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/src/nwfilter/nwfilter_dhcpsnoop.c b/src/nwfilter/nwfilter_dhcpsnoop.c index 8e955150fa..6069e70460 100644 --- a/src/nwfilter/nwfilter_dhcpsnoop.c +++ b/src/nwfilter/nwfilter_dhcpsnoop.c @@ -147,6 +147,7 @@ struct _virNWFilterSnoopReq { virNWFilterSnoopIPLeasePtr start; virNWFilterSnoopIPLeasePtr end; char *threadkey; + virErrorPtr threadError; virNWFilterSnoopThreadStatus threadStatus; virCond threadStatusCond; @@ -639,6 +640,7 @@ virNWFilterSnoopReqFree(virNWFilterSnoopReqPtr req) virMutexDestroy(&req->lock); virCondDestroy(&req->threadStatusCond); + virFreeError(req->threadError); VIR_FREE(req); } @@ -1404,10 +1406,12 @@ virNWFilterDHCPSnoopThread(void *req0) /* let creator know how well we initialized */ if (error || !threadkey || tmp < 0 || !worker || - ifindex != req->ifindex) + ifindex != req->ifindex) { + virErrorPreserveLast(&req->threadError); req->threadStatus = THREAD_STATUS_FAIL; - else + } else { req->threadStatus = THREAD_STATUS_OK; + } virCondSignal(&req->threadStatusCond); @@ -1713,9 +1717,16 @@ virNWFilterDHCPSnoopReq(virNWFilterTechDriverPtr techdriver, } /* sync with thread */ - if (virCondWait(&req->threadStatusCond, &req->lock) < 0 || - req->threadStatus != THREAD_STATUS_OK) + if (virCondWait(&req->threadStatusCond, &req->lock) < 0) { + virReportSystemError(errno, "%s", + _("unable to wait on dhcp snoop thread")); goto exit_snoop_cancel; + } + + if (req->threadStatus != THREAD_STATUS_OK) { + virErrorRestore(&req->threadError); + goto exit_snoop_cancel; + } virNWFilterSnoopReqUnlock(req); -- 2.14.3