On 12/14/18 2:04 PM, Daniel P. Berrangé wrote: > The virtualization driver has two connections to the virtlogd daemon, > one pipe fd for writing to the log file, and one socket fd for making > RPC calls. The typical sequence is to write some data to the pipe fd and > then make an RPC call to determine the current log file offset. > > Unfortunately these two operations are not guaranteed to be handling in > order by virtlogd. The event loop for virtlogd may identify an incoming > event on both the pipe fd and socket fd in the same iteration of the > event loop. It is then entirely possible that it will process the socket > fd RPC call before reading the pending log data from the pipe fd. > > As a result the virtualization driver will get an outdated log file > offset reported back. > > This can be seen with the QEMU driver where, when a guest fails to > start, it will randomly include too much data in the error message it > has fetched from the log file. > > The solution is to ensure we have drained all pending data from the pipe > fd before reporting the log file offset. The pipe fd is always in > blocking mode, so cares needs to be taken to avoid blocking. When > draining this is taken care of by using poll(). The extra complication > is that they might already be an event loop dispatch pending on the pipe > fd. If we have just drained the pipe this pending event will be invalid > so must be discarded. > > Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx> > --- > src/logging/log_handler.c | 48 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 48 insertions(+) ACK Michal -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
