From 9ca66ec59882ea673a6fae9ae2e36989d88dc9d1 Mon Sep 17 00:00:00 2001 From: Dean Deng Date: Fri, 23 Oct 2020 09:14:52 -0700 Subject: Rewrite reference leak checker without finalizers. Our current reference leak checker uses finalizers to verify whether an object has reached zero references before it is garbage collected. There are multiple problems with this mechanism, so a rewrite is in order. With finalizers, there is no way to guarantee that a finalizer will run before the program exits. When an unreachable object with a finalizer is garbage collected, its finalizer will be added to a queue and run asynchronously. The best we can do is run garbage collection upon sandbox exit to make sure that all finalizers are enqueued. Furthermore, if there is a chain of finalized objects, e.g. A points to B points to C, garbage collection needs to run multiple times before all of the finalizers are enqueued. The first GC run will register the finalizer for A but not free it. It takes another GC run to free A, at which point B's finalizer can be registered. As a result, we need to run GC as many times as the length of the longest such chain to have a somewhat reliable leak checker. Finally, a cyclical chain of structs pointing to one another will never be garbage collected if a finalizer is set. This is a well-known issue with Go finalizers (https://github.com/golang/go/issues/7358). Using leak checking on filesystem objects that produce cycles will not work and even result in memory leaks. The new leak checker stores reference counted objects in a global map when leak check is enabled and removes them once they are destroyed. At sandbox exit, any remaining objects in the map are considered as leaked. This provides a deterministic way of detecting leaks without relying on the complexities of finalizers and garbage collection. This approach has several benefits over the former, including: - Always detects leaks of objects that should be destroyed very close to sandbox exit. The old checker very rarely detected these leaks, because it relied on garbage collection to be run in a short window of time. - Panics if we forgot to enable leak check on a ref-counted object (we will try to remove it from the map when it is destroyed, but it will never have been added). - Can store extra logging information in the map values without adding to the size of the ref count struct itself. With the size of just an int64, the ref count object remains compact, meaning frequent operations like IncRef/DecRef are more cache-efficient. - Can aggregate leak results in a single report after the sandbox exits. Instead of having warnings littered in the log, which were non-deterministically triggered by garbage collection, we can print all warning messages at once. Note that this could also be a limitation--the sandbox must exit properly for leaks to be detected. Some basic benchmarking indicates that this change does not significantly affect performance when leak checking is enabled, which is understandable since registering/unregistering is only done once for each filesystem object. Updates #1486. PiperOrigin-RevId: 338685972 --- runsc/boot/BUILD | 1 + runsc/boot/loader.go | 7 +++++++ 2 files changed, 8 insertions(+) (limited to 'runsc/boot') diff --git a/runsc/boot/BUILD b/runsc/boot/BUILD index 248f77c34..b97dc3c47 100644 --- a/runsc/boot/BUILD +++ b/runsc/boot/BUILD @@ -38,6 +38,7 @@ go_library( "//pkg/memutil", "//pkg/rand", "//pkg/refs", + "//pkg/refsvfs2", "//pkg/sentry/arch", "//pkg/sentry/arch:registers_go_proto", "//pkg/sentry/control", diff --git a/runsc/boot/loader.go b/runsc/boot/loader.go index a971d20ec..8c6ab213d 100644 --- a/runsc/boot/loader.go +++ b/runsc/boot/loader.go @@ -35,6 +35,7 @@ import ( "gvisor.dev/gvisor/pkg/memutil" "gvisor.dev/gvisor/pkg/rand" "gvisor.dev/gvisor/pkg/refs" + "gvisor.dev/gvisor/pkg/refsvfs2" "gvisor.dev/gvisor/pkg/sentry/arch" "gvisor.dev/gvisor/pkg/sentry/control" "gvisor.dev/gvisor/pkg/sentry/fdimport" @@ -476,6 +477,12 @@ func (l *Loader) Destroy() { // save/restore. l.k.Release() + // All sentry-created resources should have been released at this point; + // check for reference leaks. + if refsvfs2.LeakCheckEnabled() { + refsvfs2.DoLeakCheck() + } + // In the success case, stdioFDs and goferFDs will only contain // released/closed FDs that ownership has been passed over to host FDs and // gofer sessions. Close them here in case of failure. -- cgit v1.2.3