diff options
| -rw-r--r-- | pkg/sentry/kernel/cgroup.go | 29 | ||||
| -rw-r--r-- | test/syscalls/linux/cgroup.cc | 35 |
2 files changed, 60 insertions, 4 deletions
diff --git a/pkg/sentry/kernel/cgroup.go b/pkg/sentry/kernel/cgroup.go index 0fbf27f64..c93ef6ac1 100644 --- a/pkg/sentry/kernel/cgroup.go +++ b/pkg/sentry/kernel/cgroup.go @@ -181,7 +181,23 @@ func (r *CgroupRegistry) FindHierarchy(ctypes []CgroupControllerType) *vfs.Files for _, h := range r.hierarchies { if h.match(ctypes) { - h.fs.IncRef() + if !h.fs.TryIncRef() { + // Racing with filesystem destruction, namely h.fs.Release. + // Since we hold r.mu, we know the hierarchy hasn't been + // unregistered yet, but its associated filesystem is tearing + // down. + // + // If we simply indicate the hierarchy wasn't found without + // cleaning up the registry, the caller can race with the + // unregister and find itself temporarily unable to create a new + // hierarchy with a subset of the relevant controllers. + // + // To keep the result of FindHierarchy consistent with the + // uniqueness of controllers enforced by Register, drop the + // dying hierarchy now. The eventual unregister by the FS + // teardown will become a no-op. + return nil + } return h.fs } } @@ -230,12 +246,17 @@ func (r *CgroupRegistry) Register(cs []CgroupController, fs cgroupFS) error { return nil } -// Unregister removes a previously registered hierarchy from the registry. If -// the controller was not previously registered, Unregister is a no-op. +// Unregister removes a previously registered hierarchy from the registry. If no +// such hierarchy is registered, Unregister is a no-op. func (r *CgroupRegistry) Unregister(hid uint32) { r.mu.Lock() - defer r.mu.Unlock() + r.unregisterLocked(hid) + r.mu.Unlock() +} +// Precondition: Caller must hold r.mu. +// +checklocks:r.mu +func (r *CgroupRegistry) unregisterLocked(hid uint32) { if h, ok := r.hierarchies[hid]; ok { for name, _ := range h.controllers { delete(r.controllers, name) diff --git a/test/syscalls/linux/cgroup.cc b/test/syscalls/linux/cgroup.cc index a009ade7e..f29891571 100644 --- a/test/syscalls/linux/cgroup.cc +++ b/test/syscalls/linux/cgroup.cc @@ -227,6 +227,41 @@ TEST(Cgroup, MountRace) { EXPECT_NO_ERRNO(c.ContainsCallingProcess()); } +TEST(Cgroup, MountUnmountRace) { + SKIP_IF(!CgroupsAvailable()); + + TempPath mountpoint = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir()); + + const DisableSave ds; // Too many syscalls. + + auto mount_thread = [&mountpoint]() { + for (int i = 0; i < 100; ++i) { + mount("none", mountpoint.path().c_str(), "cgroup", 0, 0); + } + }; + auto unmount_thread = [&mountpoint]() { + for (int i = 0; i < 100; ++i) { + umount(mountpoint.path().c_str()); + } + }; + std::list<ScopedThread> threads; + for (int i = 0; i < 10; ++i) { + threads.emplace_back(mount_thread); + } + for (int i = 0; i < 10; ++i) { + threads.emplace_back(unmount_thread); + } + for (auto& t : threads) { + t.Join(); + } + + // We don't know how many mount refs are remaining, since the count depends on + // the ordering of mount and umount calls. Keep calling unmount until it + // returns an error. + while (umount(mountpoint.path().c_str()) == 0) { + } +} + TEST(Cgroup, UnmountRepeated) { SKIP_IF(!CgroupsAvailable()); |