1 files changed, 383 insertions, 0 deletions
diff --git a/pkg/sentry/vfs/epoll.go b/pkg/sentry/vfs/epoll.go
new file mode 100644
index 000000000..599c3131c
--- /dev/null
+++ b/pkg/sentry/vfs/epoll.go
@@ -0,0 +1,383 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package vfs
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+// epollCycleMu serializes attempts to register EpollInstances with other
+// EpollInstances in order to check for cycles.
+var epollCycleMu sync.Mutex
+
+// EpollInstance represents an epoll instance, as described by epoll(7).
+type EpollInstance struct {
+	vfsfd FileDescription
+	FileDescriptionDefaultImpl
+	DentryMetadataFileDescriptionImpl
+	NoLockFD
+
+	// q holds waiters on this EpollInstance.
+	q waiter.Queue
+
+	// interest is the set of file descriptors that are registered with the
+	// EpollInstance for monitoring. interest is protected by interestMu.
+	interestMu sync.Mutex
+	interest   map[epollInterestKey]*epollInterest
+
+	// mu protects fields in registered epollInterests.
+	mu sync.Mutex
+
+	// ready is the set of file descriptors that may be "ready" for I/O. Note
+	// that this must be an ordered list, not a map: "If more than maxevents
+	// file descriptors are ready when epoll_wait() is called, then successive
+	// epoll_wait() calls will round robin through the set of ready file
+	// descriptors. This behavior helps avoid starvation scenarios, where a
+	// process fails to notice that additional file descriptors are ready
+	// because it focuses on a set of file descriptors that are already known
+	// to be ready." - epoll_wait(2)
+	ready epollInterestList
+}
+
+type epollInterestKey struct {
+	// file is the registered FileDescription. No reference is held on file;
+	// instead, when the last reference is dropped, FileDescription.DecRef()
+	// removes the FileDescription from all EpollInstances. file is immutable.
+	file *FileDescription
+
+	// num is the file descriptor number with which this entry was registered.
+	// num is immutable.
+	num int32
+}
+
+// epollInterest represents an EpollInstance's interest in a file descriptor.
+type epollInterest struct {
+	// epoll is the owning EpollInstance. epoll is immutable.
+	epoll *EpollInstance
+
+	// key is the file to which this epollInterest applies. key is immutable.
+	key epollInterestKey
+
+	// waiter is registered with key.file. entry is protected by epoll.mu.
+	waiter waiter.Entry
+
+	// mask is the event mask associated with this registration, including
+	// flags EPOLLET and EPOLLONESHOT. mask is protected by epoll.mu.
+	mask uint32
+
+	// ready is true if epollInterestEntry is linked into epoll.ready. ready
+	// and epollInterestEntry are protected by epoll.mu.
+	ready bool
+	epollInterestEntry
+
+	// userData is the struct epoll_event::data associated with this
+	// epollInterest. userData is protected by epoll.mu.
+	userData [2]int32
+}
+
+// NewEpollInstanceFD returns a FileDescription representing a new epoll
+// instance. A reference is taken on the returned FileDescription.
+func (vfs *VirtualFilesystem) NewEpollInstanceFD() (*FileDescription, error) {
+	vd := vfs.NewAnonVirtualDentry("[eventpoll]")
+	defer vd.DecRef()
+	ep := &EpollInstance{
+		interest: make(map[epollInterestKey]*epollInterest),
+	}
+	if err := ep.vfsfd.Init(ep, linux.O_RDWR, vd.Mount(), vd.Dentry(), &FileDescriptionOptions{
+		DenyPRead:         true,
+		DenyPWrite:        true,
+		UseDentryMetadata: true,
+	}); err != nil {
+		return nil, err
+	}
+	return &ep.vfsfd, nil
+}
+
+// Release implements FileDescriptionImpl.Release.
+func (ep *EpollInstance) Release() {
+	// Unregister all polled fds.
+	ep.interestMu.Lock()
+	defer ep.interestMu.Unlock()
+	for key, epi := range ep.interest {
+		file := key.file
+		file.epollMu.Lock()
+		delete(file.epolls, epi)
+		file.epollMu.Unlock()
+		file.EventUnregister(&epi.waiter)
+	}
+	ep.interest = nil
+}
+
+// Readiness implements waiter.Waitable.Readiness.
+func (ep *EpollInstance) Readiness(mask waiter.EventMask) waiter.EventMask {
+	if mask&waiter.EventIn == 0 {
+		return 0
+	}
+	ep.mu.Lock()
+	for epi := ep.ready.Front(); epi != nil; epi = epi.Next() {
+		wmask := waiter.EventMaskFromLinux(epi.mask)
+		if epi.key.file.Readiness(wmask)&wmask != 0 {
+			ep.mu.Unlock()
+			return waiter.EventIn
+		}
+	}
+	ep.mu.Unlock()
+	return 0
+}
+
+// EventRegister implements waiter.Waitable.EventRegister.
+func (ep *EpollInstance) EventRegister(e *waiter.Entry, mask waiter.EventMask) {
+	ep.q.EventRegister(e, mask)
+}
+
+// EventUnregister implements waiter.Waitable.EventUnregister.
+func (ep *EpollInstance) EventUnregister(e *waiter.Entry) {
+	ep.q.EventUnregister(e)
+}
+
+// Seek implements FileDescriptionImpl.Seek.
+func (ep *EpollInstance) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	// Linux: fs/eventpoll.c:eventpoll_fops.llseek == noop_llseek
+	return 0, nil
+}
+
+// AddInterest implements the semantics of EPOLL_CTL_ADD.
+//
+// Preconditions: A reference must be held on file.
+func (ep *EpollInstance) AddInterest(file *FileDescription, num int32, event linux.EpollEvent) error {
+	// Check for cyclic polling if necessary.
+	subep, _ := file.impl.(*EpollInstance)
+	if subep != nil {
+		epollCycleMu.Lock()
+		// epollCycleMu must be locked for the rest of AddInterest to ensure
+		// that cyclic polling is not introduced after the check.
+		defer epollCycleMu.Unlock()
+		if subep.mightPoll(ep) {
+			return syserror.ELOOP
+		}
+	}
+
+	ep.interestMu.Lock()
+	defer ep.interestMu.Unlock()
+
+	// Fail if the key is already registered.
+	key := epollInterestKey{
+		file: file,
+		num:  num,
+	}
+	if _, ok := ep.interest[key]; ok {
+		return syserror.EEXIST
+	}
+
+	// Register interest in file.
+	mask := event.Events | linux.EPOLLERR | linux.EPOLLRDHUP
+	epi := &epollInterest{
+		epoll:    ep,
+		key:      key,
+		mask:     mask,
+		userData: event.Data,
+	}
+	epi.waiter.Callback = epi
+	ep.interest[key] = epi
+	wmask := waiter.EventMaskFromLinux(mask)
+	file.EventRegister(&epi.waiter, wmask)
+
+	// Check if the file is already ready.
+	if file.Readiness(wmask)&wmask != 0 {
+		epi.Callback(nil)
+	}
+
+	// Add epi to file.epolls so that it is removed when the last
+	// FileDescription reference is dropped.
+	file.epollMu.Lock()
+	if file.epolls == nil {
+		file.epolls = make(map[*epollInterest]struct{})
+	}
+	file.epolls[epi] = struct{}{}
+	file.epollMu.Unlock()
+
+	return nil
+}
+
+func (ep *EpollInstance) mightPoll(ep2 *EpollInstance) bool {
+	return ep.mightPollRecursive(ep2, 4) // Linux: fs/eventpoll.c:EP_MAX_NESTS
+}
+
+func (ep *EpollInstance) mightPollRecursive(ep2 *EpollInstance, remainingRecursion int) bool {
+	ep.interestMu.Lock()
+	defer ep.interestMu.Unlock()
+	for key := range ep.interest {
+		nextep, ok := key.file.impl.(*EpollInstance)
+		if !ok {
+			continue
+		}
+		if nextep == ep2 {
+			return true
+		}
+		if remainingRecursion == 0 {
+			return true
+		}
+		if nextep.mightPollRecursive(ep2, remainingRecursion-1) {
+			return true
+		}
+	}
+	return false
+}
+
+// ModifyInterest implements the semantics of EPOLL_CTL_MOD.
+//
+// Preconditions: A reference must be held on file.
+func (ep *EpollInstance) ModifyInterest(file *FileDescription, num int32, event linux.EpollEvent) error {
+	ep.interestMu.Lock()
+	defer ep.interestMu.Unlock()
+
+	// Fail if the key is not already registered.
+	epi, ok := ep.interest[epollInterestKey{
+		file: file,
+		num:  num,
+	}]
+	if !ok {
+		return syserror.ENOENT
+	}
+
+	// Update epi for the next call to ep.ReadEvents().
+	mask := event.Events | linux.EPOLLERR | linux.EPOLLRDHUP
+	ep.mu.Lock()
+	epi.mask = mask
+	epi.userData = event.Data
+	ep.mu.Unlock()
+
+	// Re-register with the new mask.
+	file.EventUnregister(&epi.waiter)
+	wmask := waiter.EventMaskFromLinux(mask)
+	file.EventRegister(&epi.waiter, wmask)
+
+	// Check if the file is already ready with the new mask.
+	if file.Readiness(wmask)&wmask != 0 {
+		epi.Callback(nil)
+	}
+
+	return nil
+}
+
+// DeleteInterest implements the semantics of EPOLL_CTL_DEL.
+//
+// Preconditions: A reference must be held on file.
+func (ep *EpollInstance) DeleteInterest(file *FileDescription, num int32) error {
+	ep.interestMu.Lock()
+	defer ep.interestMu.Unlock()
+
+	// Fail if the key is not already registered.
+	epi, ok := ep.interest[epollInterestKey{
+		file: file,
+		num:  num,
+	}]
+	if !ok {
+		return syserror.ENOENT
+	}
+
+	// Unregister from the file so that epi will no longer be readied.
+	file.EventUnregister(&epi.waiter)
+
+	// Forget about epi.
+	ep.removeLocked(epi)
+
+	file.epollMu.Lock()
+	delete(file.epolls, epi)
+	file.epollMu.Unlock()
+
+	return nil
+}
+
+// Callback implements waiter.EntryCallback.Callback.
+func (epi *epollInterest) Callback(*waiter.Entry) {
+	newReady := false
+	epi.epoll.mu.Lock()
+	if !epi.ready {
+		newReady = true
+		epi.ready = true
+		epi.epoll.ready.PushBack(epi)
+	}
+	epi.epoll.mu.Unlock()
+	if newReady {
+		epi.epoll.q.Notify(waiter.EventIn)
+	}
+}
+
+// Preconditions: ep.interestMu must be locked.
+func (ep *EpollInstance) removeLocked(epi *epollInterest) {
+	delete(ep.interest, epi.key)
+	ep.mu.Lock()
+	if epi.ready {
+		epi.ready = false
+		ep.ready.Remove(epi)
+	}
+	ep.mu.Unlock()
+}
+
+// ReadEvents reads up to len(events) ready events into events and returns the
+// number of events read.
+//
+// Preconditions: len(events) != 0.
+func (ep *EpollInstance) ReadEvents(events []linux.EpollEvent) int {
+	i := 0
+	// Hot path: avoid defer.
+	ep.mu.Lock()
+	var next *epollInterest
+	var requeue epollInterestList
+	for epi := ep.ready.Front(); epi != nil; epi = next {
+		next = epi.Next()
+		// Regardless of what else happens, epi is initially removed from the
+		// ready list.
+		ep.ready.Remove(epi)
+		wmask := waiter.EventMaskFromLinux(epi.mask)
+		ievents := epi.key.file.Readiness(wmask) & wmask
+		if ievents == 0 {
+			// Leave epi off the ready list.
+			epi.ready = false
+			continue
+		}
+		// Determine what we should do with epi.
+		switch {
+		case epi.mask&linux.EPOLLONESHOT != 0:
+			// Clear all events from the mask; they must be re-added by
+			// EPOLL_CTL_MOD.
+			epi.mask &= linux.EP_PRIVATE_BITS
+			fallthrough
+		case epi.mask&linux.EPOLLET != 0:
+			// Leave epi off the ready list.
+			epi.ready = false
+		default:
+			// Queue epi to be moved to the end of the ready list.
+			requeue.PushBack(epi)
+		}
+		// Report ievents.
+		events[i] = linux.EpollEvent{
+			Events: ievents.ToLinux(),
+			Data:   epi.userData,
+		}
+		i++
+		if i == len(events) {
+			break
+		}
+	}
+	ep.ready.PushBackList(&requeue)
+	ep.mu.Unlock()
+	return i
+}