Implement F_GETLK fcntl.

Fixes #5113.

PiperOrigin-RevId: 353313374

15 files changed, 329 insertions, 160 deletions

diff --git a/pkg/sentry/fs/lock/BUILD b/pkg/sentry/fs/lock/BUILD
index ae3331737..4d3b216d8 100644
--- a/pkg/sentry/fs/lock/BUILD
+++ b/pkg/sentry/fs/lock/BUILD
@@ -41,6 +41,8 @@ go_library(
     ],
     visibility = ["//pkg/sentry:internal"],
     deps = [
+        "//pkg/abi/linux",
+        "//pkg/context",
         "//pkg/log",
         "//pkg/sync",
         "//pkg/waiter",
diff --git a/pkg/sentry/fs/lock/lock.go b/pkg/sentry/fs/lock/lock.go
index 8a5d9c7eb..57686ce07 100644
--- a/pkg/sentry/fs/lock/lock.go
+++ b/pkg/sentry/fs/lock/lock.go
@@ -54,6 +54,8 @@ import (
 	"math"
 	"syscall"
 
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
@@ -83,6 +85,17 @@ const (
 // offset 0 to LockEOF.
 const LockEOF = math.MaxUint64
 
+// OwnerInfo describes the owner of a lock.
+//
+// TODO(gvisor.dev/issue/5264): We may need to add other fields in the future
+// (e.g., Linux's file_lock.fl_flags to support open file-descriptor locks).
+//
+// +stateify savable
+type OwnerInfo struct {
+	// PID is the process ID of the lock owner.
+	PID int32
+}
+
 // Lock is a regional file lock.  It consists of either a single writer
 // or a set of readers.
 //
@@ -92,14 +105,20 @@ const LockEOF = math.MaxUint64
 // A Lock may be downgraded from a write lock to a read lock only if
 // the write lock's uid is the same as the read lock.
 //
+// Accesses to Lock are synchronized through the Locks object to which it
+// belongs.
+//
 // +stateify savable
 type Lock struct {
 	// Readers are the set of read lock holders identified by UniqueID.
-	// If len(Readers) > 0 then HasWriter must be false.
-	Readers map[UniqueID]bool
+	// If len(Readers) > 0 then Writer must be nil.
+	Readers map[UniqueID]OwnerInfo
 
 	// Writer holds the writer unique ID. It's nil if there are no writers.
 	Writer UniqueID
+
+	// WriterInfo describes the writer. It is only meaningful if Writer != nil.
+	WriterInfo OwnerInfo
 }
 
 // Locks is a thread-safe wrapper around a LockSet.
@@ -135,14 +154,14 @@ const (
 // acquiring the lock in a non-blocking mode or "interrupted" if in a blocking mode.
 // Blocker is the interface used to provide blocking behavior, passing a nil Blocker
 // will result in non-blocking behavior.
-func (l *Locks) LockRegion(uid UniqueID, t LockType, r LockRange, block Blocker) bool {
+func (l *Locks) LockRegion(uid UniqueID, ownerPID int32, t LockType, r LockRange, block Blocker) bool {
 	for {
 		l.mu.Lock()
 
 		// Blocking locks must run in a loop because we'll be woken up whenever an unlock event
 		// happens for this lock. We will then attempt to take the lock again and if it fails
 		// continue blocking.
-		res := l.locks.lock(uid, t, r)
+		res := l.locks.lock(uid, ownerPID, t, r)
 		if !res && block != nil {
 			e, ch := waiter.NewChannelEntry(nil)
 			l.blockedQueue.EventRegister(&e, EventMaskAll)
@@ -161,6 +180,14 @@ func (l *Locks) LockRegion(uid UniqueID, t LockType, r LockRange, block Blocker)
 	}
 }
 
+// LockRegionVFS1 is a wrapper around LockRegion for VFS1, which does not implement
+// F_GETLK (and does not care about storing PIDs as a result).
+//
+// TODO(gvisor.dev/issue/1624): Delete.
+func (l *Locks) LockRegionVFS1(uid UniqueID, t LockType, r LockRange, block Blocker) bool {
+	return l.LockRegion(uid, 0 /* ownerPID */, t, r, block)
+}
+
 // UnlockRegion attempts to release a lock for the uid on a region of a file.
 // This operation is always successful, even if there did not exist a lock on
 // the requested region held by uid in the first place.
@@ -175,13 +202,14 @@ func (l *Locks) UnlockRegion(uid UniqueID, r LockRange) {
 
 // makeLock returns a new typed Lock that has either uid as its only reader
 // or uid as its only writer.
-func makeLock(uid UniqueID, t LockType) Lock {
-	value := Lock{Readers: make(map[UniqueID]bool)}
+func makeLock(uid UniqueID, ownerPID int32, t LockType) Lock {
+	value := Lock{Readers: make(map[UniqueID]OwnerInfo)}
 	switch t {
 	case ReadLock:
-		value.Readers[uid] = true
+		value.Readers[uid] = OwnerInfo{PID: ownerPID}
 	case WriteLock:
 		value.Writer = uid
+		value.WriterInfo = OwnerInfo{PID: ownerPID}
 	default:
 		panic(fmt.Sprintf("makeLock: invalid lock type %d", t))
 	}
@@ -190,17 +218,20 @@ func makeLock(uid UniqueID, t LockType) Lock {
 
 // isHeld returns true if uid is a holder of Lock.
 func (l Lock) isHeld(uid UniqueID) bool {
-	return l.Writer == uid || l.Readers[uid]
+	if _, ok := l.Readers[uid]; ok {
+		return true
+	}
+	return l.Writer == uid
 }
 
 // lock sets uid as a holder of a typed lock on Lock.
 //
 // Preconditions: canLock is true for the range containing this Lock.
-func (l *Lock) lock(uid UniqueID, t LockType) {
+func (l *Lock) lock(uid UniqueID, ownerPID int32, t LockType) {
 	switch t {
 	case ReadLock:
 		// If we are already a reader, then this is a no-op.
-		if l.Readers[uid] {
+		if _, ok := l.Readers[uid]; ok {
 			return
 		}
 		// We cannot downgrade a write lock to a read lock unless the
@@ -210,11 +241,11 @@ func (l *Lock) lock(uid UniqueID, t LockType) {
 				panic(fmt.Sprintf("lock: cannot downgrade write lock to read lock for uid %d, writer is %d", uid, l.Writer))
 			}
 			// Ensure that there is only one reader if upgrading.
-			l.Readers = make(map[UniqueID]bool)
+			l.Readers = make(map[UniqueID]OwnerInfo)
 			// Ensure that there is no longer a writer.
 			l.Writer = nil
 		}
-		l.Readers[uid] = true
+		l.Readers[uid] = OwnerInfo{PID: ownerPID}
 		return
 	case WriteLock:
 		// If we are already the writer, then this is a no-op.
@@ -228,13 +259,14 @@ func (l *Lock) lock(uid UniqueID, t LockType) {
 			if readers != 1 {
 				panic(fmt.Sprintf("lock: cannot upgrade read lock to write lock for uid %d, too many readers %v", uid, l.Readers))
 			}
-			if !l.Readers[uid] {
+			if _, ok := l.Readers[uid]; !ok {
 				panic(fmt.Sprintf("lock: cannot upgrade read lock to write lock for uid %d, conflicting reader %v", uid, l.Readers))
 			}
 		}
 		// Ensure that there is only a writer.
-		l.Readers = make(map[UniqueID]bool)
+		l.Readers = make(map[UniqueID]OwnerInfo)
 		l.Writer = uid
+		l.WriterInfo = OwnerInfo{PID: ownerPID}
 	default:
 		panic(fmt.Sprintf("lock: invalid lock type %d", t))
 	}
@@ -247,7 +279,7 @@ func (l LockSet) lockable(r LockRange, check func(value Lock) bool) bool {
 	// Get our starting point.
 	seg := l.LowerBoundSegment(r.Start)
 	for seg.Ok() && seg.Start() < r.End {
-		// Note that we don't care about overruning the end of the
+		// Note that we don't care about overrunning the end of the
 		// last segment because if everything checks out we'll just
 		// split the last segment.
 		if !check(seg.Value()) {
@@ -281,7 +313,7 @@ func (l LockSet) canLock(uid UniqueID, t LockType, r LockRange) bool {
 			if value.Writer == nil {
 				// Then this uid can only take a write lock if this is a private
 				// upgrade, meaning that the only reader is uid.
-				return len(value.Readers) == 1 && value.Readers[uid]
+				return value.isOnlyReader(uid)
 			}
 			// If the uid is already a writer on this region, then
 			// adding a write lock would be a no-op.
@@ -292,11 +324,19 @@ func (l LockSet) canLock(uid UniqueID, t LockType, r LockRange) bool {
 	}
 }
 
+func (l *Lock) isOnlyReader(uid UniqueID) bool {
+	if len(l.Readers) != 1 {
+		return false
+	}
+	_, ok := l.Readers[uid]
+	return ok
+}
+
 // lock returns true if uid took a lock of type t on the entire range of
 // LockRange.
 //
 // Preconditions: r.Start <= r.End (will panic otherwise).
-func (l *LockSet) lock(uid UniqueID, t LockType, r LockRange) bool {
+func (l *LockSet) lock(uid UniqueID, ownerPID int32, t LockType, r LockRange) bool {
 	if r.Start > r.End {
 		panic(fmt.Sprintf("lock: r.Start %d > r.End %d", r.Start, r.End))
 	}
@@ -317,7 +357,7 @@ func (l *LockSet) lock(uid UniqueID, t LockType, r LockRange) bool {
 	seg, gap := l.Find(r.Start)
 	if gap.Ok() {
 		// Fill in the gap and get the next segment to modify.
-		seg = l.Insert(gap, gap.Range().Intersect(r), makeLock(uid, t)).NextSegment()
+		seg = l.Insert(gap, gap.Range().Intersect(r), makeLock(uid, ownerPID, t)).NextSegment()
 	} else if seg.Start() < r.Start {
 		// Get our first segment to modify.
 		_, seg = l.Split(seg, r.Start)
@@ -331,12 +371,12 @@ func (l *LockSet) lock(uid UniqueID, t LockType, r LockRange) bool {
 		// Set the lock on the segment. This is guaranteed to
 		// always be safe, given canLock above.
 		value := seg.ValuePtr()
-		value.lock(uid, t)
+		value.lock(uid, ownerPID, t)
 
 		// Fill subsequent gaps.
 		gap = seg.NextGap()
 		if gr := gap.Range().Intersect(r); gr.Length() > 0 {
-			seg = l.Insert(gap, gr, makeLock(uid, t)).NextSegment()
+			seg = l.Insert(gap, gr, makeLock(uid, ownerPID, t)).NextSegment()
 		} else {
 			seg = gap.NextSegment()
 		}
@@ -380,7 +420,7 @@ func (l *LockSet) unlock(uid UniqueID, r LockRange) {
 			// only ever be one writer and no readers, then this
 			// lock should always be removed from the set.
 			remove = true
-		} else if value.Readers[uid] {
+		} else if _, ok := value.Readers[uid]; ok {
 			// If uid is the last reader, then just remove the entire
 			// segment.
 			if len(value.Readers) == 1 {
@@ -390,7 +430,7 @@ func (l *LockSet) unlock(uid UniqueID, r LockRange) {
 				// affecting any other segment's readers.  To do
 				// this, we need to make a copy of the Readers map
 				// and not add this uid.
-				newValue := Lock{Readers: make(map[UniqueID]bool)}
+				newValue := Lock{Readers: make(map[UniqueID]OwnerInfo)}
 				for k, v := range value.Readers {
 					if k != uid {
 						newValue.Readers[k] = v
@@ -451,3 +491,72 @@ func ComputeRange(start, length, offset int64) (LockRange, error) {
 	// Offset is guaranteed to be positive at this point.
 	return LockRange{Start: uint64(offset), End: end}, nil
 }
+
+// TestRegion checks whether the lock holder identified by uid can hold a lock
+// of type t on range r. It returns a Flock struct representing this
+// information as the F_GETLK fcntl does.
+//
+// Note that the PID returned in the flock structure is relative to the root PID
+// namespace. It needs to be converted to the caller's PID namespace before
+// returning to userspace.
+//
+// TODO(gvisor.dev/issue/5264): we don't support OFD locks through fcntl, which
+// would return a struct with pid = -1.
+func (l *Locks) TestRegion(ctx context.Context, uid UniqueID, t LockType, r LockRange) linux.Flock {
+	f := linux.Flock{Type: linux.F_UNLCK}
+	switch t {
+	case ReadLock:
+		l.testRegion(r, func(lock Lock, start, length uint64) bool {
+			if lock.Writer == nil || lock.Writer == uid {
+				return true
+			}
+			f.Type = linux.F_WRLCK
+			f.PID = lock.WriterInfo.PID
+			f.Start = int64(start)
+			f.Len = int64(length)
+			return false
+		})
+	case WriteLock:
+		l.testRegion(r, func(lock Lock, start, length uint64) bool {
+			if lock.Writer == nil {
+				for k, v := range lock.Readers {
+					if k != uid {
+						// Stop at the first conflict detected.
+						f.Type = linux.F_RDLCK
+						f.PID = v.PID
+						f.Start = int64(start)
+						f.Len = int64(length)
+						return false
+					}
+				}
+				return true
+			}
+			if lock.Writer == uid {
+				return true
+			}
+			f.Type = linux.F_WRLCK
+			f.PID = lock.WriterInfo.PID
+			f.Start = int64(start)
+			f.Len = int64(length)
+			return false
+		})
+	default:
+		panic(fmt.Sprintf("TestRegion: invalid lock type %d", t))
+	}
+	return f
+}
+
+func (l *Locks) testRegion(r LockRange, check func(lock Lock, start, length uint64) bool) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	seg := l.locks.LowerBoundSegment(r.Start)
+	for seg.Ok() && seg.Start() < r.End {
+		lock := seg.Value()
+		if !check(lock, seg.Start(), seg.End()-seg.Start()) {
+			// Stop at the first conflict detected.
+			return
+		}
+		seg = seg.NextSegment()
+	}
+}
diff --git a/pkg/sentry/fs/lock/lock_set_functions.go b/pkg/sentry/fs/lock/lock_set_functions.go
index 50a16e662..dcc17c0dc 100644
--- a/pkg/sentry/fs/lock/lock_set_functions.go
+++ b/pkg/sentry/fs/lock/lock_set_functions.go
@@ -40,7 +40,7 @@ func (lockSetFunctions) Merge(r1 LockRange, val1 Lock, r2 LockRange, val2 Lock)
 		return Lock{}, false
 	}
 	for k := range val1.Readers {
-		if !val2.Readers[k] {
+		if _, ok := val2.Readers[k]; !ok {
 			return Lock{}, false
 		}
 	}
@@ -53,11 +53,12 @@ func (lockSetFunctions) Merge(r1 LockRange, val1 Lock, r2 LockRange, val2 Lock)
 func (lockSetFunctions) Split(r LockRange, val Lock, split uint64) (Lock, Lock) {
 	// Copy the segment so that split segments don't contain map references
 	// to other segments.
-	val0 := Lock{Readers: make(map[UniqueID]bool)}
+	val0 := Lock{Readers: make(map[UniqueID]OwnerInfo)}
 	for k, v := range val.Readers {
 		val0.Readers[k] = v
 	}
 	val0.Writer = val.Writer
+	val0.WriterInfo = val.WriterInfo
 
 	return val, val0
 }
diff --git a/pkg/sentry/fs/lock/lock_test.go b/pkg/sentry/fs/lock/lock_test.go
index fad90984b..9878c04e1 100644
--- a/pkg/sentry/fs/lock/lock_test.go
+++ b/pkg/sentry/fs/lock/lock_test.go
@@ -30,12 +30,12 @@ func equals(e0, e1 []entry) bool {
 	}
 	for i := range e0 {
 		for k := range e0[i].Lock.Readers {
-			if !e1[i].Lock.Readers[k] {
+			if _, ok := e1[i].Lock.Readers[k]; !ok {
 				return false
 			}
 		}
 		for k := range e1[i].Lock.Readers {
-			if !e0[i].Lock.Readers[k] {
+			if _, ok := e0[i].Lock.Readers[k]; !ok {
 				return false
 			}
 		}
@@ -90,15 +90,15 @@ func TestCanLock(t *testing.T) {
 	// 0             1024         2048             3072        4096
 	l := fill([]entry{
 		{
-			Lock:      Lock{Readers: map[UniqueID]bool{1: true, 2: true}},
+			Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 2: OwnerInfo{}}},
 			LockRange: LockRange{0, 1024},
 		},
 		{
-			Lock:      Lock{Readers: map[UniqueID]bool{1: true}},
+			Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}}},
 			LockRange: LockRange{1024, 2048},
 		},
 		{
-			Lock:      Lock{Readers: map[UniqueID]bool{1: true, 3: true}},
+			Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 3: OwnerInfo{}}},
 			LockRange: LockRange{2048, 3072},
 		},
 		{
@@ -220,7 +220,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -266,7 +266,7 @@ func TestSetLock(t *testing.T) {
 			// 0          4096                    max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, 4096},
 				},
 				{
@@ -283,7 +283,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -302,7 +302,7 @@ func TestSetLock(t *testing.T) {
 					LockRange: LockRange{0, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -333,7 +333,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -351,7 +351,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -366,11 +366,11 @@ func TestSetLock(t *testing.T) {
 			// 0             4096                    max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -383,7 +383,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -398,15 +398,15 @@ func TestSetLock(t *testing.T) {
 			// 0         4096             8192    max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{4096, 8192},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{8192, LockEOF},
 				},
 			},
@@ -419,7 +419,7 @@ func TestSetLock(t *testing.T) {
 			// 0        1024                     max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, LockEOF},
 				},
 			},
@@ -434,7 +434,7 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -447,7 +447,7 @@ func TestSetLock(t *testing.T) {
 			// 0                           4096
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, 4096},
 				},
 			},
@@ -467,11 +467,11 @@ func TestSetLock(t *testing.T) {
 			// 0                                  max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, LockEOF},
 				},
 			},
@@ -484,7 +484,7 @@ func TestSetLock(t *testing.T) {
 			// 0        1024                     max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, LockEOF},
 				},
 			},
@@ -499,15 +499,15 @@ func TestSetLock(t *testing.T) {
 			// 0        1024            4096      max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{1024, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -520,15 +520,15 @@ func TestSetLock(t *testing.T) {
 			// 0            1024        2048  4096      max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, 2048},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -543,7 +543,7 @@ func TestSetLock(t *testing.T) {
 			// 0            1024       4096      max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
@@ -551,7 +551,7 @@ func TestSetLock(t *testing.T) {
 					LockRange: LockRange{1024, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -564,15 +564,15 @@ func TestSetLock(t *testing.T) {
 			// 0            1024        2048  4096      max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, 2048},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -587,7 +587,7 @@ func TestSetLock(t *testing.T) {
 			// 0            1024       3072  4096      max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
@@ -595,7 +595,7 @@ func TestSetLock(t *testing.T) {
 					LockRange: LockRange{1024, 3072},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -608,11 +608,11 @@ func TestSetLock(t *testing.T) {
 			// 0            1024        2048
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, 2048},
 				},
 			},
@@ -634,15 +634,15 @@ func TestSetLock(t *testing.T) {
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{1024, 2048},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{2048, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -676,7 +676,7 @@ func TestSetLock(t *testing.T) {
 			l := fill(test.before)
 
 			r := LockRange{Start: test.start, End: test.end}
-			success := l.lock(test.uid, test.lockType, r)
+			success := l.lock(test.uid, 0 /* ownerPID */, test.lockType, r)
 			var got []entry
 			for seg := l.FirstSegment(); seg.Ok(); seg = seg.NextSegment() {
 				got = append(got, entry{
@@ -739,7 +739,7 @@ func TestUnlock(t *testing.T) {
 			// 0                    max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -752,7 +752,7 @@ func TestUnlock(t *testing.T) {
 			// 0                    max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -765,7 +765,7 @@ func TestUnlock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -797,7 +797,7 @@ func TestUnlock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -810,7 +810,7 @@ func TestUnlock(t *testing.T) {
 			// 0     4096                    max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -849,7 +849,7 @@ func TestUnlock(t *testing.T) {
 			// 0                                  max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -862,7 +862,7 @@ func TestUnlock(t *testing.T) {
 			// 0                          4096
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}}},
 					LockRange: LockRange{0, 4096},
 				},
 			},
@@ -901,7 +901,7 @@ func TestUnlock(t *testing.T) {
 			// 0            1024        4096            max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
@@ -909,7 +909,7 @@ func TestUnlock(t *testing.T) {
 					LockRange: LockRange{1024, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -922,11 +922,11 @@ func TestUnlock(t *testing.T) {
 			// 0        1024  4096        max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -939,7 +939,7 @@ func TestUnlock(t *testing.T) {
 			// 0                        max uint64
 			before: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{0, LockEOF},
 				},
 			},
@@ -952,15 +952,15 @@ func TestUnlock(t *testing.T) {
 			// 0                1024            4096          max uint64
 			after: []entry{
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{1024, 4096},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true, 2: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}, 2: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -977,7 +977,7 @@ func TestUnlock(t *testing.T) {
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -994,7 +994,7 @@ func TestUnlock(t *testing.T) {
 					LockRange: LockRange{0, 8},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -1011,7 +1011,7 @@ func TestUnlock(t *testing.T) {
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{4096, LockEOF},
 				},
 			},
@@ -1028,11 +1028,11 @@ func TestUnlock(t *testing.T) {
 					LockRange: LockRange{0, 1024},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{1: OwnerInfo{}}},
 					LockRange: LockRange{4096, 8192},
 				},
 				{
-					Lock:      Lock{Readers: map[UniqueID]bool{0: true, 1: true}},
+					Lock:      Lock{Readers: map[UniqueID]OwnerInfo{0: OwnerInfo{}, 1: OwnerInfo{}}},
 					LockRange: LockRange{8192, LockEOF},
 				},
 			},
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index a43364b55..98f7bc52f 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -1944,19 +1944,19 @@ func (fd *fileDescription) RemoveXattr(ctx context.Context, name string) error {
 }
 
 // LockBSD implements vfs.FileDescriptionImpl.LockBSD.
-func (fd *fileDescription) LockBSD(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
+func (fd *fileDescription) LockBSD(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, block fslock.Blocker) error {
 	fd.lockLogging.Do(func() {
 		log.Infof("File lock using gofer file handled internally.")
 	})
-	return fd.LockFD.LockBSD(ctx, uid, t, block)
+	return fd.LockFD.LockBSD(ctx, uid, ownerPID, t, block)
 }
 
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
-func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
+func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
 	fd.lockLogging.Do(func() {
 		log.Infof("Range lock using gofer file handled internally.")
 	})
-	return fd.Locks().LockPOSIX(ctx, uid, t, r, block)
+	return fd.Locks().LockPOSIX(ctx, uid, ownerPID, t, r, block)
 }
 
 // UnlockPOSIX implements vfs.FileDescriptionImpl.UnlockPOSIX.
diff --git a/pkg/sentry/fsimpl/tmpfs/regular_file_test.go b/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
index 7a8e1f5e0..4393cc13b 100644
--- a/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
+++ b/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
@@ -140,32 +140,32 @@ func TestLocks(t *testing.T) {
 
 	uid1 := 123
 	uid2 := 456
-	if err := fd.Impl().LockBSD(ctx, uid1, lock.ReadLock, nil); err != nil {
+	if err := fd.Impl().LockBSD(ctx, uid1, 0 /* ownerPID */, lock.ReadLock, nil); err != nil {
 		t.Fatalf("fd.Impl().LockBSD failed: err = %v", err)
 	}
-	if err := fd.Impl().LockBSD(ctx, uid2, lock.ReadLock, nil); err != nil {
+	if err := fd.Impl().LockBSD(ctx, uid2, 0 /* ownerPID */, lock.ReadLock, nil); err != nil {
 		t.Fatalf("fd.Impl().LockBSD failed: err = %v", err)
 	}
-	if got, want := fd.Impl().LockBSD(ctx, uid2, lock.WriteLock, nil), syserror.ErrWouldBlock; got != want {
+	if got, want := fd.Impl().LockBSD(ctx, uid2, 0 /* ownerPID */, lock.WriteLock, nil), syserror.ErrWouldBlock; got != want {
 		t.Fatalf("fd.Impl().LockBSD failed: got = %v, want = %v", got, want)
 	}
 	if err := fd.Impl().UnlockBSD(ctx, uid1); err != nil {
 		t.Fatalf("fd.Impl().UnlockBSD failed: err = %v", err)
 	}
-	if err := fd.Impl().LockBSD(ctx, uid2, lock.WriteLock, nil); err != nil {
+	if err := fd.Impl().LockBSD(ctx, uid2, 0 /* ownerPID */, lock.WriteLock, nil); err != nil {
 		t.Fatalf("fd.Impl().LockBSD failed: err = %v", err)
 	}
 
-	if err := fd.Impl().LockPOSIX(ctx, uid1, lock.ReadLock, lock.LockRange{Start: 0, End: 1}, nil); err != nil {
+	if err := fd.Impl().LockPOSIX(ctx, uid1, 0 /* ownerPID */, lock.ReadLock, lock.LockRange{Start: 0, End: 1}, nil); err != nil {
 		t.Fatalf("fd.Impl().LockPOSIX failed: err = %v", err)
 	}
-	if err := fd.Impl().LockPOSIX(ctx, uid2, lock.ReadLock, lock.LockRange{Start: 1, End: 2}, nil); err != nil {
+	if err := fd.Impl().LockPOSIX(ctx, uid2, 0 /* ownerPID */, lock.ReadLock, lock.LockRange{Start: 1, End: 2}, nil); err != nil {
 		t.Fatalf("fd.Impl().LockPOSIX failed: err = %v", err)
 	}
-	if err := fd.Impl().LockPOSIX(ctx, uid1, lock.WriteLock, lock.LockRange{Start: 0, End: 1}, nil); err != nil {
+	if err := fd.Impl().LockPOSIX(ctx, uid1, 0 /* ownerPID */, lock.WriteLock, lock.LockRange{Start: 0, End: 1}, nil); err != nil {
 		t.Fatalf("fd.Impl().LockPOSIX failed: err = %v", err)
 	}
-	if got, want := fd.Impl().LockPOSIX(ctx, uid2, lock.ReadLock, lock.LockRange{Start: 0, End: 1}, nil), syserror.ErrWouldBlock; got != want {
+	if got, want := fd.Impl().LockPOSIX(ctx, uid2, 0 /* ownerPID */, lock.ReadLock, lock.LockRange{Start: 0, End: 1}, nil), syserror.ErrWouldBlock; got != want {
 		t.Fatalf("fd.Impl().LockPOSIX failed: got = %v, want = %v", got, want)
 	}
 	if err := fd.Impl().UnlockPOSIX(ctx, uid1, lock.LockRange{Start: 0, End: 1}); err != nil {
diff --git a/pkg/sentry/fsimpl/verity/verity.go b/pkg/sentry/fsimpl/verity/verity.go
index 9ce4bb12b..8645078a0 100644
--- a/pkg/sentry/fsimpl/verity/verity.go
+++ b/pkg/sentry/fsimpl/verity/verity.go
@@ -1104,8 +1104,8 @@ func (fd *fileDescription) Write(ctx context.Context, src usermem.IOSequence, op
 }
 
 // LockBSD implements vfs.FileDescriptionImpl.LockBSD.
-func (fd *fileDescription) LockBSD(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
-	return fd.lowerFD.LockBSD(ctx, t, block)
+func (fd *fileDescription) LockBSD(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, block fslock.Blocker) error {
+	return fd.lowerFD.LockBSD(ctx, ownerPID, t, block)
 }
 
 // UnlockBSD implements vfs.FileDescriptionImpl.UnlockBSD.
@@ -1114,8 +1114,8 @@ func (fd *fileDescription) UnlockBSD(ctx context.Context, uid fslock.UniqueID) e
 }
 
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
-func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
-	return fd.lowerFD.LockPOSIX(ctx, uid, t, r, block)
+func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
+	return fd.lowerFD.LockPOSIX(ctx, uid, ownerPID, t, r, block)
 }
 
 // UnlockPOSIX implements vfs.FileDescriptionImpl.UnlockPOSIX.
@@ -1123,6 +1123,11 @@ func (fd *fileDescription) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID,
 	return fd.lowerFD.UnlockPOSIX(ctx, uid, r)
 }
 
+// TestPOSIX implements vfs.FileDescriptionImpl.TestPOSIX.
+func (fd *fileDescription) TestPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange) (linux.Flock, error) {
+	return fd.lowerFD.TestPOSIX(ctx, uid, t, r)
+}
+
 // FileReadWriteSeeker is a helper struct to pass a vfs.FileDescription as
 // io.Reader/io.Writer/io.ReadSeeker/io.ReaderAt/io.WriterAt/etc.
 type FileReadWriteSeeker struct {
diff --git a/pkg/sentry/kernel/BUILD b/pkg/sentry/kernel/BUILD
index 0ee60569c..8a5b11d40 100644
--- a/pkg/sentry/kernel/BUILD
+++ b/pkg/sentry/kernel/BUILD
@@ -240,7 +240,6 @@ go_library(
         "//pkg/sentry/fs/lock",
         "//pkg/sentry/fs/timerfd",
         "//pkg/sentry/fsbridge",
-        "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/fsimpl/pipefs",
         "//pkg/sentry/fsimpl/sockfs",
         "//pkg/sentry/fsimpl/timerfd",
diff --git a/pkg/sentry/kernel/threads.go b/pkg/sentry/kernel/threads.go
index fdadb52c0..e9da99067 100644
--- a/pkg/sentry/kernel/threads.go
+++ b/pkg/sentry/kernel/threads.go
@@ -216,7 +216,7 @@ func (ns *PIDNamespace) TaskWithID(tid ThreadID) *Task {
 	return t
 }
 
-// ThreadGroupWithID returns the thread group lead by the task with thread ID
+// ThreadGroupWithID returns the thread group led by the task with thread ID
 // tid in PID namespace ns. If no task has that TID, or if the task with that
 // TID is not a thread group leader, ThreadGroupWithID returns nil.
 func (ns *PIDNamespace) ThreadGroupWithID(tid ThreadID) *ThreadGroup {
@@ -292,6 +292,11 @@ func (ns *PIDNamespace) UserNamespace() *auth.UserNamespace {
 	return ns.userns
 }
 
+// Root returns the root PID namespace of ns.
+func (ns *PIDNamespace) Root() *PIDNamespace {
+	return ns.owner.Root
+}
+
 // A threadGroupNode defines the relationship between a thread group and the
 // rest of the system. Conceptually, threadGroupNode is data belonging to the
 // owning TaskSet, as if TaskSet contained a field `nodes
@@ -485,3 +490,8 @@ func (t *Task) Parent() *Task {
 func (t *Task) ThreadID() ThreadID {
 	return t.tg.pidns.IDOfTask(t)
 }
+
+// TGIDInRoot returns t's TGID in the root PID namespace.
+func (t *Task) TGIDInRoot() ThreadID {
+	return t.tg.pidns.owner.Root.IDOfThreadGroup(t.tg)
+}
diff --git a/pkg/sentry/syscalls/linux/sys_file.go b/pkg/sentry/syscalls/linux/sys_file.go
index c33571f43..a6253626e 100644
--- a/pkg/sentry/syscalls/linux/sys_file.go
+++ b/pkg/sentry/syscalls/linux/sys_file.go
@@ -1014,12 +1014,12 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 			}
 			if cmd == linux.F_SETLK {
 				// Non-blocking lock, provide a nil lock.Blocker.
-				if !file.Dirent.Inode.LockCtx.Posix.LockRegion(t.FDTable(), lock.ReadLock, rng, nil) {
+				if !file.Dirent.Inode.LockCtx.Posix.LockRegionVFS1(t.FDTable(), lock.ReadLock, rng, nil) {
 					return 0, nil, syserror.EAGAIN
 				}
 			} else {
 				// Blocking lock, pass in the task to satisfy the lock.Blocker interface.
-				if !file.Dirent.Inode.LockCtx.Posix.LockRegion(t.FDTable(), lock.ReadLock, rng, t) {
+				if !file.Dirent.Inode.LockCtx.Posix.LockRegionVFS1(t.FDTable(), lock.ReadLock, rng, t) {
 					return 0, nil, syserror.EINTR
 				}
 			}
@@ -1030,12 +1030,12 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 			}
 			if cmd == linux.F_SETLK {
 				// Non-blocking lock, provide a nil lock.Blocker.
-				if !file.Dirent.Inode.LockCtx.Posix.LockRegion(t.FDTable(), lock.WriteLock, rng, nil) {
+				if !file.Dirent.Inode.LockCtx.Posix.LockRegionVFS1(t.FDTable(), lock.WriteLock, rng, nil) {
 					return 0, nil, syserror.EAGAIN
 				}
 			} else {
 				// Blocking lock, pass in the task to satisfy the lock.Blocker interface.
-				if !file.Dirent.Inode.LockCtx.Posix.LockRegion(t.FDTable(), lock.WriteLock, rng, t) {
+				if !file.Dirent.Inode.LockCtx.Posix.LockRegionVFS1(t.FDTable(), lock.WriteLock, rng, t) {
 					return 0, nil, syserror.EINTR
 				}
 			}
@@ -2167,24 +2167,24 @@ func Flock(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 	case linux.LOCK_EX:
 		if nonblocking {
 			// Since we're nonblocking we pass a nil lock.Blocker implementation.
-			if !file.Dirent.Inode.LockCtx.BSD.LockRegion(file, lock.WriteLock, rng, nil) {
+			if !file.Dirent.Inode.LockCtx.BSD.LockRegionVFS1(file, lock.WriteLock, rng, nil) {
 				return 0, nil, syserror.EWOULDBLOCK
 			}
 		} else {
 			// Because we're blocking we will pass the task to satisfy the lock.Blocker interface.
-			if !file.Dirent.Inode.LockCtx.BSD.LockRegion(file, lock.WriteLock, rng, t) {
+			if !file.Dirent.Inode.LockCtx.BSD.LockRegionVFS1(file, lock.WriteLock, rng, t) {
 				return 0, nil, syserror.EINTR
 			}
 		}
 	case linux.LOCK_SH:
 		if nonblocking {
 			// Since we're nonblocking we pass a nil lock.Blocker implementation.
-			if !file.Dirent.Inode.LockCtx.BSD.LockRegion(file, lock.ReadLock, rng, nil) {
+			if !file.Dirent.Inode.LockCtx.BSD.LockRegionVFS1(file, lock.ReadLock, rng, nil) {
 				return 0, nil, syserror.EWOULDBLOCK
 			}
 		} else {
 			// Because we're blocking we will pass the task to satisfy the lock.Blocker interface.
-			if !file.Dirent.Inode.LockCtx.BSD.LockRegion(file, lock.ReadLock, rng, t) {
+			if !file.Dirent.Inode.LockCtx.BSD.LockRegionVFS1(file, lock.ReadLock, rng, t) {
 				return 0, nil, syserror.EINTR
 			}
 		}
diff --git a/pkg/sentry/syscalls/linux/vfs2/fd.go b/pkg/sentry/syscalls/linux/vfs2/fd.go
index 9281952cb..e39f074f2 100644
--- a/pkg/sentry/syscalls/linux/vfs2/fd.go
+++ b/pkg/sentry/syscalls/linux/vfs2/fd.go
@@ -205,8 +205,12 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		}
 		err := tmpfs.AddSeals(file, args[2].Uint())
 		return 0, nil, err
-	case linux.F_SETLK, linux.F_SETLKW:
-		return 0, nil, posixLock(t, args, file, cmd)
+	case linux.F_SETLK:
+		return 0, nil, posixLock(t, args, file, false /* blocking */)
+	case linux.F_SETLKW:
+		return 0, nil, posixLock(t, args, file, true /* blocking */)
+	case linux.F_GETLK:
+		return 0, nil, posixTestLock(t, args, file)
 	case linux.F_GETSIG:
 		a := file.AsyncHandler()
 		if a == nil {
@@ -292,7 +296,49 @@ func setAsyncOwner(t *kernel.Task, fd int, file *vfs.FileDescription, ownerType,
 	}
 }
 
-func posixLock(t *kernel.Task, args arch.SyscallArguments, file *vfs.FileDescription, cmd int32) error {
+func posixTestLock(t *kernel.Task, args arch.SyscallArguments, file *vfs.FileDescription) error {
+	// Copy in the lock request.
+	flockAddr := args[2].Pointer()
+	var flock linux.Flock
+	if _, err := flock.CopyIn(t, flockAddr); err != nil {
+		return err
+	}
+	var typ lock.LockType
+	switch flock.Type {
+	case linux.F_RDLCK:
+		typ = lock.ReadLock
+	case linux.F_WRLCK:
+		typ = lock.WriteLock
+	default:
+		return syserror.EINVAL
+	}
+	r, err := file.ComputeLockRange(t, uint64(flock.Start), uint64(flock.Len), flock.Whence)
+	if err != nil {
+		return err
+	}
+
+	newFlock, err := file.TestPOSIX(t, t.FDTable(), typ, r)
+	if err != nil {
+		return err
+	}
+	newFlock.PID = translatePID(t.PIDNamespace().Root(), t.PIDNamespace(), newFlock.PID)
+	if _, err = newFlock.CopyOut(t, flockAddr); err != nil {
+		return err
+	}
+	return nil
+}
+
+// translatePID translates a pid from one namespace to another. Note that this
+// may race with task termination/creation, in which case the original task
+// corresponding to pid may no longer exist. This is used to implement the
+// F_GETLK fcntl, which has the same potential race in Linux as well (i.e.,
+// there is no synchronization between retrieving the lock PID and translating
+// it). See fs/locks.c:posix_lock_to_flock.
+func translatePID(old, new *kernel.PIDNamespace, pid int32) int32 {
+	return int32(new.IDOfTask(old.TaskWithID(kernel.ThreadID(pid))))
+}
+
+func posixLock(t *kernel.Task, args arch.SyscallArguments, file *vfs.FileDescription, blocking bool) error {
 	// Copy in the lock request.
 	flockAddr := args[2].Pointer()
 	var flock linux.Flock
@@ -301,7 +347,7 @@ func posixLock(t *kernel.Task, args arch.SyscallArguments, file *vfs.FileDescrip
 	}
 
 	var blocker lock.Blocker
-	if cmd == linux.F_SETLKW {
+	if blocking {
 		blocker = t
 	}
 
@@ -315,13 +361,13 @@ func posixLock(t *kernel.Task, args arch.SyscallArguments, file *vfs.FileDescrip
 		if !file.IsReadable() {
 			return syserror.EBADF
 		}
-		return file.LockPOSIX(t, t.FDTable(), lock.ReadLock, r, blocker)
+		return file.LockPOSIX(t, t.FDTable(), int32(t.TGIDInRoot()), lock.ReadLock, r, blocker)
 
 	case linux.F_WRLCK:
 		if !file.IsWritable() {
 			return syserror.EBADF
 		}
-		return file.LockPOSIX(t, t.FDTable(), lock.WriteLock, r, blocker)
+		return file.LockPOSIX(t, t.FDTable(), int32(t.TGIDInRoot()), lock.WriteLock, r, blocker)
 
 	case linux.F_UNLCK:
 		return file.UnlockPOSIX(t, t.FDTable(), r)
diff --git a/pkg/sentry/syscalls/linux/vfs2/lock.go b/pkg/sentry/syscalls/linux/vfs2/lock.go
index b910b5a74..d1452a04d 100644
--- a/pkg/sentry/syscalls/linux/vfs2/lock.go
+++ b/pkg/sentry/syscalls/linux/vfs2/lock.go
@@ -44,11 +44,11 @@ func Flock(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 
 	switch operation {
 	case linux.LOCK_EX:
-		if err := file.LockBSD(t, lock.WriteLock, blocker); err != nil {
+		if err := file.LockBSD(t, int32(t.TGIDInRoot()), lock.WriteLock, blocker); err != nil {
 			return 0, nil, err
 		}
 	case linux.LOCK_SH:
-		if err := file.LockBSD(t, lock.ReadLock, blocker); err != nil {
+		if err := file.LockBSD(t, int32(t.TGIDInRoot()), lock.ReadLock, blocker); err != nil {
 			return 0, nil, err
 		}
 	case linux.LOCK_UN:
diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index dbf8425b4..eb6c2e36b 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -448,16 +448,19 @@ type FileDescriptionImpl interface {
 	RemoveXattr(ctx context.Context, name string) error
 
 	// LockBSD tries to acquire a BSD-style advisory file lock.
-	LockBSD(ctx context.Context, uid lock.UniqueID, t lock.LockType, block lock.Blocker) error
+	LockBSD(ctx context.Context, uid lock.UniqueID, ownerPID int32, t lock.LockType, block lock.Blocker) error
 
 	// UnlockBSD releases a BSD-style advisory file lock.
 	UnlockBSD(ctx context.Context, uid lock.UniqueID) error
 
 	// LockPOSIX tries to acquire a POSIX-style advisory file lock.
-	LockPOSIX(ctx context.Context, uid lock.UniqueID, t lock.LockType, r lock.LockRange, block lock.Blocker) error
+	LockPOSIX(ctx context.Context, uid lock.UniqueID, ownerPID int32, t lock.LockType, r lock.LockRange, block lock.Blocker) error
 
 	// UnlockPOSIX releases a POSIX-style advisory file lock.
 	UnlockPOSIX(ctx context.Context, uid lock.UniqueID, ComputeLockRange lock.LockRange) error
+
+	// TestPOSIX returns information about whether the specified lock can be held, in the style of the F_GETLK fcntl.
+	TestPOSIX(ctx context.Context, uid lock.UniqueID, t lock.LockType, r lock.LockRange) (linux.Flock, error)
 }
 
 // Dirent holds the information contained in struct linux_dirent64.
@@ -791,9 +794,9 @@ func (fd *FileDescription) Msync(ctx context.Context, mr memmap.MappableRange) e
 }
 
 // LockBSD tries to acquire a BSD-style advisory file lock.
-func (fd *FileDescription) LockBSD(ctx context.Context, lockType lock.LockType, blocker lock.Blocker) error {
+func (fd *FileDescription) LockBSD(ctx context.Context, ownerPID int32, lockType lock.LockType, blocker lock.Blocker) error {
 	atomic.StoreUint32(&fd.usedLockBSD, 1)
-	return fd.impl.LockBSD(ctx, fd, lockType, blocker)
+	return fd.impl.LockBSD(ctx, fd, ownerPID, lockType, blocker)
 }
 
 // UnlockBSD releases a BSD-style advisory file lock.
@@ -802,8 +805,8 @@ func (fd *FileDescription) UnlockBSD(ctx context.Context) error {
 }
 
 // LockPOSIX locks a POSIX-style file range lock.
-func (fd *FileDescription) LockPOSIX(ctx context.Context, uid lock.UniqueID, t lock.LockType, r lock.LockRange, block lock.Blocker) error {
-	return fd.impl.LockPOSIX(ctx, uid, t, r, block)
+func (fd *FileDescription) LockPOSIX(ctx context.Context, uid lock.UniqueID, ownerPID int32, t lock.LockType, r lock.LockRange, block lock.Blocker) error {
+	return fd.impl.LockPOSIX(ctx, uid, ownerPID, t, r, block)
 }
 
 // UnlockPOSIX unlocks a POSIX-style file range lock.
@@ -811,6 +814,11 @@ func (fd *FileDescription) UnlockPOSIX(ctx context.Context, uid lock.UniqueID, r
 	return fd.impl.UnlockPOSIX(ctx, uid, r)
 }
 
+// TestPOSIX returns information about whether the specified lock can be held.
+func (fd *FileDescription) TestPOSIX(ctx context.Context, uid lock.UniqueID, t lock.LockType, r lock.LockRange) (linux.Flock, error) {
+	return fd.impl.TestPOSIX(ctx, uid, t, r)
+}
+
 // ComputeLockRange computes the range of a file lock based on the given values.
 func (fd *FileDescription) ComputeLockRange(ctx context.Context, start uint64, length uint64, whence int16) (lock.LockRange, error) {
 	var off int64
diff --git a/pkg/sentry/vfs/file_description_impl_util.go b/pkg/sentry/vfs/file_description_impl_util.go
index 9d13e2ccb..eb7d2fd3b 100644
--- a/pkg/sentry/vfs/file_description_impl_util.go
+++ b/pkg/sentry/vfs/file_description_impl_util.go
@@ -419,8 +419,8 @@ func (fd *LockFD) Locks() *FileLocks {
 }
 
 // LockBSD implements vfs.FileDescriptionImpl.LockBSD.
-func (fd *LockFD) LockBSD(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
-	return fd.locks.LockBSD(uid, t, block)
+func (fd *LockFD) LockBSD(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, block fslock.Blocker) error {
+	return fd.locks.LockBSD(ctx, uid, ownerPID, t, block)
 }
 
 // UnlockBSD implements vfs.FileDescriptionImpl.UnlockBSD.
@@ -430,8 +430,8 @@ func (fd *LockFD) UnlockBSD(ctx context.Context, uid fslock.UniqueID) error {
 }
 
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
-func (fd *LockFD) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
-	return fd.locks.LockPOSIX(ctx, uid, t, r, block)
+func (fd *LockFD) LockPOSIX(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
+	return fd.locks.LockPOSIX(ctx, uid, ownerPID, t, r, block)
 }
 
 // UnlockPOSIX implements vfs.FileDescriptionImpl.UnlockPOSIX.
@@ -439,6 +439,11 @@ func (fd *LockFD) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, r fslock
 	return fd.locks.UnlockPOSIX(ctx, uid, r)
 }
 
+// TestPOSIX implements vfs.FileDescriptionImpl.TestPOSIX.
+func (fd *LockFD) TestPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange) (linux.Flock, error) {
+	return fd.locks.TestPOSIX(ctx, uid, t, r)
+}
+
 // NoLockFD implements Lock*/Unlock* portion of FileDescriptionImpl interface
 // returning ENOLCK.
 //
@@ -446,7 +451,7 @@ func (fd *LockFD) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, r fslock
 type NoLockFD struct{}
 
 // LockBSD implements vfs.FileDescriptionImpl.LockBSD.
-func (NoLockFD) LockBSD(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
+func (NoLockFD) LockBSD(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, block fslock.Blocker) error {
 	return syserror.ENOLCK
 }
 
@@ -456,7 +461,7 @@ func (NoLockFD) UnlockBSD(ctx context.Context, uid fslock.UniqueID) error {
 }
 
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
-func (NoLockFD) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
+func (NoLockFD) LockPOSIX(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
 	return syserror.ENOLCK
 }
 
@@ -464,3 +469,8 @@ func (NoLockFD) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.Loc
 func (NoLockFD) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, r fslock.LockRange) error {
 	return syserror.ENOLCK
 }
+
+// TestPOSIX implements vfs.FileDescriptionImpl.TestPOSIX.
+func (NoLockFD) TestPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange) (linux.Flock, error) {
+	return linux.Flock{}, syserror.ENOLCK
+}
diff --git a/pkg/sentry/vfs/lock.go b/pkg/sentry/vfs/lock.go
index bcff35792..cbe4d8c2d 100644
--- a/pkg/sentry/vfs/lock.go
+++ b/pkg/sentry/vfs/lock.go
@@ -39,8 +39,8 @@ type FileLocks struct {
 }
 
 // LockBSD tries to acquire a BSD-style lock on the entire file.
-func (fl *FileLocks) LockBSD(uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
-	if fl.bsd.LockRegion(uid, t, fslock.LockRange{0, fslock.LockEOF}, block) {
+func (fl *FileLocks) LockBSD(ctx context.Context, uid fslock.UniqueID, ownerID int32, t fslock.LockType, block fslock.Blocker) error {
+	if fl.bsd.LockRegion(uid, ownerID, t, fslock.LockRange{0, fslock.LockEOF}, block) {
 		return nil
 	}
 
@@ -61,8 +61,8 @@ func (fl *FileLocks) UnlockBSD(uid fslock.UniqueID) {
 }
 
 // LockPOSIX tries to acquire a POSIX-style lock on a file region.
-func (fl *FileLocks) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
-	if fl.posix.LockRegion(uid, t, r, block) {
+func (fl *FileLocks) LockPOSIX(ctx context.Context, uid fslock.UniqueID, ownerPID int32, t fslock.LockType, r fslock.LockRange, block fslock.Blocker) error {
+	if fl.posix.LockRegion(uid, ownerPID, t, r, block) {
 		return nil
 	}
 
@@ -83,28 +83,7 @@ func (fl *FileLocks) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, r fsl
 	return nil
 }
 
-func computeRange(ctx context.Context, fd *FileDescription, start uint64, length uint64, whence int16) (fslock.LockRange, error) {
-	var off int64
-	switch whence {
-	case linux.SEEK_SET:
-		off = 0
-	case linux.SEEK_CUR:
-		// Note that Linux does not hold any mutexes while retrieving the file
-		// offset, see fs/locks.c:flock_to_posix_lock and fs/locks.c:fcntl_setlk.
-		curOff, err := fd.Seek(ctx, 0, linux.SEEK_CUR)
-		if err != nil {
-			return fslock.LockRange{}, err
-		}
-		off = curOff
-	case linux.SEEK_END:
-		stat, err := fd.Stat(ctx, StatOptions{Mask: linux.STATX_SIZE})
-		if err != nil {
-			return fslock.LockRange{}, err
-		}
-		off = int64(stat.Size)
-	default:
-		return fslock.LockRange{}, syserror.EINVAL
-	}
-
-	return fslock.ComputeRange(int64(start), int64(length), off)
+// TestPOSIX returns information about whether the specified lock can be held, in the style of the F_GETLK fcntl.
+func (fl *FileLocks) TestPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, r fslock.LockRange) (linux.Flock, error) {
+	return fl.posix.TestRegion(ctx, uid, t, r), nil
 }