Implement `fcntl` options `F_GETSIG` and `F_SETSIG`.

These options allow overriding the signal that gets sent to the process when
I/O operations are available on the file descriptor, rather than the default
`SIGIO` signal. Doing so also populates `siginfo` to contain extra information
about which file descriptor caused the event (`si_fd`) and what events happened
on it (`si_band`). The logic around which FD is populated within `si_fd`
matches Linux's, which means it has some weird edge cases where that value may
not actually refer to a file descriptor that is still valid.

This CL also ports extra S/R logic regarding async handler in VFS2.
Without this, async I/O handlers aren't properly re-registered after S/R.

PiperOrigin-RevId: 345436598

16 files changed, 610 insertions, 118 deletions

diff --git a/pkg/abi/linux/fcntl.go b/pkg/abi/linux/fcntl.go
index cc3571fad..d1ca56370 100644
--- a/pkg/abi/linux/fcntl.go
+++ b/pkg/abi/linux/fcntl.go
@@ -25,6 +25,8 @@ const (
 	F_SETLKW        = 7
 	F_SETOWN        = 8
 	F_GETOWN        = 9
+	F_SETSIG        = 10
+	F_GETSIG        = 11
 	F_SETOWN_EX     = 15
 	F_GETOWN_EX     = 16
 	F_DUPFD_CLOEXEC = 1024 + 6
diff --git a/pkg/sentry/arch/signal.go b/pkg/sentry/arch/signal.go
index c9fb55d00..5138f3bf5 100644
--- a/pkg/sentry/arch/signal.go
+++ b/pkg/sentry/arch/signal.go
@@ -251,3 +251,26 @@ func (s *SignalInfo) Arch() uint32 {
 func (s *SignalInfo) SetArch(val uint32) {
 	usermem.ByteOrder.PutUint32(s.Fields[12:16], val)
 }
+
+// Band returns the si_band field.
+func (s *SignalInfo) Band() int64 {
+	return int64(usermem.ByteOrder.Uint64(s.Fields[0:8]))
+}
+
+// SetBand mutates the si_band field.
+func (s *SignalInfo) SetBand(val int64) {
+	// Note: this assumes the platform uses `long` as `__ARCH_SI_BAND_T`.
+	// On some platforms, which gVisor doesn't support, `__ARCH_SI_BAND_T` is
+	// `int`. See siginfo.h.
+	usermem.ByteOrder.PutUint64(s.Fields[0:8], uint64(val))
+}
+
+// FD returns the si_fd field.
+func (s *SignalInfo) FD() uint32 {
+	return usermem.ByteOrder.Uint32(s.Fields[8:12])
+}
+
+// SetFD mutates the si_fd field.
+func (s *SignalInfo) SetFD(val uint32) {
+	usermem.ByteOrder.PutUint32(s.Fields[8:12], val)
+}
diff --git a/pkg/sentry/fsimpl/overlay/regular_file.go b/pkg/sentry/fsimpl/overlay/regular_file.go
index 2b89a7a6d..25c785fd4 100644
--- a/pkg/sentry/fsimpl/overlay/regular_file.go
+++ b/pkg/sentry/fsimpl/overlay/regular_file.go
@@ -103,8 +103,8 @@ func (fd *regularFileFD) currentFDLocked(ctx context.Context) (*vfs.FileDescript
 			for e, mask := range fd.lowerWaiters {
 				fd.cachedFD.EventUnregister(e)
 				upperFD.EventRegister(e, mask)
-				if ready&mask != 0 {
-					e.Callback.Callback(e)
+				if m := ready & mask; m != 0 {
+					e.Callback.Callback(e, m)
 				}
 			}
 		}
diff --git a/pkg/sentry/kernel/epoll/epoll.go b/pkg/sentry/kernel/epoll/epoll.go
index 15519f0df..61aeca044 100644
--- a/pkg/sentry/kernel/epoll/epoll.go
+++ b/pkg/sentry/kernel/epoll/epoll.go
@@ -273,7 +273,7 @@ func (e *EventPoll) ReadEvents(max int) []linux.EpollEvent {
 //
 // Callback is called when one of the files we're polling becomes ready. It
 // moves said file to the readyList if it's currently in the waiting list.
-func (p *pollEntry) Callback(*waiter.Entry) {
+func (p *pollEntry) Callback(*waiter.Entry, waiter.EventMask) {
 	e := p.epoll
 
 	e.listsMu.Lock()
@@ -306,9 +306,8 @@ func (e *EventPoll) initEntryReadiness(entry *pollEntry) {
 	f.EventRegister(&entry.waiter, entry.mask)
 
 	// Check if the file happens to already be in a ready state.
-	ready := f.Readiness(entry.mask) & entry.mask
-	if ready != 0 {
-		entry.Callback(&entry.waiter)
+	if ready := f.Readiness(entry.mask) & entry.mask; ready != 0 {
+		entry.Callback(&entry.waiter, ready)
 	}
 }
 
diff --git a/pkg/sentry/kernel/fasync/BUILD b/pkg/sentry/kernel/fasync/BUILD
index 2b3955598..f855f038b 100644
--- a/pkg/sentry/kernel/fasync/BUILD
+++ b/pkg/sentry/kernel/fasync/BUILD
@@ -8,11 +8,13 @@ go_library(
     visibility = ["//:sandbox"],
     deps = [
         "//pkg/abi/linux",
+        "//pkg/sentry/arch",
         "//pkg/sentry/fs",
         "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/vfs",
         "//pkg/sync",
+        "//pkg/syserror",
         "//pkg/waiter",
     ],
 )
diff --git a/pkg/sentry/kernel/fasync/fasync.go b/pkg/sentry/kernel/fasync/fasync.go
index 153d2cd9b..b66d61c6f 100644
--- a/pkg/sentry/kernel/fasync/fasync.go
+++ b/pkg/sentry/kernel/fasync/fasync.go
@@ -17,22 +17,45 @@ package fasync
 
 import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
-// New creates a new fs.FileAsync.
-func New() fs.FileAsync {
-	return &FileAsync{}
+// Table to convert waiter event masks into si_band siginfo codes.
+// Taken from fs/fcntl.c:band_table.
+var bandTable = map[waiter.EventMask]int64{
+	// POLL_IN
+	waiter.EventIn: linux.EPOLLIN | linux.EPOLLRDNORM,
+	// POLL_OUT
+	waiter.EventOut: linux.EPOLLOUT | linux.EPOLLWRNORM | linux.EPOLLWRBAND,
+	// POLL_ERR
+	waiter.EventErr: linux.EPOLLERR,
+	// POLL_PRI
+	waiter.EventPri: linux.EPOLLPRI | linux.EPOLLRDBAND,
+	// POLL_HUP
+	waiter.EventHUp: linux.EPOLLHUP | linux.EPOLLERR,
 }
 
-// NewVFS2 creates a new vfs.FileAsync.
-func NewVFS2() vfs.FileAsync {
-	return &FileAsync{}
+// New returns a function that creates a new fs.FileAsync with the given file
+// descriptor.
+func New(fd int) func() fs.FileAsync {
+	return func() fs.FileAsync {
+		return &FileAsync{fd: fd}
+	}
+}
+
+// NewVFS2 returns a function that creates a new vfs.FileAsync with the given
+// file descriptor.
+func NewVFS2(fd int) func() vfs.FileAsync {
+	return func() vfs.FileAsync {
+		return &FileAsync{fd: fd}
+	}
 }
 
 // FileAsync sends signals when the registered file is ready for IO.
@@ -42,6 +65,12 @@ type FileAsync struct {
 	// e is immutable after first use (which is protected by mu below).
 	e waiter.Entry
 
+	// fd is the file descriptor to notify about.
+	// It is immutable, set at allocation time. This matches Linux semantics in
+	// fs/fcntl.c:fasync_helper.
+	// The fd value is passed to the signal recipient in siginfo.si_fd.
+	fd int
+
 	// regMu protects registeration and unregistration actions on e.
 	//
 	// regMu must be held while registration decisions are being made
@@ -56,6 +85,10 @@ type FileAsync struct {
 	mu         sync.Mutex `state:"nosave"`
 	requester  *auth.Credentials
 	registered bool
+	// signal is the signal to deliver upon I/O being available.
+	// The default value ("zero signal") means the default SIGIO signal will be
+	// delivered.
+	signal linux.Signal
 
 	// Only one of the following is allowed to be non-nil.
 	recipientPG *kernel.ProcessGroup
@@ -64,10 +97,10 @@ type FileAsync struct {
 }
 
 // Callback sends a signal.
-func (a *FileAsync) Callback(e *waiter.Entry) {
+func (a *FileAsync) Callback(e *waiter.Entry, mask waiter.EventMask) {
 	a.mu.Lock()
+	defer a.mu.Unlock()
 	if !a.registered {
-		a.mu.Unlock()
 		return
 	}
 	t := a.recipientT
@@ -80,19 +113,34 @@ func (a *FileAsync) Callback(e *waiter.Entry) {
 	}
 	if t == nil {
 		// No recipient has been registered.
-		a.mu.Unlock()
 		return
 	}
 	c := t.Credentials()
 	// Logic from sigio_perm in fs/fcntl.c.
-	if a.requester.EffectiveKUID == 0 ||
+	permCheck := (a.requester.EffectiveKUID == 0 ||
 		a.requester.EffectiveKUID == c.SavedKUID ||
 		a.requester.EffectiveKUID == c.RealKUID ||
 		a.requester.RealKUID == c.SavedKUID ||
-		a.requester.RealKUID == c.RealKUID {
-		t.SendSignal(kernel.SignalInfoPriv(linux.SIGIO))
+		a.requester.RealKUID == c.RealKUID)
+	if !permCheck {
+		return
 	}
-	a.mu.Unlock()
+	signalInfo := &arch.SignalInfo{
+		Signo: int32(linux.SIGIO),
+		Code:  arch.SignalInfoKernel,
+	}
+	if a.signal != 0 {
+		signalInfo.Signo = int32(a.signal)
+		signalInfo.SetFD(uint32(a.fd))
+		var band int64
+		for m, bandCode := range bandTable {
+			if m&mask != 0 {
+				band |= bandCode
+			}
+		}
+		signalInfo.SetBand(band)
+	}
+	t.SendSignal(signalInfo)
 }
 
 // Register sets the file which will be monitored for IO events.
@@ -186,3 +234,25 @@ func (a *FileAsync) ClearOwner() {
 	a.recipientTG = nil
 	a.recipientPG = nil
 }
+
+// Signal returns which signal will be sent to the signal recipient.
+// A value of zero means the signal to deliver wasn't customized, which means
+// the default signal (SIGIO) will be delivered.
+func (a *FileAsync) Signal() linux.Signal {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	return a.signal
+}
+
+// SetSignal overrides which signal to send when I/O is available.
+// The default behavior can be reset by specifying signal zero, which means
+// to send SIGIO.
+func (a *FileAsync) SetSignal(signal linux.Signal) error {
+	if signal != 0 && !signal.IsValid() {
+		return syserror.EINVAL
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.signal = signal
+	return nil
+}
diff --git a/pkg/sentry/syscalls/linux/sys_file.go b/pkg/sentry/syscalls/linux/sys_file.go
index 519066a47..8db587401 100644
--- a/pkg/sentry/syscalls/linux/sys_file.go
+++ b/pkg/sentry/syscalls/linux/sys_file.go
@@ -646,7 +646,7 @@ func Ioctl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		if _, err := primitive.CopyInt32In(t, args[2].Pointer(), &set); err != nil {
 			return 0, nil, err
 		}
-		fSetOwn(t, file, set)
+		fSetOwn(t, int(fd), file, set)
 		return 0, nil, nil
 
 	case linux.FIOGETOWN, linux.SIOCGPGRP:
@@ -901,8 +901,8 @@ func fGetOwn(t *kernel.Task, file *fs.File) int32 {
 //
 // If who is positive, it represents a PID. If negative, it represents a PGID.
 // If the PID or PGID is invalid, the owner is silently unset.
-func fSetOwn(t *kernel.Task, file *fs.File, who int32) error {
-	a := file.Async(fasync.New).(*fasync.FileAsync)
+func fSetOwn(t *kernel.Task, fd int, file *fs.File, who int32) error {
+	a := file.Async(fasync.New(fd)).(*fasync.FileAsync)
 	if who < 0 {
 		// Check for overflow before flipping the sign.
 		if who-1 > who {
@@ -1049,7 +1049,7 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 	case linux.F_GETOWN:
 		return uintptr(fGetOwn(t, file)), nil, nil
 	case linux.F_SETOWN:
-		return 0, nil, fSetOwn(t, file, args[2].Int())
+		return 0, nil, fSetOwn(t, int(fd), file, args[2].Int())
 	case linux.F_GETOWN_EX:
 		addr := args[2].Pointer()
 		owner := fGetOwnEx(t, file)
@@ -1062,7 +1062,7 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		if err != nil {
 			return 0, nil, err
 		}
-		a := file.Async(fasync.New).(*fasync.FileAsync)
+		a := file.Async(fasync.New(int(fd))).(*fasync.FileAsync)
 		switch owner.Type {
 		case linux.F_OWNER_TID:
 			task := t.PIDNamespace().TaskWithID(kernel.ThreadID(owner.PID))
@@ -1111,6 +1111,12 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		}
 		n, err := sz.SetFifoSize(int64(args[2].Int()))
 		return uintptr(n), nil, err
+	case linux.F_GETSIG:
+		a := file.Async(fasync.New(int(fd))).(*fasync.FileAsync)
+		return uintptr(a.Signal()), nil, nil
+	case linux.F_SETSIG:
+		a := file.Async(fasync.New(int(fd))).(*fasync.FileAsync)
+		return 0, nil, a.SetSignal(linux.Signal(args[2].Int()))
 	default:
 		// Everything else is not yet supported.
 		return 0, nil, syserror.EINVAL
diff --git a/pkg/sentry/syscalls/linux/vfs2/fd.go b/pkg/sentry/syscalls/linux/vfs2/fd.go
index 36e89700e..7dd9ef857 100644
--- a/pkg/sentry/syscalls/linux/vfs2/fd.go
+++ b/pkg/sentry/syscalls/linux/vfs2/fd.go
@@ -165,7 +165,7 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 			ownerType = linux.F_OWNER_PGRP
 			who = -who
 		}
-		return 0, nil, setAsyncOwner(t, file, ownerType, who)
+		return 0, nil, setAsyncOwner(t, int(fd), file, ownerType, who)
 	case linux.F_GETOWN_EX:
 		owner, hasOwner := getAsyncOwner(t, file)
 		if !hasOwner {
@@ -179,7 +179,7 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		if err != nil {
 			return 0, nil, err
 		}
-		return 0, nil, setAsyncOwner(t, file, owner.Type, owner.PID)
+		return 0, nil, setAsyncOwner(t, int(fd), file, owner.Type, owner.PID)
 	case linux.F_SETPIPE_SZ:
 		pipefile, ok := file.Impl().(*pipe.VFSPipeFD)
 		if !ok {
@@ -207,6 +207,16 @@ func Fcntl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 		return 0, nil, err
 	case linux.F_SETLK, linux.F_SETLKW:
 		return 0, nil, posixLock(t, args, file, cmd)
+	case linux.F_GETSIG:
+		a := file.AsyncHandler()
+		if a == nil {
+			// Default behavior aka SIGIO.
+			return 0, nil, nil
+		}
+		return uintptr(a.(*fasync.FileAsync).Signal()), nil, nil
+	case linux.F_SETSIG:
+		a := file.SetAsyncHandler(fasync.NewVFS2(int(fd))).(*fasync.FileAsync)
+		return 0, nil, a.SetSignal(linux.Signal(args[2].Int()))
 	default:
 		// Everything else is not yet supported.
 		return 0, nil, syserror.EINVAL
@@ -241,7 +251,7 @@ func getAsyncOwner(t *kernel.Task, fd *vfs.FileDescription) (ownerEx linux.FOwne
 	}
 }
 
-func setAsyncOwner(t *kernel.Task, fd *vfs.FileDescription, ownerType, pid int32) error {
+func setAsyncOwner(t *kernel.Task, fd int, file *vfs.FileDescription, ownerType, pid int32) error {
 	switch ownerType {
 	case linux.F_OWNER_TID, linux.F_OWNER_PID, linux.F_OWNER_PGRP:
 		// Acceptable type.
@@ -249,7 +259,7 @@ func setAsyncOwner(t *kernel.Task, fd *vfs.FileDescription, ownerType, pid int32
 		return syserror.EINVAL
 	}
 
-	a := fd.SetAsyncHandler(fasync.NewVFS2).(*fasync.FileAsync)
+	a := file.SetAsyncHandler(fasync.NewVFS2(fd)).(*fasync.FileAsync)
 	if pid == 0 {
 		a.ClearOwner()
 		return nil
diff --git a/pkg/sentry/syscalls/linux/vfs2/ioctl.go b/pkg/sentry/syscalls/linux/vfs2/ioctl.go
index 2806c3f6f..20c264fef 100644
--- a/pkg/sentry/syscalls/linux/vfs2/ioctl.go
+++ b/pkg/sentry/syscalls/linux/vfs2/ioctl.go
@@ -100,7 +100,7 @@ func Ioctl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 			ownerType = linux.F_OWNER_PGRP
 			who = -who
 		}
-		return 0, nil, setAsyncOwner(t, file, ownerType, who)
+		return 0, nil, setAsyncOwner(t, int(fd), file, ownerType, who)
 	}
 
 	ret, err := file.Ioctl(t, t.MemoryManager(), args)
diff --git a/pkg/sentry/vfs/epoll.go b/pkg/sentry/vfs/epoll.go
index a98aac52b..072655fe8 100644
--- a/pkg/sentry/vfs/epoll.go
+++ b/pkg/sentry/vfs/epoll.go
@@ -204,8 +204,8 @@ func (ep *EpollInstance) AddInterest(file *FileDescription, num int32, event lin
 	file.EventRegister(&epi.waiter, wmask)
 
 	// Check if the file is already ready.
-	if file.Readiness(wmask)&wmask != 0 {
-		epi.Callback(nil)
+	if m := file.Readiness(wmask) & wmask; m != 0 {
+		epi.Callback(nil, m)
 	}
 
 	// Add epi to file.epolls so that it is removed when the last
@@ -274,8 +274,8 @@ func (ep *EpollInstance) ModifyInterest(file *FileDescription, num int32, event
 	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)
+	if m := file.Readiness(wmask) & wmask; m != 0 {
+		epi.Callback(nil, m)
 	}
 
 	return nil
@@ -311,7 +311,7 @@ func (ep *EpollInstance) DeleteInterest(file *FileDescription, num int32) error
 }
 
 // Callback implements waiter.EntryCallback.Callback.
-func (epi *epollInterest) Callback(*waiter.Entry) {
+func (epi *epollInterest) Callback(*waiter.Entry, waiter.EventMask) {
 	newReady := false
 	epi.epoll.mu.Lock()
 	if !epi.ready {
diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index 51fddbaff..2153382e9 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -43,7 +43,7 @@ import (
 type FileDescription struct {
 	FileDescriptionRefs
 
-	// flagsMu protects statusFlags and asyncHandler below.
+	// flagsMu protects `statusFlags`, `saved`, and `asyncHandler` below.
 	flagsMu sync.Mutex `state:"nosave"`
 
 	// statusFlags contains status flags, "initialized by open(2) and possibly
@@ -52,6 +52,11 @@ type FileDescription struct {
 	// access to asyncHandler.
 	statusFlags uint32
 
+	// saved is true after beforeSave is called. This is used to prevent
+	// double-unregistration of asyncHandler. This does not work properly for
+	// save-resume, which is not currently supported in gVisor (see b/26588733).
+	saved bool `state:"nosave"`
+
 	// asyncHandler handles O_ASYNC signal generation. It is set with the
 	// F_SETOWN or F_SETOWN_EX fcntls. For asyncHandler to be used, O_ASYNC must
 	// also be set by fcntl(2).
@@ -184,7 +189,7 @@ func (fd *FileDescription) DecRef(ctx context.Context) {
 		}
 		fd.vd.DecRef(ctx)
 		fd.flagsMu.Lock()
-		if fd.statusFlags&linux.O_ASYNC != 0 && fd.asyncHandler != nil {
+		if !fd.saved && fd.statusFlags&linux.O_ASYNC != 0 && fd.asyncHandler != nil {
 			fd.asyncHandler.Unregister(fd)
 		}
 		fd.asyncHandler = nil
diff --git a/pkg/sentry/vfs/save_restore.go b/pkg/sentry/vfs/save_restore.go
index 7723ed643..8f070ed53 100644
--- a/pkg/sentry/vfs/save_restore.go
+++ b/pkg/sentry/vfs/save_restore.go
@@ -18,8 +18,10 @@ import (
 	"fmt"
 	"sync/atomic"
 
+	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/refsvfs2"
+	"gvisor.dev/gvisor/pkg/waiter"
 )
 
 // FilesystemImplSaveRestoreExtension is an optional extension to
@@ -120,5 +122,20 @@ func (mnt *Mount) afterLoad() {
 func (epi *epollInterest) afterLoad() {
 	// Mark all epollInterests as ready after restore so that the next call to
 	// EpollInstance.ReadEvents() rechecks their readiness.
-	epi.Callback(nil)
+	epi.Callback(nil, waiter.EventMaskFromLinux(epi.mask))
+}
+
+// beforeSave is called by stateify.
+func (fd *FileDescription) beforeSave() {
+	fd.saved = true
+	if fd.statusFlags&linux.O_ASYNC != 0 && fd.asyncHandler != nil {
+		fd.asyncHandler.Unregister(fd)
+	}
+}
+
+// afterLoad is called by stateify.
+func (fd *FileDescription) afterLoad() {
+	if fd.statusFlags&linux.O_ASYNC != 0 && fd.asyncHandler != nil {
+		fd.asyncHandler.Register(fd)
+	}
 }
diff --git a/pkg/waiter/waiter.go b/pkg/waiter/waiter.go
index 08519d986..83d4f893a 100644
--- a/pkg/waiter/waiter.go
+++ b/pkg/waiter/waiter.go
@@ -119,7 +119,10 @@ type EntryCallback interface {
 	// The callback is supposed to perform minimal work, and cannot call
 	// any method on the queue itself because it will be locked while the
 	// callback is running.
-	Callback(e *Entry)
+	//
+	// The mask indicates the events that occurred and that the entry is
+	// interested in.
+	Callback(e *Entry, mask EventMask)
 }
 
 // Entry represents a waiter that can be add to the a wait queue. It can
@@ -140,7 +143,7 @@ type channelCallback struct {
 }
 
 // Callback implements EntryCallback.Callback.
-func (c *channelCallback) Callback(*Entry) {
+func (c *channelCallback) Callback(*Entry, EventMask) {
 	select {
 	case c.ch <- struct{}{}:
 	default:
@@ -193,8 +196,8 @@ func (q *Queue) EventUnregister(e *Entry) {
 func (q *Queue) Notify(mask EventMask) {
 	q.mu.RLock()
 	for e := q.list.Front(); e != nil; e = e.Next() {
-		if mask&e.mask != 0 {
-			e.Callback.Callback(e)
+		if m := mask & e.mask; m != 0 {
+			e.Callback.Callback(e, m)
 		}
 	}
 	q.mu.RUnlock()
diff --git a/pkg/waiter/waiter_test.go b/pkg/waiter/waiter_test.go
index c1b94a4f3..6928f28b4 100644
--- a/pkg/waiter/waiter_test.go
+++ b/pkg/waiter/waiter_test.go
@@ -20,12 +20,12 @@ import (
 )
 
 type callbackStub struct {
-	f func(e *Entry)
+	f func(e *Entry, m EventMask)
 }
 
 // Callback implements EntryCallback.Callback.
-func (c *callbackStub) Callback(e *Entry) {
-	c.f(e)
+func (c *callbackStub) Callback(e *Entry, m EventMask) {
+	c.f(e, m)
 }
 
 func TestEmptyQueue(t *testing.T) {
@@ -36,7 +36,7 @@ func TestEmptyQueue(t *testing.T) {
 
 	// Register then unregister a waiter, then notify the queue.
 	cnt := 0
-	e := Entry{Callback: &callbackStub{func(*Entry) { cnt++ }}}
+	e := Entry{Callback: &callbackStub{func(*Entry, EventMask) { cnt++ }}}
 	q.EventRegister(&e, EventIn)
 	q.EventUnregister(&e)
 	q.Notify(EventIn)
@@ -49,7 +49,7 @@ func TestMask(t *testing.T) {
 	// Register a waiter.
 	var q Queue
 	var cnt int
-	e := Entry{Callback: &callbackStub{func(*Entry) { cnt++ }}}
+	e := Entry{Callback: &callbackStub{func(*Entry, EventMask) { cnt++ }}}
 	q.EventRegister(&e, EventIn|EventErr)
 
 	// Notify with an overlapping mask.
@@ -101,11 +101,14 @@ func TestConcurrentRegistration(t *testing.T) {
 	for i := 0; i < concurrency; i++ {
 		go func() {
 			var e Entry
-			e.Callback = &callbackStub{func(entry *Entry) {
+			e.Callback = &callbackStub{func(entry *Entry, mask EventMask) {
 				cnt++
 				if entry != &e {
 					t.Errorf("entry = %p, want %p", entry, &e)
 				}
+				if mask != EventIn {
+					t.Errorf("mask = %#x want %#x", mask, EventIn)
+				}
 			}}
 
 			// Wait for notification, then register.
@@ -158,11 +161,14 @@ func TestConcurrentNotification(t *testing.T) {
 	// Register waiters.
 	for i := 0; i < waiterCount; i++ {
 		var e Entry
-		e.Callback = &callbackStub{func(entry *Entry) {
+		e.Callback = &callbackStub{func(entry *Entry, mask EventMask) {
 			atomic.AddInt32(&cnt, 1)
 			if entry != &e {
 				t.Errorf("entry = %p, want %p", entry, &e)
 			}
+			if mask != EventIn {
+				t.Errorf("mask = %#x want %#x", mask, EventIn)
+			}
 		}}
 
 		q.EventRegister(&e, EventIn|EventErr)
diff --git a/test/syscalls/linux/BUILD b/test/syscalls/linux/BUILD
index 946d06cd6..dec8c8100 100644
--- a/test/syscalls/linux/BUILD
+++ b/test/syscalls/linux/BUILD
@@ -799,8 +799,8 @@ cc_binary(
     deps = [
         ":socket_test_util",
         "//test/util:cleanup",
-        "//test/util:epoll_util",
         "//test/util:eventfd_util",
+        "//test/util:file_descriptor",
         "//test/util:fs_util",
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/flags:flag",
@@ -811,6 +811,7 @@ cc_binary(
         "//test/util:multiprocess_util",
         "//test/util:posix_error",
         "//test/util:save_util",
+        "//test/util:signal_util",
         "//test/util:temp_path",
         "//test/util:test_util",
         "//test/util:thread_util",
diff --git a/test/syscalls/linux/fcntl.cc b/test/syscalls/linux/fcntl.cc
index 34016d4bd..4b581045b 100644
--- a/test/syscalls/linux/fcntl.cc
+++ b/test/syscalls/linux/fcntl.cc
@@ -14,10 +14,13 @@
 
 #include <fcntl.h>
 #include <signal.h>
+#include <sys/epoll.h>
 #include <sys/types.h>
 #include <syscall.h>
 #include <unistd.h>
 
+#include <atomic>
+#include <deque>
 #include <iostream>
 #include <list>
 #include <string>
@@ -34,25 +37,27 @@
 #include "test/syscalls/linux/socket_test_util.h"
 #include "test/util/cleanup.h"
 #include "test/util/eventfd_util.h"
+#include "test/util/file_descriptor.h"
 #include "test/util/fs_util.h"
 #include "test/util/multiprocess_util.h"
 #include "test/util/posix_error.h"
 #include "test/util/save_util.h"
+#include "test/util/signal_util.h"
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
 #include "test/util/timer_util.h"
 
-ABSL_FLAG(std::string, child_setlock_on, "",
+ABSL_FLAG(std::string, child_set_lock_on, "",
           "Contains the path to try to set a file lock on.");
-ABSL_FLAG(bool, child_setlock_write, false,
+ABSL_FLAG(bool, child_set_lock_write, false,
           "Whether to set a writable lock (otherwise readable)");
 ABSL_FLAG(bool, blocking, false,
           "Whether to set a blocking lock (otherwise non-blocking).");
 ABSL_FLAG(bool, retry_eintr, false,
           "Whether to retry in the subprocess on EINTR.");
-ABSL_FLAG(uint64_t, child_setlock_start, 0, "The value of struct flock start");
-ABSL_FLAG(uint64_t, child_setlock_len, 0, "The value of struct flock len");
+ABSL_FLAG(uint64_t, child_set_lock_start, 0, "The value of struct flock start");
+ABSL_FLAG(uint64_t, child_set_lock_len, 0, "The value of struct flock len");
 ABSL_FLAG(int32_t, socket_fd, -1,
           "A socket to use for communicating more state back "
           "to the parent.");
@@ -60,6 +65,11 @@ ABSL_FLAG(int32_t, socket_fd, -1,
 namespace gvisor {
 namespace testing {
 
+std::function<void(int, siginfo_t*, void*)> setsig_signal_handle;
+void setsig_signal_handler(int signum, siginfo_t* siginfo, void* ucontext) {
+  setsig_signal_handle(signum, siginfo, ucontext);
+}
+
 class FcntlLockTest : public ::testing::Test {
  public:
   void SetUp() override {
@@ -84,18 +94,93 @@ class FcntlLockTest : public ::testing::Test {
   int fds_[2] = {};
 };
 
+struct SignalDelivery {
+  int num;
+  siginfo_t info;
+};
+
+class FcntlSignalTest : public ::testing::Test {
+ public:
+  void SetUp() override {
+    int pipe_fds[2];
+    ASSERT_THAT(pipe2(pipe_fds, O_NONBLOCK), SyscallSucceeds());
+    pipe_read_fd_ = pipe_fds[0];
+    pipe_write_fd_ = pipe_fds[1];
+  }
+
+  PosixErrorOr<Cleanup> RegisterSignalHandler(int signum) {
+    struct sigaction handler;
+    handler.sa_sigaction = setsig_signal_handler;
+    setsig_signal_handle = [&](int signum, siginfo_t* siginfo,
+                               void* unused_ucontext) {
+      SignalDelivery sig;
+      sig.num = signum;
+      sig.info = *siginfo;
+      signals_received_.push_back(sig);
+      num_signals_received_++;
+    };
+    sigemptyset(&handler.sa_mask);
+    handler.sa_flags = SA_SIGINFO;
+    return ScopedSigaction(signum, handler);
+  }
+
+  void FlushAndCloseFD(int fd) {
+    char buf;
+    int read_bytes;
+    do {
+      read_bytes = read(fd, &buf, 1);
+    } while (read_bytes > 0);
+    // read() can also fail with EWOULDBLOCK since the pipe is open in
+    // non-blocking mode. This is not an error.
+    EXPECT_TRUE(read_bytes == 0 || (read_bytes == -1 && errno == EWOULDBLOCK));
+    EXPECT_THAT(close(fd), SyscallSucceeds());
+  }
+
+  void DupReadFD() {
+    ASSERT_THAT(pipe_read_fd_dup_ = dup(pipe_read_fd_), SyscallSucceeds());
+    max_expected_signals++;
+  }
+
+  void RegisterFD(int fd, int signum) {
+    ASSERT_THAT(fcntl(fd, F_SETOWN, getpid()), SyscallSucceeds());
+    ASSERT_THAT(fcntl(fd, F_SETSIG, signum), SyscallSucceeds());
+    int old_flags;
+    ASSERT_THAT(old_flags = fcntl(fd, F_GETFL), SyscallSucceeds());
+    ASSERT_THAT(fcntl(fd, F_SETFL, old_flags | O_ASYNC), SyscallSucceeds());
+  }
+
+  void GenerateIOEvent() {
+    ASSERT_THAT(write(pipe_write_fd_, "test", 4), SyscallSucceedsWithValue(4));
+  }
+
+  void WaitForSignalDelivery(absl::Duration timeout) {
+    absl::Time wait_start = absl::Now();
+    while (num_signals_received_ < max_expected_signals &&
+           absl::Now() - wait_start < timeout) {
+      absl::SleepFor(absl::Milliseconds(10));
+    }
+  }
+
+  int pipe_read_fd_ = -1;
+  int pipe_read_fd_dup_ = -1;
+  int pipe_write_fd_ = -1;
+  int max_expected_signals = 1;
+  std::deque<SignalDelivery> signals_received_;
+  std::atomic<int> num_signals_received_ = 0;
+};
+
 namespace {
 
 PosixErrorOr<Cleanup> SubprocessLock(std::string const& path, bool for_write,
                                      bool blocking, bool retry_eintr, int fd,
                                      off_t start, off_t length, pid_t* child) {
   std::vector<std::string> args = {
-      "/proc/self/exe",        "--child_setlock_on", path,
-      "--child_setlock_start", absl::StrCat(start),  "--child_setlock_len",
-      absl::StrCat(length),    "--socket_fd",        absl::StrCat(fd)};
+      "/proc/self/exe",         "--child_set_lock_on", path,
+      "--child_set_lock_start", absl::StrCat(start),   "--child_set_lock_len",
+      absl::StrCat(length),     "--socket_fd",         absl::StrCat(fd)};
 
   if (for_write) {
-    args.push_back("--child_setlock_write");
+    args.push_back("--child_set_lock_write");
   }
 
   if (blocking) {
@@ -965,7 +1050,6 @@ TEST(FcntlTest, GetOwnNone) {
   // into F_{GET,SET}OWN_EX.
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(0));
-  MaybeSave();
 }
 
 TEST(FcntlTest, GetOwnExNone) {
@@ -1009,7 +1093,6 @@ TEST(FcntlTest, SetOwnPid) {
 
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(pid));
-  MaybeSave();
 }
 
 TEST(FcntlTest, SetOwnPgrp) {
@@ -1030,7 +1113,6 @@ TEST(FcntlTest, SetOwnPgrp) {
               SyscallSucceedsWithValue(0));
   EXPECT_EQ(got_owner.type, F_OWNER_PGRP);
   EXPECT_EQ(got_owner.pid, pgid);
-  MaybeSave();
 }
 
 TEST(FcntlTest, SetOwnUnset) {
@@ -1058,7 +1140,6 @@ TEST(FcntlTest, SetOwnUnset) {
 
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(0));
-  MaybeSave();
 }
 
 // F_SETOWN flips the sign of negative values, an operation that is guarded
@@ -1130,7 +1211,6 @@ TEST(FcntlTest, SetOwnExTid) {
 
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(owner.pid));
-  MaybeSave();
 }
 
 TEST(FcntlTest, SetOwnExPid) {
@@ -1146,7 +1226,6 @@ TEST(FcntlTest, SetOwnExPid) {
 
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(owner.pid));
-  MaybeSave();
 }
 
 TEST(FcntlTest, SetOwnExPgrp) {
@@ -1168,7 +1247,6 @@ TEST(FcntlTest, SetOwnExPgrp) {
               SyscallSucceedsWithValue(0));
   EXPECT_EQ(got_owner.type, set_owner.type);
   EXPECT_EQ(got_owner.pid, set_owner.pid);
-  MaybeSave();
 }
 
 TEST(FcntlTest, SetOwnExUnset) {
@@ -1201,7 +1279,6 @@ TEST(FcntlTest, SetOwnExUnset) {
 
   EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETOWN),
               SyscallSucceedsWithValue(0));
-  MaybeSave();
 }
 
 TEST(FcntlTest, GetOwnExTid) {
@@ -1258,9 +1335,269 @@ TEST(FcntlTest, GetOwnExPgrp) {
   EXPECT_EQ(got_owner.pid, set_owner.pid);
 }
 
+TEST(FcntlTest, SetSig) {
+  FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
+
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGUSR1),
+              SyscallSucceedsWithValue(0));
+  EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(SIGUSR1));
+}
+
+TEST(FcntlTest, SetSigDefaultsToZero) {
+  FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
+
+  // Defaults to returning the zero value, indicating default behavior (SIGIO).
+  EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(0));
+}
+
+TEST(FcntlTest, SetSigToDefault) {
+  FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
+
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGIO),
+              SyscallSucceedsWithValue(0));
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(SIGIO));
+
+  // Can be reset to the default behavior.
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, 0),
+              SyscallSucceedsWithValue(0));
+  EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(0));
+}
+
+TEST(FcntlTest, SetSigInvalid) {
+  FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
+
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGRTMAX + 1),
+              SyscallFailsWithErrno(EINVAL));
+  EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(0));
+}
+
+TEST(FcntlTest, SetSigInvalidDoesNotResetPreviousChoice) {
+  FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
+
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGUSR1),
+              SyscallSucceedsWithValue(0));
+  ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGRTMAX + 1),
+              SyscallFailsWithErrno(EINVAL));
+  EXPECT_THAT(syscall(__NR_fcntl, s.get(), F_GETSIG),
+              SyscallSucceedsWithValue(SIGUSR1));
+}
+
+TEST_F(FcntlSignalTest, SetSigDefault) {
+  const auto signal_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGIO));
+  RegisterFD(pipe_read_fd_, 0);  // Zero = default behavior
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  signals_received_.pop_front();
+  EXPECT_EQ(sig.num, SIGIO);
+  EXPECT_EQ(sig.info.si_signo, SIGIO);
+  // siginfo contents is undefined in this case.
+}
+
+TEST_F(FcntlSignalTest, SetSigCustom) {
+  const auto signal_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  signals_received_.pop_front();
+  EXPECT_EQ(sig.num, SIGUSR1);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR1);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigUnregisterStillGetsSigio) {
+  const auto sigio_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGIO));
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  RegisterFD(pipe_read_fd_, 0);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  signals_received_.pop_front();
+  EXPECT_EQ(sig.num, SIGIO);
+  // siginfo contents is undefined in this case.
+}
+
+TEST_F(FcntlSignalTest, SetSigWithSigioStillGetsSiginfo) {
+  const auto signal_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGIO));
+  RegisterFD(pipe_read_fd_, SIGIO);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  EXPECT_EQ(sig.num, SIGIO);
+  EXPECT_EQ(sig.info.si_signo, SIGIO);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupThenCloseOld) {
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  FlushAndCloseFD(pipe_read_fd_);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the **old** FD (even though it is closed).
+  EXPECT_EQ(sig.num, SIGUSR1);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR1);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupThenCloseNew) {
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  FlushAndCloseFD(pipe_read_fd_dup_);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the old FD.
+  EXPECT_EQ(sig.num, SIGUSR1);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR1);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupOldRegistered) {
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the old FD.
+  EXPECT_EQ(sig.num, SIGUSR1);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR1);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupNewRegistered) {
+  const auto sigusr2_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR2));
+  DupReadFD();
+  RegisterFD(pipe_read_fd_dup_, SIGUSR2);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the new FD.
+  EXPECT_EQ(sig.num, SIGUSR2);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR2);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_dup_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupBothRegistered) {
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  const auto sigusr2_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR2));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  RegisterFD(pipe_read_fd_dup_, SIGUSR2);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the **new** signal number, but the **old** FD.
+  EXPECT_EQ(sig.num, SIGUSR2);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR2);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupBothRegisteredAfterDup) {
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  const auto sigusr2_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR2));
+  DupReadFD();
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  RegisterFD(pipe_read_fd_dup_, SIGUSR2);
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with the **new** signal number, but the **old** FD.
+  EXPECT_EQ(sig.num, SIGUSR2);
+  EXPECT_EQ(sig.info.si_signo, SIGUSR2);
+  EXPECT_EQ(sig.info.si_fd, pipe_read_fd_);
+  EXPECT_EQ(sig.info.si_band, EPOLLIN | EPOLLRDNORM);
+}
+
+TEST_F(FcntlSignalTest, SetSigDupUnregisterOld) {
+  const auto sigio_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGIO));
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  const auto sigusr2_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR2));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  RegisterFD(pipe_read_fd_dup_, SIGUSR2);
+  RegisterFD(pipe_read_fd_, 0);  // Should go back to SIGIO behavior.
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with SIGIO.
+  EXPECT_EQ(sig.num, SIGIO);
+  // siginfo is undefined in this case.
+}
+
+TEST_F(FcntlSignalTest, SetSigDupUnregisterNew) {
+  const auto sigio_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGIO));
+  const auto sigusr1_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR1));
+  const auto sigusr2_cleanup =
+      ASSERT_NO_ERRNO_AND_VALUE(RegisterSignalHandler(SIGUSR2));
+  RegisterFD(pipe_read_fd_, SIGUSR1);
+  DupReadFD();
+  RegisterFD(pipe_read_fd_dup_, SIGUSR2);
+  RegisterFD(pipe_read_fd_dup_, 0);  // Should go back to SIGIO behavior.
+  GenerateIOEvent();
+  WaitForSignalDelivery(absl::Seconds(1));
+  ASSERT_EQ(num_signals_received_, 1);
+  SignalDelivery sig = signals_received_.front();
+  // We get a signal with SIGIO.
+  EXPECT_EQ(sig.num, SIGIO);
+  // siginfo is undefined in this case.
+}
+
 // Make sure that making multiple concurrent changes to async signal generation
 // does not cause any race issues.
-TEST(FcntlTest, SetFlSetOwnDoNotRace) {
+TEST(FcntlTest, SetFlSetOwnSetSigDoNotRace) {
   FileDescriptor s = ASSERT_NO_ERRNO_AND_VALUE(
       Socket(AF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK | SOCK_CLOEXEC, 0));
 
@@ -1268,32 +1605,40 @@ TEST(FcntlTest, SetFlSetOwnDoNotRace) {
   EXPECT_THAT(pid = getpid(), SyscallSucceeds());
 
   constexpr absl::Duration runtime = absl::Milliseconds(300);
-  auto setAsync = [&s, &runtime] {
+  auto set_async = [&s, &runtime] {
     for (auto start = absl::Now(); absl::Now() - start < runtime;) {
       ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETFL, O_ASYNC),
                   SyscallSucceeds());
       sched_yield();
     }
   };
-  auto resetAsync = [&s, &runtime] {
+  auto reset_async = [&s, &runtime] {
     for (auto start = absl::Now(); absl::Now() - start < runtime;) {
       ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETFL, 0), SyscallSucceeds());
       sched_yield();
     }
   };
-  auto setOwn = [&s, &pid, &runtime] {
+  auto set_own = [&s, &pid, &runtime] {
     for (auto start = absl::Now(); absl::Now() - start < runtime;) {
       ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETOWN, pid),
                   SyscallSucceeds());
       sched_yield();
     }
   };
+  auto set_sig = [&s, &runtime] {
+    for (auto start = absl::Now(); absl::Now() - start < runtime;) {
+      ASSERT_THAT(syscall(__NR_fcntl, s.get(), F_SETSIG, SIGUSR1),
+                  SyscallSucceeds());
+      sched_yield();
+    }
+  };
 
   std::list<ScopedThread> threads;
   for (int i = 0; i < 10; i++) {
-    threads.emplace_back(setAsync);
-    threads.emplace_back(resetAsync);
-    threads.emplace_back(setOwn);
+    threads.emplace_back(set_async);
+    threads.emplace_back(reset_async);
+    threads.emplace_back(set_own);
+    threads.emplace_back(set_sig);
   }
 }
 
@@ -1302,57 +1647,60 @@ TEST(FcntlTest, SetFlSetOwnDoNotRace) {
 }  // namespace testing
 }  // namespace gvisor
 
-int main(int argc, char** argv) {
-  gvisor::testing::TestInit(&argc, &argv);
-
-  const std::string setlock_on = absl::GetFlag(FLAGS_child_setlock_on);
-  if (!setlock_on.empty()) {
-    int socket_fd = absl::GetFlag(FLAGS_socket_fd);
-    int fd = open(setlock_on.c_str(), O_RDWR, 0666);
-    if (fd == -1 && errno != 0) {
-      int err = errno;
-      std::cerr << "CHILD open " << setlock_on << " failed " << err
-                << std::endl;
-      exit(err);
-    }
+int set_lock() {
+  const std::string set_lock_on = absl::GetFlag(FLAGS_child_set_lock_on);
+  int socket_fd = absl::GetFlag(FLAGS_socket_fd);
+  int fd = open(set_lock_on.c_str(), O_RDWR, 0666);
+  if (fd == -1 && errno != 0) {
+    int err = errno;
+    std::cerr << "CHILD open " << set_lock_on << " failed: " << err
+              << std::endl;
+    return err;
+  }
 
-    struct flock fl;
-    if (absl::GetFlag(FLAGS_child_setlock_write)) {
-      fl.l_type = F_WRLCK;
-    } else {
-      fl.l_type = F_RDLCK;
-    }
-    fl.l_whence = SEEK_SET;
-    fl.l_start = absl::GetFlag(FLAGS_child_setlock_start);
-    fl.l_len = absl::GetFlag(FLAGS_child_setlock_len);
+  struct flock fl;
+  if (absl::GetFlag(FLAGS_child_set_lock_write)) {
+    fl.l_type = F_WRLCK;
+  } else {
+    fl.l_type = F_RDLCK;
+  }
+  fl.l_whence = SEEK_SET;
+  fl.l_start = absl::GetFlag(FLAGS_child_set_lock_start);
+  fl.l_len = absl::GetFlag(FLAGS_child_set_lock_len);
+
+  // Test the fcntl.
+  int err = 0;
+  int ret = 0;
+
+  gvisor::testing::MonotonicTimer timer;
+  timer.Start();
+  do {
+    ret = fcntl(fd, absl::GetFlag(FLAGS_blocking) ? F_SETLKW : F_SETLK, &fl);
+  } while (absl::GetFlag(FLAGS_retry_eintr) && ret == -1 && errno == EINTR);
+  auto usec = absl::ToInt64Microseconds(timer.Duration());
+
+  if (ret == -1 && errno != 0) {
+    err = errno;
+    std::cerr << "CHILD lock " << set_lock_on << " failed " << err << std::endl;
+  }
 
-    // Test the fcntl.
-    int err = 0;
-    int ret = 0;
+  // If there is a socket fd let's send back the time in microseconds it took
+  // to execute this syscall.
+  if (socket_fd != -1) {
+    gvisor::testing::WriteFd(socket_fd, reinterpret_cast<void*>(&usec),
+                                   sizeof(usec));
+    close(socket_fd);
+  }
 
-    gvisor::testing::MonotonicTimer timer;
-    timer.Start();
-    do {
-      ret = fcntl(fd, absl::GetFlag(FLAGS_blocking) ? F_SETLKW : F_SETLK, &fl);
-    } while (absl::GetFlag(FLAGS_retry_eintr) && ret == -1 && errno == EINTR);
-    auto usec = absl::ToInt64Microseconds(timer.Duration());
-
-    if (ret == -1 && errno != 0) {
-      err = errno;
-      std::cerr << "CHILD lock " << setlock_on << " failed " << err
-                << std::endl;
-    }
+  close(fd);
+  return err;
+}
 
-    // If there is a socket fd let's send back the time in microseconds it took
-    // to execute this syscall.
-    if (socket_fd != -1) {
-      gvisor::testing::WriteFd(socket_fd, reinterpret_cast<void*>(&usec),
-                                     sizeof(usec));
-      close(socket_fd);
-    }
+int main(int argc, char** argv) {
+  gvisor::testing::TestInit(&argc, &argv);
 
-    close(fd);
-    exit(err);
+  if (!absl::GetFlag(FLAGS_child_set_lock_on).empty()) {
+    exit(set_lock());
   }
 
   return gvisor::testing::RunAllTests();