Handle gofer blocking opens of host named pipes in VFS2.

Using tee instead of read to detect when a O_RDONLY|O_NONBLOCK pipe FD has a
writer circumvents the problem of what to do with the byte read from the pipe,
avoiding much of the complexity of the fdpipe package.

PiperOrigin-RevId: 314216146

4 files changed, 206 insertions, 27 deletions

diff --git a/pkg/sentry/fsimpl/gofer/BUILD b/pkg/sentry/fsimpl/gofer/BUILD
index 67e916525..f5f35a3bc 100644
--- a/pkg/sentry/fsimpl/gofer/BUILD
+++ b/pkg/sentry/fsimpl/gofer/BUILD
@@ -35,6 +35,7 @@ go_library(
         "fstree.go",
         "gofer.go",
         "handle.go",
+        "host_named_pipe.go",
         "p9file.go",
         "regular_file.go",
         "socket.go",
@@ -47,6 +48,7 @@ go_library(
         "//pkg/abi/linux",
         "//pkg/context",
         "//pkg/fd",
+        "//pkg/fdnotifier",
         "//pkg/fspath",
         "//pkg/log",
         "//pkg/p9",
@@ -71,6 +73,7 @@ go_library(
         "//pkg/unet",
         "//pkg/usermem",
         "//pkg/waiter",
+        "@org_golang_x_sys//unix:go_default_library",
     ],
 )
 
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 7f2181216..0000ca6ba 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -873,19 +873,37 @@ func (d *dentry) openSpecialFileLocked(ctx context.Context, mnt *vfs.Mount, opts
 	if opts.Flags&linux.O_DIRECT != 0 {
 		return nil, syserror.EINVAL
 	}
-	h, err := openHandle(ctx, d.file, ats&vfs.MayRead != 0, ats&vfs.MayWrite != 0, opts.Flags&linux.O_TRUNC != 0)
+	// We assume that the server silently inserts O_NONBLOCK in the open flags
+	// for all named pipes (because all existing gofers do this).
+	//
+	// NOTE(b/133875563): This makes named pipe opens racy, because the
+	// mechanisms for translating nonblocking to blocking opens can only detect
+	// the instantaneous presence of a peer holding the other end of the pipe
+	// open, not whether the pipe was *previously* opened by a peer that has
+	// since closed its end.
+	isBlockingOpenOfNamedPipe := d.fileType() == linux.S_IFIFO && opts.Flags&linux.O_NONBLOCK == 0
+retry:
+	h, err := openHandle(ctx, d.file, ats.MayRead(), ats.MayWrite(), opts.Flags&linux.O_TRUNC != 0)
 	if err != nil {
+		if isBlockingOpenOfNamedPipe && ats == vfs.MayWrite && err == syserror.ENXIO {
+			// An attempt to open a named pipe with O_WRONLY|O_NONBLOCK fails
+			// with ENXIO if opening the same named pipe with O_WRONLY would
+			// block because there are no readers of the pipe.
+			if err := sleepBetweenNamedPipeOpenChecks(ctx); err != nil {
+				return nil, err
+			}
+			goto retry
+		}
 		return nil, err
 	}
-	seekable := d.fileType() == linux.S_IFREG
-	fd := &specialFileFD{
-		handle:   h,
-		seekable: seekable,
+	if isBlockingOpenOfNamedPipe && ats == vfs.MayRead && h.fd >= 0 {
+		if err := blockUntilNonblockingPipeHasWriter(ctx, h.fd); err != nil {
+			h.close(ctx)
+			return nil, err
+		}
 	}
-	if err := fd.vfsfd.Init(fd, opts.Flags, mnt, &d.vfsd, &vfs.FileDescriptionOptions{
-		DenyPRead:  !seekable,
-		DenyPWrite: !seekable,
-	}); err != nil {
+	fd, err := newSpecialFileFD(h, mnt, d, opts.Flags)
+	if err != nil {
 		h.close(ctx)
 		return nil, err
 	}
@@ -981,22 +999,16 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 		}
 		childVFSFD = &fd.vfsfd
 	} else {
-		seekable := child.fileType() == linux.S_IFREG
-		fd := &specialFileFD{
-			handle: handle{
-				file: openFile,
-				fd:   -1,
-			},
-			seekable: seekable,
+		h := handle{
+			file: openFile,
+			fd:   -1,
 		}
 		if fdobj != nil {
-			fd.handle.fd = int32(fdobj.Release())
+			h.fd = int32(fdobj.Release())
 		}
-		if err := fd.vfsfd.Init(fd, opts.Flags, mnt, &child.vfsd, &vfs.FileDescriptionOptions{
-			DenyPRead:  !seekable,
-			DenyPWrite: !seekable,
-		}); err != nil {
-			fd.handle.close(ctx)
+		fd, err := newSpecialFileFD(h, mnt, child, opts.Flags)
+		if err != nil {
+			h.close(ctx)
 			return nil, err
 		}
 		childVFSFD = &fd.vfsfd
diff --git a/pkg/sentry/fsimpl/gofer/host_named_pipe.go b/pkg/sentry/fsimpl/gofer/host_named_pipe.go
new file mode 100644
index 000000000..7294de7d6
--- /dev/null
+++ b/pkg/sentry/fsimpl/gofer/host_named_pipe.go
@@ -0,0 +1,97 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gofer
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// Global pipe used by blockUntilNonblockingPipeHasWriter since we can't create
+// pipes after sentry initialization due to syscall filters.
+var (
+	tempPipeMu      sync.Mutex
+	tempPipeReadFD  int
+	tempPipeWriteFD int
+	tempPipeBuf     [1]byte
+)
+
+func init() {
+	var pipeFDs [2]int
+	if err := unix.Pipe(pipeFDs[:]); err != nil {
+		panic(fmt.Sprintf("failed to create pipe for gofer.blockUntilNonblockingPipeHasWriter: %v", err))
+	}
+	tempPipeReadFD = pipeFDs[0]
+	tempPipeWriteFD = pipeFDs[1]
+}
+
+func blockUntilNonblockingPipeHasWriter(ctx context.Context, fd int32) error {
+	for {
+		ok, err := nonblockingPipeHasWriter(fd)
+		if err != nil {
+			return err
+		}
+		if ok {
+			return nil
+		}
+		if err := sleepBetweenNamedPipeOpenChecks(ctx); err != nil {
+			return err
+		}
+	}
+}
+
+func nonblockingPipeHasWriter(fd int32) (bool, error) {
+	tempPipeMu.Lock()
+	defer tempPipeMu.Unlock()
+	// Copy 1 byte from fd into the temporary pipe.
+	n, err := unix.Tee(int(fd), tempPipeWriteFD, 1, unix.SPLICE_F_NONBLOCK)
+	if err == syserror.EAGAIN {
+		// The pipe represented by fd is empty, but has a writer.
+		return true, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	if n == 0 {
+		// The pipe represented by fd is empty and has no writer.
+		return false, nil
+	}
+	// The pipe represented by fd is non-empty, so it either has, or has
+	// previously had, a writer. Remove the byte copied to the temporary pipe
+	// before returning.
+	if n, err := unix.Read(tempPipeReadFD, tempPipeBuf[:]); err != nil || n != 1 {
+		panic(fmt.Sprintf("failed to drain pipe for gofer.blockUntilNonblockingPipeHasWriter: got (%d, %v), wanted (1, nil)", n, err))
+	}
+	return true, nil
+}
+
+func sleepBetweenNamedPipeOpenChecks(ctx context.Context) error {
+	t := time.NewTimer(100 * time.Millisecond)
+	defer t.Stop()
+	cancel := ctx.SleepStart()
+	select {
+	case <-t.C:
+		ctx.SleepFinish(true)
+		return nil
+	case <-cancel:
+		ctx.SleepFinish(false)
+		return syserror.ErrInterrupted
+	}
+}
diff --git a/pkg/sentry/fsimpl/gofer/special_file.go b/pkg/sentry/fsimpl/gofer/special_file.go
index a464e6a94..ff6126b87 100644
--- a/pkg/sentry/fsimpl/gofer/special_file.go
+++ b/pkg/sentry/fsimpl/gofer/special_file.go
@@ -19,17 +19,18 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fdnotifier"
 	"gvisor.dev/gvisor/pkg/safemem"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/usermem"
+	"gvisor.dev/gvisor/pkg/waiter"
 )
 
-// specialFileFD implements vfs.FileDescriptionImpl for files other than
-// regular files, directories, and symlinks: pipes, sockets, etc. It is also
-// used for regular files when filesystemOptions.specialRegularFiles is in
-// effect. specialFileFD differs from regularFileFD by using per-FD handles
-// instead of shared per-dentry handles, and never buffering I/O.
+// specialFileFD implements vfs.FileDescriptionImpl for pipes, sockets, device
+// special files, and (when filesystemOptions.specialRegularFiles is in effect)
+// regular files. specialFileFD differs from regularFileFD by using per-FD
+// handles instead of shared per-dentry handles, and never buffering I/O.
 type specialFileFD struct {
 	fileDescription
 
@@ -40,13 +41,47 @@ type specialFileFD struct {
 	// file offset is significant, i.e. a regular file. seekable is immutable.
 	seekable bool
 
+	// mayBlock is true if this file description represents a file for which
+	// queue may send I/O readiness events. mayBlock is immutable.
+	mayBlock bool
+	queue    waiter.Queue
+
 	// If seekable is true, off is the file offset. off is protected by mu.
 	mu  sync.Mutex
 	off int64
 }
 
+func newSpecialFileFD(h handle, mnt *vfs.Mount, d *dentry, flags uint32) (*specialFileFD, error) {
+	ftype := d.fileType()
+	seekable := ftype == linux.S_IFREG
+	mayBlock := ftype == linux.S_IFIFO || ftype == linux.S_IFSOCK
+	fd := &specialFileFD{
+		handle:   h,
+		seekable: seekable,
+		mayBlock: mayBlock,
+	}
+	if mayBlock && h.fd >= 0 {
+		if err := fdnotifier.AddFD(h.fd, &fd.queue); err != nil {
+			return nil, err
+		}
+	}
+	if err := fd.vfsfd.Init(fd, flags, mnt, &d.vfsd, &vfs.FileDescriptionOptions{
+		DenyPRead:  !seekable,
+		DenyPWrite: !seekable,
+	}); err != nil {
+		if mayBlock && h.fd >= 0 {
+			fdnotifier.RemoveFD(h.fd)
+		}
+		return nil, err
+	}
+	return fd, nil
+}
+
 // Release implements vfs.FileDescriptionImpl.Release.
 func (fd *specialFileFD) Release() {
+	if fd.mayBlock && fd.handle.fd >= 0 {
+		fdnotifier.RemoveFD(fd.handle.fd)
+	}
 	fd.handle.close(context.Background())
 	fs := fd.vfsfd.Mount().Filesystem().Impl().(*filesystem)
 	fs.syncMu.Lock()
@@ -62,6 +97,32 @@ func (fd *specialFileFD) OnClose(ctx context.Context) error {
 	return fd.handle.file.flush(ctx)
 }
 
+// Readiness implements waiter.Waitable.Readiness.
+func (fd *specialFileFD) Readiness(mask waiter.EventMask) waiter.EventMask {
+	if fd.mayBlock {
+		return fdnotifier.NonBlockingPoll(fd.handle.fd, mask)
+	}
+	return fd.fileDescription.Readiness(mask)
+}
+
+// EventRegister implements waiter.Waitable.EventRegister.
+func (fd *specialFileFD) EventRegister(e *waiter.Entry, mask waiter.EventMask) {
+	if fd.mayBlock {
+		fd.queue.EventRegister(e, mask)
+		return
+	}
+	fd.fileDescription.EventRegister(e, mask)
+}
+
+// EventUnregister implements waiter.Waitable.EventUnregister.
+func (fd *specialFileFD) EventUnregister(e *waiter.Entry) {
+	if fd.mayBlock {
+		fd.queue.EventUnregister(e)
+		return
+	}
+	fd.fileDescription.EventUnregister(e)
+}
+
 // PRead implements vfs.FileDescriptionImpl.PRead.
 func (fd *specialFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
 	if fd.seekable && offset < 0 {
@@ -81,6 +142,9 @@ func (fd *specialFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offs
 	}
 	buf := make([]byte, dst.NumBytes())
 	n, err := fd.handle.readToBlocksAt(ctx, safemem.BlockSeqOf(safemem.BlockFromSafeSlice(buf)), uint64(offset))
+	if err == syserror.EAGAIN {
+		err = syserror.ErrWouldBlock
+	}
 	if n == 0 {
 		return 0, err
 	}
@@ -130,6 +194,9 @@ func (fd *specialFileFD) PWrite(ctx context.Context, src usermem.IOSequence, off
 		return 0, err
 	}
 	n, err := fd.handle.writeFromBlocksAt(ctx, safemem.BlockSeqOf(safemem.BlockFromSafeSlice(buf)), uint64(offset))
+	if err == syserror.EAGAIN {
+		err = syserror.ErrWouldBlock
+	}
 	return int64(n), err
 }