Add socket files to tmpfs VFS2.

Updates #1476.

PiperOrigin-RevId: 305024274

pkg/sentry/fsimpl/tmpfs/BUILD

@@ -24,6 +24,7 @@ go_library(
         "filesystem.go",
         "named_pipe.go",
         "regular_file.go",
+        "socket_file.go",
         "symlink.go",
         "tmpfs.go",
     ],

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -261,8 +261,7 @@ func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		case linux.S_IFCHR:
 			childInode = fs.newDeviceFile(rp.Credentials(), opts.Mode, vfs.CharDevice, opts.DevMajor, opts.DevMinor)
 		case linux.S_IFSOCK:
-			// Not yet supported.
-			return syserror.EPERM
+			childInode = fs.newSocketFile(rp.Credentials(), opts.Mode, opts.Endpoint)
 		default:
 			return syserror.EINVAL
 		}
@@ -396,6 +395,8 @@ func (d *dentry) open(ctx context.Context, rp *vfs.ResolvingPath, opts *vfs.Open
 		return newNamedPipeFD(ctx, impl, rp, &d.vfsd, opts.Flags)
 	case *deviceFile:
 		return rp.VirtualFilesystem().OpenDeviceSpecialFile(ctx, rp.Mount(), &d.vfsd, impl.kind, impl.major, impl.minor, opts)
+	case *socketFile:
+		return nil, syserror.ENXIO
 	default:
 		panic(fmt.Sprintf("unknown inode type: %T", d.inode.impl))
 	}
@@ -679,10 +680,19 @@ func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error
 }
 
 // BoundEndpointAt implements FilesystemImpl.BoundEndpointAt.
-//
-// TODO(gvisor.dev/issue/1476): Implement BoundEndpointAt.
 func (fs *filesystem) BoundEndpointAt(ctx context.Context, rp *vfs.ResolvingPath) (transport.BoundEndpoint, error) {
-	return nil, syserror.ECONNREFUSED
+	fs.mu.RLock()
+	defer fs.mu.RUnlock()
+	d, err := resolveLocked(rp)
+	if err != nil {
+		return nil, err
+	}
+	switch impl := d.inode.impl.(type) {
+	case *socketFile:
+		return impl.ep, nil
+	default:
+		return nil, syserror.ECONNREFUSED
+	}
 }
 
 // ListxattrAt implements vfs.FilesystemImpl.ListxattrAt.

pkg/sentry/fsimpl/tmpfs/socket_file.go

@@ -0,0 +1,34 @@
+// 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 tmpfs
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
+)
+
+// socketFile is a socket (=S_IFSOCK) tmpfs file.
+type socketFile struct {
+	inode inode
+	ep    transport.BoundEndpoint
+}
+
+func (fs *filesystem) newSocketFile(creds *auth.Credentials, mode linux.FileMode, ep transport.BoundEndpoint) *inode {
+	file := &socketFile{ep: ep}
+	file.inode.init(file, fs, creds, mode)
+	file.inode.nlink = 1 // from parent directory
+	return &file.inode
+}

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -331,7 +331,7 @@ func (i *inode) statTo(stat *linux.Statx) {
 	case *deviceFile:
 		stat.RdevMajor = impl.major
 		stat.RdevMinor = impl.minor
-	case *directory, *namedPipe:
+	case *socketFile, *directory, *namedPipe:
 		// Nothing to do.
 	default:
 		panic(fmt.Sprintf("unknown inode type: %T", i.impl))
@@ -479,6 +479,8 @@ func (i *inode) direntType() uint8 {
 		return linux.DT_DIR
 	case *symlink:
 		return linux.DT_LNK
+	case *socketFile:
+		return linux.DT_SOCK
 	case *deviceFile:
 		switch impl.kind {
 		case vfs.BlockDevice: