Add FileDescriptionImpl for Unix sockets.

This change involves several steps:
- Refactor the VFS1 unix socket implementation to share methods between VFS1
  and VFS2 where possible. Re-implement the rest.
- Override the default PRead, Read, PWrite, Write, Ioctl, Release methods in
  FileDescriptionDefaultImpl.
- Add functions to create and initialize a new Dentry/Inode and FileDescription
  for a Unix socket file.

Updates #1476

PiperOrigin-RevId: 304689796

2 files changed, 30 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/sockfs/BUILD b/pkg/sentry/fsimpl/sockfs/BUILD
index 790d50e65..52084ddb5 100644
--- a/pkg/sentry/fsimpl/sockfs/BUILD
+++ b/pkg/sentry/fsimpl/sockfs/BUILD
@@ -7,6 +7,7 @@ go_library(
     srcs = ["sockfs.go"],
     visibility = ["//pkg/sentry:internal"],
     deps = [
+        "//pkg/abi/linux",
         "//pkg/context",
         "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/kernel/auth",
diff --git a/pkg/sentry/fsimpl/sockfs/sockfs.go b/pkg/sentry/fsimpl/sockfs/sockfs.go
index c13511de2..3f7ad1d65 100644
--- a/pkg/sentry/fsimpl/sockfs/sockfs.go
+++ b/pkg/sentry/fsimpl/sockfs/sockfs.go
@@ -16,6 +16,7 @@
 package sockfs
 
 import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -60,6 +61,10 @@ type filesystem struct {
 }
 
 // inode implements kernfs.Inode.
+//
+// TODO(gvisor.dev/issue/1476): Add device numbers to this inode (which are
+// not included in InodeAttrs) to store the numbers of the appropriate
+// socket device. Override InodeAttrs.Stat() accordingly.
 type inode struct {
 	kernfs.InodeNotDirectory
 	kernfs.InodeNotSymlink
@@ -71,3 +76,27 @@ type inode struct {
 func (i *inode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
 	return nil, syserror.ENXIO
 }
+
+// InitSocket initializes a socket FileDescription, with a corresponding
+// Dentry in mnt.
+//
+// fd should be the FileDescription associated with socketImpl, i.e. its first
+// field. mnt should be the global socket mount, Kernel.socketMount.
+func InitSocket(socketImpl vfs.FileDescriptionImpl, fd *vfs.FileDescription, mnt *vfs.Mount, creds *auth.Credentials) error {
+	fsimpl := mnt.Filesystem().Impl()
+	fs := fsimpl.(*kernfs.Filesystem)
+
+	// File mode matches net/socket.c:sock_alloc.
+	filemode := linux.FileMode(linux.S_IFSOCK | 0600)
+	i := &inode{}
+	i.Init(creds, fs.NextIno(), filemode)
+
+	d := &kernfs.Dentry{}
+	d.Init(i)
+
+	opts := &vfs.FileDescriptionOptions{UseDentryMetadata: true}
+	if err := fd.Init(socketImpl, linux.O_RDWR, mnt, d.VFSDentry(), opts); err != nil {
+		return err
+	}
+	return nil
+}