Add plumbing for importing fds in VFS2, along with non-socket, non-TTY impl.

In VFS2, imported file descriptors are stored in a kernfs-based filesystem.
Upon calling ImportFD, the host fd can be accessed in two ways:
1. a FileDescription that can be added to the FDTable, and
2. a Dentry in the host.filesystem mount, which we will want to access through
magic symlinks in /proc/[pid]/fd/.

An implementation of the kernfs.Inode interface stores a unique host fd. This
inode can be inserted into file descriptions as well as dentries.

This change also plumbs in three FileDescriptionImpls corresponding to fds for
sockets, TTYs, and other files (only the latter is implemented here).
These implementations will mostly make corresponding syscalls to the host.
Where possible, the logic is ported over from pkg/sentry/fs/host.

Updates #1672

PiperOrigin-RevId: 299417263

1 files changed, 3 insertions, 4 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/kernfs.go b/pkg/sentry/fsimpl/kernfs/kernfs.go
index c74fa999b..a8ab2a2ba 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs.go
@@ -176,8 +176,6 @@ type Dentry struct {
 	vfsd  vfs.Dentry
 	inode Inode
 
-	refs uint64
-
 	// flags caches useful information about the dentry from the inode. See the
 	// dflags* consts above. Must be accessed by atomic ops.
 	flags uint32
@@ -302,7 +300,8 @@ type Inode interface {
 	// this inode. The returned file description should hold a reference on the
 	// inode for its lifetime.
 	//
-	// Precondition: !rp.Done(). vfsd.Impl() must be a kernfs Dentry.
+	// Precondition: rp.Done(). vfsd.Impl() must be the kernfs Dentry containing
+	// the inode on which Open() is being called.
 	Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error)
 }
 
@@ -328,7 +327,7 @@ type inodeMetadata interface {
 
 	// Stat returns the metadata for this inode. This corresponds to
 	// vfs.FilesystemImpl.StatAt.
-	Stat(fs *vfs.Filesystem) linux.Statx
+	Stat(fs *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error)
 
 	// SetStat updates the metadata for this inode. This corresponds to
 	// vfs.FilesystemImpl.SetStatAt.