Minor VFS2 interface changes.

- Remove the Filesystem argument from DentryImpl.*Ref(); in general DentryImpls
  that need the Filesystem for reference counting will probably also need it
  for other interface methods that don't plumb Filesystem, so it's easier to
  just store a pointer to the filesystem in the DentryImpl.

- Add a pointer to the VirtualFilesystem to Filesystem, which is needed by the
  gofer client to disown dentries for cache eviction triggered by dentry
  reference count changes.

- Rename FilesystemType.NewFilesystem to GetFilesystem; in some cases (e.g.
  sysfs, cgroupfs) it's much cleaner for there to be only one Filesystem that
  is used by all mounts, and in at least one case (devtmpfs) it's visibly
  incorrect not to do so, so NewFilesystem doesn't always actually create and
  return a *new* Filesystem.

- Require callers of FileDescription.Init() to increment Mount/Dentry
  references. This is because the gofer client may, in the OpenAt() path, take
  a reference on a dentry with 0 references, which is safe due to
  synchronization that is outside the scope of this CL, and it would be safer
  to still have its implementation of DentryImpl.IncRef() check for an
  increment for 0 references in other cases.

- Add FileDescription.TryIncRef. This is used by the gofer client to take
  references on "special file descriptions" (FDs for files such as pipes,
  sockets, and devices), which use per-FD handles (fids) instead of
  dentry-shared handles, for sync() and syncfs().

PiperOrigin-RevId: 282473364

1 files changed, 29 insertions, 27 deletions

diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 198fb8067..1c3b2e987 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -110,12 +110,12 @@ type MountNamespace struct {
 // NewMountNamespace returns a new mount namespace with a root filesystem
 // configured by the given arguments. A reference is taken on the returned
 // MountNamespace.
-func (vfs *VirtualFilesystem) NewMountNamespace(ctx context.Context, creds *auth.Credentials, source, fsTypeName string, opts *NewFilesystemOptions) (*MountNamespace, error) {
+func (vfs *VirtualFilesystem) NewMountNamespace(ctx context.Context, creds *auth.Credentials, source, fsTypeName string, opts *GetFilesystemOptions) (*MountNamespace, error) {
 	fsType := vfs.getFilesystemType(fsTypeName)
 	if fsType == nil {
 		return nil, syserror.ENODEV
 	}
-	fs, root, err := fsType.NewFilesystem(ctx, creds, source, *opts)
+	fs, root, err := fsType.GetFilesystem(ctx, vfs, creds, source, *opts)
 	if err != nil {
 		return nil, err
 	}
@@ -133,13 +133,13 @@ func (vfs *VirtualFilesystem) NewMountNamespace(ctx context.Context, creds *auth
 	return mntns, nil
 }
 
-// NewMount creates and mounts a new Filesystem.
-func (vfs *VirtualFilesystem) NewMount(ctx context.Context, creds *auth.Credentials, source string, target *PathOperation, fsTypeName string, opts *NewFilesystemOptions) error {
+// NewMount creates and mounts a Filesystem configured by the given arguments.
+func (vfs *VirtualFilesystem) NewMount(ctx context.Context, creds *auth.Credentials, source string, target *PathOperation, fsTypeName string, opts *GetFilesystemOptions) error {
 	fsType := vfs.getFilesystemType(fsTypeName)
 	if fsType == nil {
 		return syserror.ENODEV
 	}
-	fs, root, err := fsType.NewFilesystem(ctx, creds, source, *opts)
+	fs, root, err := fsType.GetFilesystem(ctx, vfs, creds, source, *opts)
 	if err != nil {
 		return err
 	}
@@ -147,8 +147,8 @@ func (vfs *VirtualFilesystem) NewMount(ctx context.Context, creds *auth.Credenti
 	// lock ordering.
 	vd, err := vfs.GetDentryAt(ctx, creds, target, &GetDentryOptions{})
 	if err != nil {
-		root.decRef(fs)
-		fs.decRef()
+		root.DecRef()
+		fs.DecRef()
 		return err
 	}
 	vfs.mountMu.Lock()
@@ -158,8 +158,8 @@ func (vfs *VirtualFilesystem) NewMount(ctx context.Context, creds *auth.Credenti
 			vd.dentry.mu.Unlock()
 			vfs.mountMu.Unlock()
 			vd.DecRef()
-			root.decRef(fs)
-			fs.decRef()
+			root.DecRef()
+			fs.DecRef()
 			return syserror.ENOENT
 		}
 		// vd might have been mounted over between vfs.GetDentryAt() and
@@ -179,7 +179,7 @@ func (vfs *VirtualFilesystem) NewMount(ctx context.Context, creds *auth.Credenti
 			break
 		}
 		// This can't fail since we're holding vfs.mountMu.
-		nextmnt.root.incRef(nextmnt.fs)
+		nextmnt.root.IncRef()
 		vd.dentry.mu.Unlock()
 		vd.DecRef()
 		vd = VirtualDentry{
@@ -229,7 +229,7 @@ type umountRecursiveOptions struct {
 // Mounts on which references must be dropped to vdsToDecRef and mountsToDecRef
 // respectively, and returns updated slices. (This is necessary because
 // filesystem locks possibly taken by DentryImpl.DecRef() may precede
-// vfs.mountMu in the lock order, and Mount.decRef() may lock vfs.mountMu.)
+// vfs.mountMu in the lock order, and Mount.DecRef() may lock vfs.mountMu.)
 //
 // umountRecursiveLocked is analogous to Linux's fs/namespace.c:umount_tree().
 //
@@ -322,13 +322,15 @@ func (mnt *Mount) tryIncMountedRef() bool {
 	}
 }
 
-func (mnt *Mount) incRef() {
+// IncRef increments mnt's reference count.
+func (mnt *Mount) IncRef() {
 	// In general, negative values for mnt.refs are valid because the MSB is
 	// the eager-unmount bit.
 	atomic.AddInt64(&mnt.refs, 1)
 }
 
-func (mnt *Mount) decRef() {
+// DecRef decrements mnt's reference count.
+func (mnt *Mount) DecRef() {
 	refs := atomic.AddInt64(&mnt.refs, -1)
 	if refs&^math.MinInt64 == 0 { // mask out MSB
 		var vd VirtualDentry
@@ -339,8 +341,8 @@ func (mnt *Mount) decRef() {
 			mnt.vfs.mounts.seq.EndWrite()
 			mnt.vfs.mountMu.Unlock()
 		}
-		mnt.root.decRef(mnt.fs)
-		mnt.fs.decRef()
+		mnt.root.DecRef()
+		mnt.fs.DecRef()
 		if vd.Ok() {
 			vd.DecRef()
 		}
@@ -368,7 +370,7 @@ func (mntns *MountNamespace) DecRef(vfs *VirtualFilesystem) {
 			vd.DecRef()
 		}
 		for _, mnt := range mountsToDecRef {
-			mnt.decRef()
+			mnt.DecRef()
 		}
 	} else if refs < 0 {
 		panic("MountNamespace.DecRef() called without holding a reference")
@@ -413,7 +415,7 @@ retryFirst:
 			// Raced with umount.
 			continue
 		}
-		mnt.decRef()
+		mnt.DecRef()
 		mnt = next
 		d = next.root
 	}
@@ -447,15 +449,15 @@ retryFirst:
 		// Raced with umount.
 		goto retryFirst
 	}
-	if !point.tryIncRef(parent.fs) {
+	if !point.TryIncRef() {
 		// Since Mount holds a reference on Mount.key.point, this can only
 		// happen due to a racing change to Mount.key.
-		parent.decRef()
+		parent.DecRef()
 		goto retryFirst
 	}
 	if !vfs.mounts.seq.ReadOk(epoch) {
-		point.decRef(parent.fs)
-		parent.decRef()
+		point.DecRef()
+		parent.DecRef()
 		goto retryFirst
 	}
 	mnt = parent
@@ -480,19 +482,19 @@ retryFirst:
 			// Raced with umount.
 			goto retryNotFirst
 		}
-		if !point.tryIncRef(parent.fs) {
+		if !point.TryIncRef() {
 			// Since Mount holds a reference on Mount.key.point, this can
 			// only happen due to a racing change to Mount.key.
-			parent.decRef()
+			parent.DecRef()
 			goto retryNotFirst
 		}
 		if !vfs.mounts.seq.ReadOk(epoch) {
-			point.decRef(parent.fs)
-			parent.decRef()
+			point.DecRef()
+			parent.DecRef()
 			goto retryNotFirst
 		}
-		d.decRef(mnt.fs)
-		mnt.decRef()
+		d.DecRef()
+		mnt.DecRef()
 		mnt = parent
 		d = point
 	}