Make mounts ReadWrite first, then later change to ReadOnly.

This lets us create "synthetic" mountpoint directories in ReadOnly mounts
during VFS setup.

Also add context.WithMountNamespace, as some filesystems (like overlay) require
a MountNamespace on ctx to handle vfs.Filesystem Operations.

PiperOrigin-RevId: 327874971

2 files changed, 39 insertions, 4 deletions

diff --git a/pkg/sentry/vfs/context.go b/pkg/sentry/vfs/context.go
index c9e724fef..97018651f 100644
--- a/pkg/sentry/vfs/context.go
+++ b/pkg/sentry/vfs/context.go
@@ -40,6 +40,30 @@ func MountNamespaceFromContext(ctx context.Context) *MountNamespace {
 	return nil
 }
 
+type mountNamespaceContext struct {
+	context.Context
+	mntns *MountNamespace
+}
+
+// Value implements Context.Value.
+func (mc mountNamespaceContext) Value(key interface{}) interface{} {
+	switch key {
+	case CtxMountNamespace:
+		mc.mntns.IncRef()
+		return mc.mntns
+	default:
+		return mc.Context.Value(key)
+	}
+}
+
+// WithMountNamespace returns a copy of ctx with the given MountNamespace.
+func WithMountNamespace(ctx context.Context, mntns *MountNamespace) context.Context {
+	return &mountNamespaceContext{
+		Context: ctx,
+		mntns:   mntns,
+	}
+}
+
 // RootFromContext returns the VFS root used by ctx. It takes a reference on
 // the returned VirtualDentry. If ctx does not have a specific VFS root,
 // RootFromContext returns a zero-value VirtualDentry.
diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 714af6907..09fea3628 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -263,16 +263,20 @@ func (vfs *VirtualFilesystem) ConnectMountAt(ctx context.Context, creds *auth.Cr
 }
 
 // MountAt creates and mounts a Filesystem configured by the given arguments.
-func (vfs *VirtualFilesystem) MountAt(ctx context.Context, creds *auth.Credentials, source string, target *PathOperation, fsTypeName string, opts *MountOptions) error {
+// The VirtualFilesystem will hold a reference to the Mount until it is unmounted.
+//
+// This method returns the mounted Mount without a reference, for convenience
+// during VFS setup when there is no chance of racing with unmount.
+func (vfs *VirtualFilesystem) MountAt(ctx context.Context, creds *auth.Credentials, source string, target *PathOperation, fsTypeName string, opts *MountOptions) (*Mount, error) {
 	mnt, err := vfs.MountDisconnected(ctx, creds, source, fsTypeName, opts)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	defer mnt.DecRef(ctx)
 	if err := vfs.ConnectMountAt(ctx, creds, mnt, target); err != nil {
-		return err
+		return nil, err
 	}
-	return nil
+	return mnt, nil
 }
 
 // UmountAt removes the Mount at the given path.
@@ -657,6 +661,13 @@ retryFirst:
 	return VirtualDentry{mnt, d}
 }
 
+// SetMountReadOnly sets the mount as ReadOnly.
+func (vfs *VirtualFilesystem) SetMountReadOnly(mnt *Mount, ro bool) error {
+	vfs.mountMu.Lock()
+	defer vfs.mountMu.Unlock()
+	return mnt.setReadOnlyLocked(ro)
+}
+
 // CheckBeginWrite increments the counter of in-progress write operations on
 // mnt. If mnt is mounted MS_RDONLY, CheckBeginWrite does nothing and returns
 // EROFS.