Add vfs.PathnameReachable().

/proc/[pid]/mount* omit mounts whose mount point is outside the chroot, which
is checked (indirectly) via __d_path().

PiperOrigin-RevId: 303434226

pkg/sentry/vfs/pathname.go

@@ -90,6 +90,49 @@ loop:
 	return b.String(), nil
 }
 
+// PathnameReachable returns an absolute pathname to vd, consistent with
+// Linux's __d_path() (as used by seq_path_root()). If vfsroot.Ok() and vd is
+// not reachable from vfsroot, such that seq_path_root() would return SEQ_SKIP
+// (causing the entire containing entry to be skipped), PathnameReachable
+// returns ("", nil).
+func (vfs *VirtualFilesystem) PathnameReachable(ctx context.Context, vfsroot, vd VirtualDentry) (string, error) {
+	b := getFSPathBuilder()
+	defer putFSPathBuilder(b)
+	haveRef := false
+	defer func() {
+		if haveRef {
+			vd.DecRef()
+		}
+	}()
+loop:
+	for {
+		err := vd.mount.fs.impl.PrependPath(ctx, vfsroot, vd, b)
+		switch err.(type) {
+		case nil:
+			if vd.mount == vfsroot.mount && vd.mount.root == vfsroot.dentry {
+				break loop
+			}
+			nextVD := vfs.getMountpointAt(vd.mount, vfsroot)
+			if !nextVD.Ok() {
+				return "", nil
+			}
+			if haveRef {
+				vd.DecRef()
+			}
+			vd = nextVD
+			haveRef = true
+		case PrependPathAtVFSRootError:
+			break loop
+		case PrependPathAtNonMountRootError, PrependPathSyntheticError:
+			return "", nil
+		default:
+			return "", err
+		}
+	}
+	b.PrependByte('/')
+	return b.String(), nil
+}
+
 // PathnameForGetcwd returns an absolute pathname to vd, consistent with
 // Linux's sys_getcwd().
 func (vfs *VirtualFilesystem) PathnameForGetcwd(ctx context.Context, vfsroot, vd VirtualDentry) (string, error) {