Pre-open container root directory

A lot of filesystem-related stuff happens inside the container root
directory, and we have used its name before. It makes sense to pre-open
it and use a *os.File handle instead.

Function names in internal/pathrs are kept as is for simplicity (and it
is an internal package), but they now accept root as *os.File.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/pathrs/mkdirall.go

@@ -31,12 +31,12 @@ import (
 // Callers need to be very careful operating on the trailing path, as trivial
 // mistakes like following symlinks can cause security bugs. Most people
 // should probably just use [MkdirAllInRoot] or [CreateInRoot].
-func MkdirAllParentInRoot(root, unsafePath string, mode os.FileMode) (*os.File, string, error) {
+func MkdirAllParentInRoot(root *os.File, unsafePath string, mode os.FileMode) (*os.File, string, error) {
 	// MkdirAllInRoot also does hallucinateUnsafePath, but we need to do it
 	// here first because when we split unsafePath into (dir, file) components
 	// we want to be doing so with the hallucinated path (so that trailing
 	// dangling symlinks are treated correctly).
-	unsafePath, err := hallucinateUnsafePath(root, unsafePath)
+	unsafePath, err := hallucinateUnsafePath(root.Name(), unsafePath)
 	if err != nil {
 		return nil, "", fmt.Errorf("failed to construct hallucinated target path: %w", err)
 	}

internal/pathrs/mkdirall_pathrslite.go

@@ -24,12 +24,11 @@ import (
 
 	"github.com/cyphar/filepath-securejoin/pathrs-lite"
 	"github.com/sirupsen/logrus"
-	"golang.org/x/sys/unix"
 )
 
 // MkdirAllInRoot attempts to make
 //
-//	path, _ := securejoin.SecureJoin(root, unsafePath)
+//	path, _ := securejoin.SecureJoin(root.Name(), unsafePath)
 //	os.MkdirAll(path, mode)
 //	os.Open(path)
 //
@@ -48,8 +47,8 @@ import (
 // handling if unsafePath has already been scoped within the rootfs (this is
 // needed for a lot of runc callers and fixing this would require reworking a
 // lot of path logic).
-func MkdirAllInRoot(root, unsafePath string, mode os.FileMode) (*os.File, error) {
-	unsafePath, err := hallucinateUnsafePath(root, unsafePath)
+func MkdirAllInRoot(root *os.File, unsafePath string, mode os.FileMode) (*os.File, error) {
+	unsafePath, err := hallucinateUnsafePath(root.Name(), unsafePath)
 	if err != nil {
 		return nil, fmt.Errorf("failed to construct hallucinated target path: %w", err)
 	}
@@ -67,13 +66,7 @@ func MkdirAllInRoot(root, unsafePath string, mode os.FileMode) (*os.File, error)
 		mode &= 0o1777
 	}
 
-	rootDir, err := os.OpenFile(root, unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
-	if err != nil {
-		return nil, fmt.Errorf("open root handle: %w", err)
-	}
-	defer rootDir.Close()
-
 	return retryEAGAIN(func() (*os.File, error) {
-		return pathrs.MkdirAllHandle(rootDir, unsafePath, mode)
+		return pathrs.MkdirAllHandle(root, unsafePath, mode)
 	})
 }

internal/pathrs/root_pathrslite.go

@@ -28,11 +28,11 @@ import (
 )
 
 // OpenInRoot opens the given path inside the root with the provided flags. It
-// is effectively shorthand for [securejoin.OpenInRoot] followed by
+// is effectively shorthand for [securejoin.OpenatInRoot] followed by
 // [securejoin.Reopen].
-func OpenInRoot(root, subpath string, flags int) (*os.File, error) {
+func OpenInRoot(root *os.File, subpath string, flags int) (*os.File, error) {
 	handle, err := retryEAGAIN(func() (*os.File, error) {
-		return pathrs.OpenInRoot(root, subpath)
+		return pathrs.OpenatInRoot(root, subpath)
 	})
 	if err != nil {
 		return nil, err
@@ -47,7 +47,7 @@ func OpenInRoot(root, subpath string, flags int) (*os.File, error) {
 // open(O_CREAT|O_NOFOLLOW) semantics. If you want the creation to use O_EXCL,
 // include it in the passed flags. The fileMode argument uses unix.* mode bits,
 // *not* os.FileMode.
-func CreateInRoot(root, subpath string, flags int, fileMode uint32) (*os.File, error) {
+func CreateInRoot(root *os.File, subpath string, flags int, fileMode uint32) (*os.File, error) {
 	dirFd, filename, err := MkdirAllParentInRoot(root, subpath, 0o755)
 	if err != nil {
 		return nil, err
@@ -63,5 +63,5 @@ func CreateInRoot(root, subpath string, flags int, fileMode uint32) (*os.File, e
 	if err != nil {
 		return nil, err
 	}
-	return os.NewFile(uintptr(fd), root+"/"+subpath), nil
+	return os.NewFile(uintptr(fd), root.Name()+"/"+subpath), nil
 }