Follow links safely during directory discovery

internal/engine/filesystem_links.go

@@ -0,0 +1,212 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+var (
+	errImportDirectoryLinkCycle = errors.New("directory link cycle")
+	errImportPathOutsideRoot    = errors.New("resolved import path is outside the configuration root")
+)
+
+// importEntryTarget describes a discovered entry without changing its lexical
+// candidate spelling. identity is resolved only to detect active traversal loops.
+type importEntryTarget struct {
+	symlink  bool
+	dir      bool
+	identity string
+}
+
+func (a *filesystemAccess) importEntryTarget(name string, entry fs.DirEntry, follow bool) (importEntryTarget, error) {
+	if a.native == nil {
+		relative, err := a.relative(name)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		info, err := a.root.Lstat(relative)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		symlink := info.Mode()&fs.ModeSymlink != 0
+		if symlink && !follow {
+			return importEntryTarget{symlink: true}, nil
+		}
+		if !symlink && !info.IsDir() {
+			return importEntryTarget{}, nil
+		}
+		resolved, err := resolveHostImportPath(a, relative)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		resolvedPath := filepath.Join(a.rootPath, filepath.FromSlash(resolved))
+		info, err = a.root.Stat(filepath.FromSlash(resolved))
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		resolvedPath = filepath.Clean(resolvedPath)
+		return importEntryTarget{symlink: symlink, dir: info.IsDir(), identity: resolvedPath}, nil
+	}
+
+	linkFS, linksKnown := a.native.(fs.ReadLinkFS)
+	if linksKnown {
+		info, err := linkFS.Lstat(name)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		symlink := info.Mode()&fs.ModeSymlink != 0
+		if symlink && !follow {
+			return importEntryTarget{symlink: true}, nil
+		}
+		if !symlink && !info.IsDir() {
+			return importEntryTarget{}, nil
+		}
+		resolved, err := resolveNativeImportPath(a, linkFS, name)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		if !a.contains(resolved) {
+			return importEntryTarget{}, errImportPathOutsideRoot
+		}
+		info, err = fs.Stat(a.native, resolved)
+		if err != nil {
+			return importEntryTarget{}, err
+		}
+		return importEntryTarget{symlink: symlink, dir: info.IsDir(), identity: resolved}, nil
+	}
+
+	symlink := entry != nil && entry.Type()&fs.ModeSymlink != 0
+	if symlink && !follow {
+		return importEntryTarget{symlink: true}, nil
+	}
+	if entry != nil && !symlink {
+		mode := entry.Type()
+		if mode == 0 {
+			info, err := entry.Info()
+			if err != nil {
+				return importEntryTarget{}, err
+			}
+			mode = info.Mode()
+			symlink = mode&fs.ModeSymlink != 0
+			if symlink && !follow {
+				return importEntryTarget{symlink: true}, nil
+			}
+		}
+		if !symlink && !mode.IsDir() {
+			return importEntryTarget{}, nil
+		}
+		if !symlink && mode.IsDir() {
+			return importEntryTarget{dir: true, identity: name}, nil
+		}
+	}
+	info, err := fs.Stat(a.native, name)
+	if err != nil {
+		return importEntryTarget{}, err
+	}
+	return importEntryTarget{symlink: symlink, dir: info.IsDir(), identity: name}, nil
+}
+
+func resolveHostImportPath(access *filesystemAccess, name string) (string, error) {
+	pending := strings.Split(filepath.ToSlash(name), "/")
+	resolved := ""
+	activeLinks := make(map[string]struct{})
+	for len(pending) > 0 {
+		segment := pending[0]
+		pending = pending[1:]
+		candidate := path.Join(resolved, segment)
+		if candidate == "" {
+			candidate = "."
+		}
+		info, err := access.root.Lstat(filepath.FromSlash(candidate))
+		if err != nil {
+			return "", err
+		}
+		if info.Mode()&fs.ModeSymlink == 0 {
+			resolved = candidate
+			continue
+		}
+		if _, exists := activeLinks[candidate]; exists {
+			return "", errImportDirectoryLinkCycle
+		}
+		activeLinks[candidate] = struct{}{}
+		target, err := access.root.Readlink(filepath.FromSlash(candidate))
+		if err != nil {
+			return "", err
+		}
+		if filepath.IsAbs(target) {
+			return "", errImportPathOutsideRoot
+		}
+		targetPath := path.Clean(path.Join(path.Dir(candidate), filepath.ToSlash(target)))
+		if !withinPortableRoot(targetPath) {
+			return "", errImportPathOutsideRoot
+		}
+		remaining := append([]string(nil), pending...)
+		pending = append(strings.Split(targetPath, "/"), remaining...)
+		resolved = ""
+	}
+	if resolved == "" {
+		resolved = "."
+	}
+	return resolved, nil
+}
+
+func withinPortableRoot(name string) bool {
+	return name == "." || name != ".." && !strings.HasPrefix(name, "../") && !strings.HasPrefix(name, "/")
+}
+
+func resolveNativeImportPath(access *filesystemAccess, filesystem fs.ReadLinkFS, name string) (string, error) {
+	if !fs.ValidPath(name) || !access.contains(name) {
+		return "", fs.ErrInvalid
+	}
+	pending := strings.Split(name, "/")
+	resolved := ""
+	activeLinks := make(map[string]struct{})
+	for len(pending) > 0 {
+		segment := pending[0]
+		pending = pending[1:]
+		candidate := path.Join(resolved, segment)
+		if candidate == "" {
+			candidate = "."
+		}
+		if !access.contains(candidate) {
+			return "", errImportPathOutsideRoot
+		}
+		info, err := filesystem.Lstat(candidate)
+		if err != nil {
+			return "", err
+		}
+		if info.Mode()&fs.ModeSymlink == 0 {
+			resolved = candidate
+			continue
+		}
+		if _, exists := activeLinks[candidate]; exists {
+			return "", errImportDirectoryLinkCycle
+		}
+		activeLinks[candidate] = struct{}{}
+		target, err := filesystem.ReadLink(candidate)
+		if err != nil {
+			return "", err
+		}
+		if target == "" || strings.HasPrefix(target, "/") {
+			return "", errImportPathOutsideRoot
+		}
+		targetPath := path.Clean(path.Join(path.Dir(candidate), target))
+		if !fs.ValidPath(targetPath) || !access.contains(targetPath) {
+			return "", errImportPathOutsideRoot
+		}
+		remaining := append([]string(nil), pending...)
+		pending = append(strings.Split(targetPath, "/"), remaining...)
+		resolved = ""
+	}
+	if resolved == "" {
+		resolved = "."
+	}
+	return resolved, nil
+}
+
+func isImportLinkCycle(err error) bool {
+	return errors.Is(err, errImportDirectoryLinkCycle)
+}