libcontainer: move CleanPath and StripRoot to internal/pathrs

These helpers will be needed for the compatibility code added in future
patches in this series, but because "internal/pathrs" is imported by
"libcontainer/utils" we need to move them so that we can avoid circular
dependencies.

Because the old functions were in a non-internal package it is possible
some downstreams use them, so add some wrappers but mark them as
deprecated.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

internal/pathrs/path.go

@@ -19,6 +19,8 @@
 package pathrs
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -32,3 +34,51 @@ func IsLexicallyInRoot(root, path string) bool {
 	path = strings.TrimRight(path, "/")
 	return strings.HasPrefix(path+"/", root+"/")
 }
+
+// LexicallyCleanPath makes a path safe for use with filepath.Join. This is
+// done by not only cleaning the path, but also (if the path is relative)
+// adding a leading '/' and cleaning it (then removing the leading '/'). This
+// ensures that a path resulting from prepending another path will always
+// resolve to lexically be a subdirectory of the prefixed path. This is all
+// done lexically, so paths that include symlinks won't be safe as a result of
+// using CleanPath.
+func LexicallyCleanPath(path string) string {
+	// Deal with empty strings nicely.
+	if path == "" {
+		return ""
+	}
+
+	// Ensure that all paths are cleaned (especially problematic ones like
+	// "/../../../../../" which can cause lots of issues).
+
+	if filepath.IsAbs(path) {
+		return filepath.Clean(path)
+	}
+
+	// If the path isn't absolute, we need to do more processing to fix paths
+	// such as "../../../../<etc>/some/path". We also shouldn't convert absolute
+	// paths to relative ones.
+	path = filepath.Clean(string(os.PathSeparator) + path)
+	// This can't fail, as (by definition) all paths are relative to root.
+	path, _ = filepath.Rel(string(os.PathSeparator), path)
+
+	return path
+}
+
+// LexicallyStripRoot returns the passed path, stripping the root path if it
+// was (lexicially) inside it. Note that both passed paths will always be
+// treated as absolute, and the returned path will also always be absolute. In
+// addition, the paths are cleaned before stripping the root.
+func LexicallyStripRoot(root, path string) string {
+	// Make the paths clean and absolute.
+	root, path = LexicallyCleanPath("/"+root), LexicallyCleanPath("/"+path)
+	switch {
+	case path == root:
+		path = "/"
+	case root == "/":
+		// do nothing
+	default:
+		path = strings.TrimPrefix(path, root+"/")
+	}
+	return LexicallyCleanPath("/" + path)
+}

internal/pathrs/path_test.go

@@ -51,3 +51,70 @@ func TestIsLexicallyInRoot(t *testing.T) {
 		})
 	}
 }
+
+func TestLexicallyCleanPath(t *testing.T) {
+	path := LexicallyCleanPath("")
+	if path != "" {
+		t.Errorf("expected to receive empty string and received %s", path)
+	}
+
+	path = LexicallyCleanPath("rootfs")
+	if path != "rootfs" {
+		t.Errorf("expected to receive 'rootfs' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("../../../var")
+	if path != "var" {
+		t.Errorf("expected to receive 'var' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/../../../var")
+	if path != "/var" {
+		t.Errorf("expected to receive '/var' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/foo/bar/")
+	if path != "/foo/bar" {
+		t.Errorf("expected to receive '/foo/bar' and received %s", path)
+	}
+
+	path = LexicallyCleanPath("/foo/bar/../")
+	if path != "/foo" {
+		t.Errorf("expected to receive '/foo' and received %s", path)
+	}
+}
+
+func TestLexicallyStripRoot(t *testing.T) {
+	for _, test := range []struct {
+		root, path, out string
+	}{
+		// Works with multiple components.
+		{"/a/b", "/a/b/c", "/c"},
+		{"/hello/world", "/hello/world/the/quick-brown/fox", "/the/quick-brown/fox"},
+		// '/' must be a no-op.
+		{"/", "/a/b/c", "/a/b/c"},
+		// Must be the correct order.
+		{"/a/b", "/a/c/b", "/a/c/b"},
+		// Must be at start.
+		{"/abc/def", "/foo/abc/def/bar", "/foo/abc/def/bar"},
+		// Must be a lexical parent.
+		{"/foo/bar", "/foo/barSAMECOMPONENT", "/foo/barSAMECOMPONENT"},
+		// Must only strip the root once.
+		{"/foo/bar", "/foo/bar/foo/bar/baz", "/foo/bar/baz"},
+		// Deal with .. in a fairly sane way.
+		{"/foo/bar", "/foo/bar/../baz", "/foo/baz"},
+		{"/foo/bar", "../../../../../../foo/bar/baz", "/baz"},
+		{"/foo/bar", "/../../../../../../foo/bar/baz", "/baz"},
+		{"/foo/bar/../baz", "/foo/baz/bar", "/bar"},
+		{"/foo/bar/../baz", "/foo/baz/../bar/../baz/./foo", "/foo"},
+		// All paths are made absolute before stripping.
+		{"foo/bar", "/foo/bar/baz/bee", "/baz/bee"},
+		{"/foo/bar", "foo/bar/baz/beef", "/baz/beef"},
+		{"foo/bar", "foo/bar/baz/beets", "/baz/beets"},
+	} {
+		got := LexicallyStripRoot(test.root, test.path)
+		if got != test.out {
+			t.Errorf("LexicallyStripRoot(%q, %q) -- got %q, expected %q", test.root, test.path, got, test.out)
+		}
+	}
+}