2 files changed, 105 insertions, 0 deletions
diff --git a/pkg/sentry/fs/path.go b/pkg/sentry/fs/path.go
index 91a9a8ffd..52139b648 100644
--- a/pkg/sentry/fs/path.go
+++ b/pkg/sentry/fs/path.go
@@ -14,6 +14,11 @@
 
 package fs
 
+import (
+	"path/filepath"
+	"strings"
+)
+
 // TrimTrailingSlashes trims any trailing slashes.
 //
 // The returned boolean indicates whether any changes were made.
@@ -90,3 +95,25 @@ func SplitFirst(path string) (current, remainder string) {
 		return current, remainder
 	}
 }
+
+// IsSubpath checks whether the first path is a (strict) descendent of the
+// second. If it is a subpath, then true is returned along with a clean
+// relative path from the second path to the first. Otherwise false is
+// returned.
+func IsSubpath(subpath, path string) (string, bool) {
+	cleanPath := filepath.Clean(path)
+	cleanSubpath := filepath.Clean(subpath)
+
+	// Add a trailing slash to the path if it does not already have one.
+	if len(cleanPath) == 0 || cleanPath[len(cleanPath)-1] != '/' {
+		cleanPath += "/"
+	}
+	if cleanPath == cleanSubpath {
+		// Paths are equal, thus not a strict subpath.
+		return "", false
+	}
+	if strings.HasPrefix(cleanSubpath, cleanPath) {
+		return strings.TrimPrefix(cleanSubpath, cleanPath), true
+	}
+	return "", false
+}
diff --git a/pkg/sentry/fs/path_test.go b/pkg/sentry/fs/path_test.go
index 391b010a7..4ba1498f6 100644
--- a/pkg/sentry/fs/path_test.go
+++ b/pkg/sentry/fs/path_test.go
@@ -209,3 +209,81 @@ func TestSplitFirst(t *testing.T) {
 		}
 	}
 }
+
+// TestIsSubpath tests the IsSubpath method.
+func TestIsSubpath(t *testing.T) {
+	tcs := []struct {
+		// Two absolute paths.
+		pathA string
+		pathB string
+
+		// Whether pathA is a subpath of pathB.
+		wantIsSubpath bool
+
+		// Relative path from pathA to pathB. Only checked if
+		// wantIsSubpath is true.
+		wantRelpath string
+	}{
+		{
+			pathA:         "/foo/bar/baz",
+			pathB:         "/foo",
+			wantIsSubpath: true,
+			wantRelpath:   "bar/baz",
+		},
+		{
+			pathA:         "/foo",
+			pathB:         "/foo/bar/baz",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/foo",
+			pathB:         "/foo",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/foobar",
+			pathB:         "/foo",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/foo",
+			pathB:         "/foobar",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/foo",
+			pathB:         "/foobar",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/",
+			pathB:         "/foo",
+			wantIsSubpath: false,
+		},
+		{
+			pathA:         "/foo",
+			pathB:         "/",
+			wantIsSubpath: true,
+			wantRelpath:   "foo",
+		},
+		{
+			pathA:         "/foo/bar/../bar",
+			pathB:         "/foo",
+			wantIsSubpath: true,
+			wantRelpath:   "bar",
+		},
+		{
+			pathA:         "/foo/bar",
+			pathB:         "/foo/../foo",
+			wantIsSubpath: true,
+			wantRelpath:   "bar",
+		},
+	}
+
+	for _, tc := range tcs {
+		gotRelpath, gotIsSubpath := IsSubpath(tc.pathA, tc.pathB)
+		if gotRelpath != tc.wantRelpath || gotIsSubpath != tc.wantIsSubpath {
+			t.Errorf("IsSubpath(%q, %q) got %q %t, want %q %t", tc.pathA, tc.pathB, gotRelpath, gotIsSubpath, tc.wantRelpath, tc.wantIsSubpath)
+		}
+	}
+}