Use a custom, world-accessible, /tmp mount

This solves two problems:

1. Using the host /tmp directly meant that concurrent tests could
   collide attempting to use the same file, and that misbehaving tests
   never have their /tmp output cleaned up.
2. Host /tmp is not world-accessible on all hosts. Some tests (e.g.,
   sticky) access files in /tmp from other users, so we need to ensure
   that its /tmp is world-accessible.

PiperOrigin-RevId: 235637873
Change-Id: I7555224685ac5b93af88c403196b09ce1bb2bfe7

test/syscalls/syscall_test_runner.go

@@ -115,10 +115,34 @@ func runTestCaseRunsc(testBin string, tc gtest.TestCase, t *testing.T) {
 	spec.Mounts = nil
 	if *useTmpfs {
 		// Forces '/tmp' to be mounted as tmpfs, otherwise test that rely on
-		// features available in gVisor's tmpfs and not gofers, may fail.
-		spec.Mounts = []specs.Mount{
-			{Destination: "/tmp", Type: "tmpfs"},
+		// features only available in gVisor's internal tmpfs may fail.
+		spec.Mounts = append(spec.Mounts, specs.Mount{
+			Destination: "/tmp",
+			Type:        "tmpfs",
+		})
+	} else {
+		// Use a gofer-backed directory as '/tmp'.
+		//
+		// Tests might be running in parallel, so make sure each has a
+		// unique test temp dir.
+		//
+		// Some tests (e.g., sticky) access this mount from other
+		// users, so make sure it is world-accessible.
+		tmpDir, err := ioutil.TempDir(testutil.TmpDir(), "")
+		if err != nil {
+			t.Fatalf("could not create temp dir: %v", err)
 		}
+		defer os.RemoveAll(tmpDir)
+
+		if err := os.Chmod(tmpDir, 0777); err != nil {
+			t.Fatalf("could not chmod temp dir: %v", err)
+		}
+
+		spec.Mounts = append(spec.Mounts, specs.Mount{
+			Type:        "bind",
+			Destination: "/tmp",
+			Source:      tmpDir,
+		})
 	}
 
 	// Set environment variable that indicates we are