bug#47628: webkitgtk-2.32.0 fails to launch without /usr/bin

[Mark H Weaver <mhw@netris.org>]

I suspect that the relevant bit that needs to be changed is line 779 of
the following file in the webkitgtk-2.32.0 source code:

  Source/WebKit/UIProcess/Launcher/glib/BubblewrapLauncher.cpp

Most likely, that line can simply be deleted.  Here's the relevant
excerpt, with line 779 marked by "==>":

--8<---------------cut here---------------start------------->8---
GRefPtr<GSubprocess> bubblewrapSpawn(GSubprocessLauncher* launcher, const ProcessLauncher::LaunchOptions& launchOptions, char** argv, GError **error)
{
    ASSERT(launcher);

    // For now we are just considering the network process trusted as it
    // requires a lot of access but doesn't execute arbitrary code like
    // the WebProcess where our focus lies.
    if (launchOptions.processType == ProcessLauncher::ProcessType::Network)
        return adoptGRef(g_subprocess_launcher_spawnv(launcher, argv, error));

    const char* runDir = g_get_user_runtime_dir();
    Vector<CString> sandboxArgs = {
        "--die-with-parent",
        "--unshare-pid",
        "--unshare-uts",

        // We assume /etc has safe permissions.
        // At a later point we can start masking privacy-concerning files.
        "--ro-bind", "/etc", "/etc",
        "--dev", "/dev",
        "--proc", "/proc",
        "--tmpfs", "/tmp",
        "--unsetenv", "TMPDIR",
        "--dir", runDir,
        "--setenv", "XDG_RUNTIME_DIR", runDir,
        "--symlink", "../run", "/var/run",
        "--symlink", "../tmp", "/var/tmp",
        "--ro-bind", "/sys/block", "/sys/block",
        "--ro-bind", "/sys/bus", "/sys/bus",
        "--ro-bind", "/sys/class", "/sys/class",
        "--ro-bind", "/sys/dev", "/sys/dev",
        "--ro-bind", "/sys/devices", "/sys/devices",

        "--ro-bind-try", "/usr/share", "/usr/share",
        "--ro-bind-try", "/usr/local/share", "/usr/local/share",
        "--ro-bind-try", DATADIR, DATADIR,

       // Bind mount the store inside the WebKitGTK sandbox.
       "--ro-bind", "@storedir@", "@storedir@",

        // We only grant access to the libdirs webkit is built with and
        // guess system libdirs. This will always have some edge cases.
        "--ro-bind-try", "/lib", "/lib",
        "--ro-bind-try", "/usr/lib", "/usr/lib",
        "--ro-bind-try", "/usr/local/lib", "/usr/local/lib",
        "--ro-bind-try", LIBDIR, LIBDIR,
        "--ro-bind-try", "/lib64", "/lib64",
        "--ro-bind-try", "/usr/lib64", "/usr/lib64",
        "--ro-bind-try", "/usr/local/lib64", "/usr/local/lib64",

        "--ro-bind-try", PKGLIBEXECDIR, PKGLIBEXECDIR,
    };

    if (launchOptions.processType == ProcessLauncher::ProcessType::DBusProxy) {
        sandboxArgs.appendVector(Vector<CString>({
==>         "--ro-bind", "/usr/bin", "/usr/bin",
            // This is a lot of access, but xdg-dbus-proxy is trusted so that's OK. It's sandboxed
            // only because we have to mount .flatpak-info in its mount namespace. The user rundir
            // is where we mount our proxy socket.
            "--bind", runDir, runDir,
        }));
    } else {
        // xdg-dbus-proxy needs access to host abstract sockets to connect to the a11y bus. Secure
        // host services must not use abstract sockets. Otherwise, only the network process should
        // have network access, and the network process is not sandboxed at all.
        sandboxArgs.appendVector(Vector<CString>({
            "--unshare-net"
        }));
    }
--8<---------------cut here---------------end--------------->8---

       Mark