Add option to propagate common signals to sandboxed process

This fixes, for example, pressing CTRL+C to send a SIGINT during interactive use
and also allows for things like passing SIGWINCH to neovim running inside the
bubblewrap to resize the window.

Extends [bubblewrap] Propagate SIGTERM and SIGINT to child #402
Borrows test from q3cpma

Co-authored-by: Earl Chew <earl_chew@yahoo.com>
Co-authored-by: q3cpma <q3cpma@posteo.net>
Signed-off-by: Aaruni Kaushik <akaushik@mathematik.uni-kl.de>

Author: 3 people

bubblewrap.c

@@ -78,6 +78,7 @@ static bool opt_unshare_cgroup_try = false;
 static bool opt_needs_devpts = false;
 static bool opt_new_session = false;
 static bool opt_die_with_parent = false;
+static bool opt_forward_signals = false;
 static uid_t opt_sandbox_uid = -1;
 static gid_t opt_sandbox_gid = -1;
 static int opt_sync_fd = -1;
@@ -373,6 +374,7 @@ usage (int ecode, FILE *out)
            "    --perms OCTAL                Set permissions of next argument (--bind-data, --file, etc.)\n"
            "    --size BYTES                 Set size of next argument (only for --tmpfs)\n"
            "    --chmod OCTAL PATH           Change permissions of PATH (must already exist)\n"
+           "    --forward-signals            Forward various commonly used signals to the child process.\n"
           );
   exit (ecode);
 }
@@ -387,6 +389,33 @@ handle_die_with_parent (void)
     die_with_error ("prctl");
 }
 
+static int forwarded_signals[] =
+{
+  SIGINT,
+  SIGTERM,
+  SIGCONT,
+  SIGHUP,
+  SIGQUIT,
+  SIGUSR1,
+  SIGUSR2,
+  SIGWINCH,
+};
+
+static void
+block_forwarded_signals (sigset_t *prevmask)
+{
+  sigset_t mask;
+  size_t i;
+
+  sigemptyset (&mask);
+
+  for (i = 0; i < N_ELEMENTS (forwarded_signals); i++)
+    sigaddset (&mask, forwarded_signals[i]);
+
+  if (sigprocmask (SIG_BLOCK, &mask, prevmask) == -1)
+    die_with_error ("sigprocmask");
+}
+
 static void
 block_sigchild (void)
 {
@@ -508,6 +537,7 @@ monitor_child (int event_fd, pid_t child_pid, int setup_finished_fd)
   int exitc;
   pid_t died_pid;
   int died_status;
+  size_t i;
 
   /* Close all extra fds in the monitoring process.
      Any passed in fds have been passed on to the child anyway. */
@@ -523,6 +553,9 @@ monitor_child (int event_fd, pid_t child_pid, int setup_finished_fd)
   sigemptyset (&mask);
   sigaddset (&mask, SIGCHLD);
 
+  for (i = 0; i < N_ELEMENTS (forwarded_signals); i++)
+    sigaddset (&mask, forwarded_signals[i]);
+
   signal_fd = signalfd (-1, &mask, SFD_CLOEXEC | SFD_NONBLOCK);
   if (signal_fd == -1)
     die_with_error ("Can't create signalfd");
@@ -561,12 +594,21 @@ monitor_child (int event_fd, pid_t child_pid, int setup_finished_fd)
         }
 
       /* We need to read the signal_fd, or it will keep polling as read,
-       * however we ignore the details as we get them from waitpid
+       * however we ignore the details for SIGCHLD as we get them from waitpid
        * below anyway */
       s = read (signal_fd, &fdsi, sizeof (struct signalfd_siginfo));
       if (s == -1 && errno != EINTR && errno != EAGAIN)
         die_with_error ("read signalfd");
 
+      /* Propagate signal to child so that it will take the correct
+       * action. This avoids the parent terminating, leaving an orphan. */
+      if (fdsi.ssi_signo != SIGCHLD)
+        {
+          s = kill (child_pid, fdsi.ssi_signo);
+          if (s != 0 && s != ESRCH)
+            die_with_error("kill child");
+        }
+
       /* We may actually get several sigchld compressed into one
          SIGCHLD, so we have to handle all of them. */
       while ((died_pid = waitpid (-1, &died_status, WNOHANG)) > 0)
@@ -2742,6 +2784,10 @@ parse_args_recurse (int          *argcp,
           argc -= 1;
           break;
         }
+      else if (strcmp (arg, "--forward-signals") == 0)
+        {
+          opt_forward_signals = true;
+        }
       else if (*arg == '-')
         {
           die ("Unknown option %s", arg);
@@ -2889,6 +2935,8 @@ main (int    argc,
   int intermediate_pids_sockets[2] = {-1, -1};
   const char *exec_path = NULL;
   int i;
+  sigset_t sigmask_before_forwarding;
+  sigemptyset (&sigmask_before_forwarding);
 
   /* Handle --version early on before we try to acquire/drop
    * any capabilities so it works in a build environment;
@@ -3062,6 +3110,10 @@ main (int    argc,
   /* We block sigchild here so that we can use signalfd in the monitor. */
   block_sigchild ();
 
+  /* We block other signals here to avoid leaving an orphan. */
+  if (opt_forward_signals)
+    block_forwarded_signals (&sigmask_before_forwarding);
+
   clone_flags = SIGCHLD | CLONE_NEWNS;
   if (opt_unshare_user)
     clone_flags |= CLONE_NEWUSER;
@@ -3212,6 +3264,13 @@ main (int    argc,
       return monitor_child (event_fd, pid, setup_finished_pipe[0]);
     }
 
+  /* Restore the state of sigmask from before the blocking. */
+  if (opt_forward_signals)
+  {
+    if (sigprocmask (SIG_SETMASK, &sigmask_before_forwarding, NULL) != 0)
+      die_with_error ("sigprocmask");
+  }
+
   if (opt_pidns_fd > 0)
     {
       if (setns (opt_pidns_fd, CLONE_NEWPID) != 0)

tests/meson.build

@@ -20,6 +20,7 @@ test_scripts = [
   'test-seccomp.py',
   'test-specifying-pidns.sh',
   'test-specifying-userns.sh',
+  'test-forward-signals.sh'
 ]
 
 test_env = environment()

tests/test-forward-signals.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+set -xeuo pipefail
+
+srcd=$(cd $(dirname "$0") && pwd)
+. ${srcd}/libtest.sh
+test_count=0
+ok () {
+    test_count=$((test_count + 1))
+    echo ok $test_count "$@"
+}
+ok_skip () {
+    ok "# SKIP" "$@"
+}
+done_testing () {
+    echo "1..$test_count"
+}
+
+sh_path=/bin/sh
+
+out=$(
+        $RUN \
+                "$sh_path" -c 'trap "echo USR1; exit" USR1; sleep 0.5' &
+        sleep 0.1
+        kill -USR1 $!
+   )
+if [ -z "$out" ]; then
+    ok "No signals forwarded without --forward-signal"
+else
+    false
+fi
+
+out=$(
+        $RUN --forward-signals \
+                "$sh_path" -c 'trap "echo USR1; exit" USR1; sleep 0.5' &
+        sleep 0.1
+        kill -USR1 $!
+   )
+   
+if [ "$out" = USR1 ]; then
+    ok "Successfully forwarded signals with --forward-signals"
+else
+    false
+fi