posix: Add terminal control setting support for posix_spawn

Currently there is no proper way to set the controlling terminal through
posix_spawn in race free manner [1].  This forces shell implementations
to keep using fork+exec when launching background process groups,
even when using posix_spawn yields better performance.

This patch adds a new GNU extension so the creating process can
configure the created process terminal group.  This is done with a new
flag, POSIX_SPAWN_TCSETPGROUP, along with two new attribute functions:
posix_spawnattr_tcsetpgrp_np, and posix_spawnattr_tcgetpgrp_np.
The function sets a new attribute, spawn-tcgroupfd, that references to
the controlling terminal.

The controlling terminal is set after the spawn-pgroup attribute, and
uses the spawn-tcgroupfd along with current creating process group
(so it is composable with POSIX_SPAWN_SETPGROUP).

To create a process and set the controlling terminal, one can use the
following sequence:

    posix_spawnattr_t attr;
    posix_spawnattr_init (&attr);
    posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP);
    posix_spawnattr_tcsetpgrp_np (&attr, tcfd);

If the idea is also to create a new process groups:

    posix_spawnattr_t attr;
    posix_spawnattr_init (&attr);
    posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP
				     | POSIX_SPAWN_SETPGROUP);
    posix_spawnattr_tcsetpgrp_np (&attr, tcfd);
    posix_spawnattr_setpgroup (&attr, 0);

The controlling terminal file descriptor is ignored if the new flag is
not set.

This interface is slight different than the one provided by QNX [2],
which only provides the POSIX_SPAWN_TCSETPGROUP flag.  The QNX
documentation does not specify how the controlling terminal is obtained
nor how it iteracts with POSIX_SPAWN_SETPGROUP.  Since a glibc
implementation is library based, it is more straightforward and avoid
requires additional file descriptor operations to request the caller
to setup the controlling terminal file descriptor (and it also allows
a bit less error handling by posix_spawn).

Checked on x86_64-linux-gnu and i686-linux-gnu.

[1] https://github.com/ksh93/ksh/issues/79
[2] https://www.qnx.com/developers/docs/7.0.0/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/posix_spawn.html

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 175 insertions, 0 deletions

diff --git a/posix/tst-spawn6.c b/posix/tst-spawn6.c
new file mode 100644
index 0000000000..5f95bd1938
--- /dev/null
+++ b/posix/tst-spawn6.c
@@ -0,0 +1,175 @@
+/* Check posix_spawn set controlling terminal extension.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <intprops.h>
+#include <paths.h>
+#include <spawn.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+#include <sys/wait.h>
+
+static int
+handle_restart (const char *argv1)
+{
+  int fd = xopen (_PATH_TTY, O_RDONLY, 0600);
+
+  /* If process group is not changed (POSIX_SPAWN_SETPGROUP), then check
+     the creating process one, otherwise check against the process group
+     itself.  */
+  pid_t pgrp;
+  if (strcmp (argv1, "setgrpr") != 0)
+    TEST_COMPARE (sscanf (argv1, "%d", &pgrp), 1);
+  else
+    {
+      pgrp = getpgrp ();
+      /* Check if a new process group was actually created.  */
+      pid_t ppid = getppid ();
+      pid_t pgid = getpgid (ppid);
+      TEST_VERIFY (pgid != pgrp);
+    }
+
+  TEST_COMPARE (tcgetpgrp (fd), pgrp);
+
+  xclose (fd);
+  return 0;
+}
+
+static int restart;
+#define CMDLINE_OPTIONS \
+  { "restart", no_argument, &restart, 1 },
+
+static void
+run_subprogram (int argc, char *argv[], const posix_spawnattr_t *attr,
+		int exp_err)
+{
+  short int flags;
+  TEST_COMPARE (posix_spawnattr_getflags (attr, &flags), 0);
+  bool setpgrp = flags & POSIX_SPAWN_SETPGROUP;
+
+  char *spargv[9];
+  char pgrp[INT_STRLEN_BOUND (pid_t)];
+
+  int i = 0;
+  for (; i < argc - 1; i++)
+    spargv[i] = argv[i + 1];
+  spargv[i++] = (char *) "--direct";
+  spargv[i++] = (char *) "--restart";
+  if (setpgrp)
+    spargv[i++] = (char *) "setgrpr";
+  else
+    {
+      snprintf (pgrp, sizeof pgrp, "%d", getpgrp ());
+      spargv[i++] = pgrp;
+    }
+  spargv[i] = NULL;
+  TEST_VERIFY_EXIT (i < array_length (spargv));
+
+  pid_t pid;
+  TEST_COMPARE (posix_spawn (&pid, argv[1], NULL, attr, spargv, environ),
+		exp_err);
+  if (exp_err != 0)
+    return;
+
+  int status;
+  TEST_COMPARE (xwaitpid (pid, &status, WUNTRACED), pid);
+  TEST_VERIFY (WIFEXITED (status));
+  TEST_VERIFY (!WIFSTOPPED (status));
+  TEST_VERIFY (!WIFSIGNALED (status));
+  TEST_COMPARE (WEXITSTATUS (status), 0);
+}
+
+static int
+do_test (int argc, char *argv[])
+{
+  /* We must have either:
+     - four parameters left if called initially:
+       + path to ld.so         optional
+       + "--library-path"      optional
+       + the library path      optional
+       + the application name
+     - six parameters left if called through re-execution:
+       + --setgrpr             optional
+   */
+
+  if (restart)
+    return handle_restart (argv[1]);
+
+  int tcfd = xopen (_PATH_TTY, O_RDONLY, 0600);
+
+  /* Check getters and setters.  */
+  {
+    posix_spawnattr_t attr;
+    TEST_COMPARE (posix_spawnattr_init (&attr), 0);
+    TEST_COMPARE (posix_spawnattr_tcsetpgrp_np (&attr, tcfd), 0);
+
+    int fd;
+    TEST_COMPARE (posix_spawnattr_tcgetpgrp_np (&attr, &fd), 0);
+    TEST_COMPARE (tcfd, fd);
+  }
+
+  /* Check setting the controlling terminal without changing the group.  */
+  {
+    posix_spawnattr_t attr;
+    TEST_COMPARE (posix_spawnattr_init (&attr), 0);
+    TEST_COMPARE (posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP),
+		  0);
+    TEST_COMPARE (posix_spawnattr_tcsetpgrp_np (&attr, tcfd), 0);
+
+    run_subprogram (argc, argv, &attr, 0);
+  }
+
+  /* Check setting both the controlling terminal and the create a new process
+     group.  */
+  {
+    posix_spawnattr_t attr;
+    TEST_COMPARE (posix_spawnattr_init (&attr), 0);
+    TEST_COMPARE (posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP
+						   | POSIX_SPAWN_SETPGROUP),
+		  0);
+    TEST_COMPARE (posix_spawnattr_setpgroup (&attr, 0), 0);
+    TEST_COMPARE (posix_spawnattr_tcsetpgrp_np (&attr, tcfd), 0);
+
+    run_subprogram (argc, argv, &attr, 0);
+  }
+
+  /* Trying to set the controlling terminal after a setsid incurs in a ENOTTY
+     from tcsetpgrp.  */
+  {
+    posix_spawnattr_t attr;
+    TEST_COMPARE (posix_spawnattr_init (&attr), 0);
+    TEST_COMPARE (posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP
+						   | POSIX_SPAWN_SETSID), 0);
+    TEST_COMPARE (posix_spawnattr_tcsetpgrp_np (&attr, tcfd), 0);
+
+    run_subprogram (argc, argv, &attr, ENOTTY);
+  }
+
+  xclose (tcfd);
+
+  return 0;
+}
+
+#define TEST_FUNCTION_ARGV do_test
+#include <support/test-driver.c>