nptl: Zero-extend arguments to SETXID syscalls [BZ #26248]

nptl has

/* Opcodes and data types for communication with the signal handler to
   change user/group IDs.  */
struct xid_command
{
  int syscall_no;
  long int id[3];
  volatile int cntr;
  volatile int error;
};

 /* This must be last, otherwise the current thread might not have
     permissions to send SIGSETXID syscall to the other threads.  */
  result = INTERNAL_SYSCALL_NCS (cmdp->syscall_no, 3,
                                 cmdp->id[0], cmdp->id[1], cmdp->id[2]);

But the second argument of setgroups syscal is a pointer:

       int setgroups (size_t size, const gid_t *list);

But on x32, pointers passed to syscall must have pointer type so that
they will be zero-extended.  The kernel XID arguments are unsigned and
do not require sign extension.  Change xid_command to

struct xid_command
{
  int syscall_no;
  unsigned long int id[3];
  volatile int cntr;
  volatile int error;
};

so that all arguments are zero-extended.  A testcase is added for x32 and
setgroups returned with EFAULT when running as root without the fix.

nptl/Makefile

@@ -310,7 +310,7 @@ tests-internal := tst-robustpi8 tst-rwlock19 tst-rwlock20 \
 		  tst-setgetname \
 
 xtests = tst-setuid1 tst-setuid1-static tst-setuid2 \
-	tst-mutexpp1 tst-mutexpp6 tst-mutexpp10
+	tst-mutexpp1 tst-mutexpp6 tst-mutexpp10 tst-setgroups
 
 # This test can run into task limits because of a linux kernel bug
 # and then cause the make process to fail too, see bug 24537.

nptl/descr.h

@@ -95,7 +95,13 @@ struct pthread_unwind_buf
 struct xid_command
 {
   int syscall_no;
-  long int id[3];
+  /* Enforce zero-extension for the pointer argument in
+
+     int setgroups (size_t size, const gid_t *list);
+
+     The kernel XID arguments are unsigned and do not require sign
+     extension.  */
+  unsigned long int id[3];
   volatile int cntr;
   volatile int error; /* -1: no call yet, 0: success seen, >0: error seen.  */
 };

nptl/tst-setgroups.c

@@ -0,0 +1,79 @@
+/* Test setgroups as root and in the presence of threads (Bug 26248)
+   Copyright (C) 2020 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 <stdlib.h>
+#include <limits.h>
+#include <grp.h>
+#include <errno.h>
+#include <error.h>
+#include <support/xthread.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+#include <support/xunistd.h>
+
+/* The purpose of this test is to test the setgroups API as root and in
+   the presence of threads.  Once we create a thread the setgroups
+   implementation must ensure that all threads are set to the same
+   group and this operation should not fail. Lastly we test setgroups
+   with a zero sized group and a bad address and verify we get EPERM.  */
+
+static void *
+start_routine (void *args)
+{
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  int size;
+  /* NB: Stack address can be at 0xfffXXXXX on 32-bit OSes.  */
+  gid_t list[NGROUPS_MAX];
+  int status = EXIT_SUCCESS;
+
+  pthread_t thread = xpthread_create (NULL, start_routine, NULL);
+
+  size = getgroups (sizeof (list) / sizeof (list[0]), list);
+  if (size < 0)
+    {
+      status = EXIT_FAILURE;
+      error (0, errno, "getgroups failed");
+    }
+  if (setgroups (size, list) < 0)
+    {
+      if (errno == EPERM)
+	status = EXIT_UNSUPPORTED;
+      else
+	{
+	  status = EXIT_FAILURE;
+	  error (0, errno, "setgroups failed");
+	}
+    }
+
+  if (status == EXIT_SUCCESS && setgroups (0, list) < 0)
+    {
+      status = EXIT_FAILURE;
+      error (0, errno, "setgroups failed");
+    }
+
+  xpthread_join (thread);
+
+  return status;
+}
+
+#include <support/test-driver.c>