Tests for minimal signal handler functionality in MINSIGSTKSZ space.

There is general agreement that the very short list of things that ISO
C says you can do in an async signal handler should all work when the
handler is running on an alternate signal stack with only MINSIGSTKSZ
space.  This patch adds tests to make sure those things do work.

To facilitate this, there is a new set of test support routines for
setting up alternate signal stacks; see support/xsignal.h for the API.

         * support/xsignal.h (xalloc_sigstack, xfree_sigstack)
         (xget_sigstack_location): New test support functions.
         * support/xsigstack.c: New file, implementing them.
         * support/tst-xsigstack.c: New test for them.
         * support/Makefile: Update.

         * signal/tst-minsigstksz-1.c
         * signal/tst-minsigstksz-2.c
         * signal/tst-minsigstksz-3.c
         * signal/tst-minsigstksz-3a.c
         * signal/tst-minsigstksz-4.c: New tests.
         * signal/Makefile: Run them.

11 files changed, 602 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 36bb3ed885..7cc7916a17 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2019-01-16  Zack Weinberg  <zackw@panix.com>
+
+	 * support/xsignal.h (xalloc_sigstack, xfree_sigstack)
+	 (xget_sigstack_location): New test support functions.
+	 * support/xsigstack.c: New file, implementing them.
+	 * support/tst-xsigstack.c: New test for them.
+	 * support/Makefile: Update.
+
+	 * signal/tst-minsigstksz-1.c
+	 * signal/tst-minsigstksz-2.c
+	 * signal/tst-minsigstksz-3.c
+	 * signal/tst-minsigstksz-3a.c
+	 * signal/tst-minsigstksz-4.c: New tests.
+	 * signal/Makefile: Run them.
+
 2019-01-16  Siddhesh Poyarekar  <siddhesh@sourceware.org>
 
 	* po/libc.pot: Regenerate.
diff --git a/signal/Makefile b/signal/Makefile
index 647ce242e6..9597287bca 100644
--- a/signal/Makefile
+++ b/signal/Makefile
@@ -47,6 +47,8 @@ routines	:= signal raise killpg \
 
 tests		:= tst-signal tst-sigset tst-sigsimple tst-raise tst-sigset2 \
 		   tst-sigwait-eintr tst-sigaction \
+		   tst-minsigstksz-1 tst-minsigstksz-2 tst-minsigstksz-3 \
+		   tst-minsigstksz-3a tst-minsigstksz-4 \
 
 include ../Rules
 
diff --git a/signal/tst-minsigstksz-1.c b/signal/tst-minsigstksz-1.c
new file mode 100644
index 0000000000..00344d5fbf
--- /dev/null
+++ b/signal/tst-minsigstksz-1.c
@@ -0,0 +1,131 @@
+/* Tests of signal delivery on an alternate stack (nonlethal).
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/check.h>
+
+/* C2011 7.4.1.1p5 specifies that only the following operations are
+   guaranteed to be well-defined inside an asynchronous signal handler:
+     * any operation on a lock-free atomic object
+     * assigning a value to an object declared as volatile sig_atomic_t
+     * calling abort, _Exit, quick_exit, or signal
+       * signal may only be called with its first argument equal to the
+         number of the signal that caused the handler to be called
+
+   We use this list as a guideline for the set of operations that ought
+   also to be safe in a _synchronous_ signal delivered on an alternate
+   signal stack with only MINSIGSTKSZ bytes of space.
+
+   This test program tests all of the above operations that do not,
+   one way or another, cause the program to be terminated.  */
+
+/* We do not try to test atomic operations exhaustively, only a simple
+   atomic counter increment.  This is only safe if atomic_[u]int is
+   unconditionally lock-free.  */
+#ifdef __STDC_NO_ATOMICS__
+# define TEST_ATOMIC_OPS 0
+#else
+# include <stdatomic.h>
+# if ATOMIC_INT_LOCK_FREE != 2
+#  define TEST_ATOMIC_OPS 0
+# else
+#  define TEST_ATOMIC_OPS 1
+# endif
+#endif
+
+static volatile sig_atomic_t signal_flag = 0;
+static volatile sig_atomic_t signal_err = 0;
+static void
+handler_set_flag (int unused)
+{
+  signal_flag = 1;
+}
+
+static void
+handler_set_flag_once (int sig)
+{
+  signal_flag = 1;
+  if (signal (sig, SIG_IGN) == SIG_ERR)
+    /* It is not safe to call FAIL_EXIT1 here.  Set another flag instead.  */
+    signal_err = 1;
+}
+
+#if TEST_ATOMIC_OPS
+static atomic_uint signal_count = 0;
+static void
+handler_count_up_1 (int unused)
+{
+  atomic_fetch_add (&signal_count, 1);
+}
+#endif
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+  struct sigaction sa;
+
+  /* Test 1: setting a volatile sig_atomic_t flag.  */
+  sa.sa_handler = handler_set_flag;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_EXIT1 ("sigaction (SIGUSR1, handler_set_flag): %m\n");
+
+  TEST_VERIFY_EXIT (signal_flag == 0);
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (signal_flag == 1);
+  signal_flag = 0;
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (signal_flag == 1);
+  signal_flag = 0;
+
+  /* Test 1: setting a volatile sig_atomic_t flag and then ignoring
+     further delivery of the signal. */
+  sa.sa_handler = handler_set_flag_once;
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_EXIT1 ("sigaction (SIGUSR1, handler_set_flag_once): %m\n");
+
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (signal_flag == 1);
+  /* Note: if signal_err is 1, a system call failed, but we can't
+     report the error code because errno is indeterminate.  */
+  TEST_VERIFY_EXIT (signal_err == 0);
+
+  signal_flag = 0;
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (signal_flag == 0);
+  TEST_VERIFY_EXIT (signal_err == 0);
+
+#if TEST_ATOMIC_OPS
+  sa.sa_handler = handler_count_up_1;
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_EXIT1 ("sigaction (SIGUSR1, handler_count_up_1): %m\n");
+
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (atomic_load (&signal_count) == 1);
+  raise (SIGUSR1);
+  TEST_VERIFY_EXIT (atomic_load (&signal_count) == 2);
+#endif
+
+  xfree_sigstack (sstk);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/signal/tst-minsigstksz-2.c b/signal/tst-minsigstksz-2.c
new file mode 100644
index 0000000000..3368dde6b8
--- /dev/null
+++ b/signal/tst-minsigstksz-2.c
@@ -0,0 +1,66 @@
+/* Tests of signal delivery on an alternate stack (abort).
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/check.h>
+#include <stdlib.h>
+
+/* C2011 7.4.1.1p5 specifies that only the following operations are
+   guaranteed to be well-defined inside an asynchronous signal handler:
+     * any operation on a lock-free atomic object
+     * assigning a value to an object declared as volatile sig_atomic_t
+     * calling abort, _Exit, quick_exit, or signal
+       * signal may only be called with its first argument equal to the
+         number of the signal that caused the handler to be called
+
+   We use this list as a guideline for the set of operations that ought
+   also to be safe in a _synchronous_ signal delivered on an alternate
+   signal stack with only MINSIGSTKSZ bytes of space.
+
+   This test program tests calls to abort.  Note that it does _not_
+   install a handler for SIGABRT, because that signal would also be
+   delivered on the alternate stack and MINSIGSTKSZ does not provide
+   enough space for delivery of nested signals.  */
+
+static void
+handler (int unused)
+{
+  abort ();
+}
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+  struct sigaction sa;
+
+  sa.sa_handler = handler;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_RET ("sigaction (SIGUSR1, handler): %m\n");
+
+  raise (SIGUSR1);
+
+  xfree_sigstack (sstk);
+  FAIL_RET ("test process was not terminated by abort in signal handler");
+}
+
+#define EXPECTED_SIGNAL SIGABRT
+#include <support/test-driver.c>
diff --git a/signal/tst-minsigstksz-3.c b/signal/tst-minsigstksz-3.c
new file mode 100644
index 0000000000..a8d9a6369c
--- /dev/null
+++ b/signal/tst-minsigstksz-3.c
@@ -0,0 +1,64 @@
+/* Tests of signal delivery on an alternate stack (_Exit).
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/check.h>
+#include <stdlib.h>
+
+/* C2011 7.4.1.1p5 specifies that only the following operations are
+   guaranteed to be well-defined inside an asynchronous signal handler:
+     * any operation on a lock-free atomic object
+     * assigning a value to an object declared as volatile sig_atomic_t
+     * calling abort, _Exit, quick_exit, or signal
+       * signal may only be called with its first argument equal to the
+         number of the signal that caused the handler to be called
+
+   We use this list as a guideline for the set of operations that ought
+   also to be safe in a _synchronous_ signal delivered on an alternate
+   signal stack with only MINSIGSTKSZ bytes of space.
+
+   This test program tests calls to _Exit.  */
+
+#define EXPECTED_STATUS 3
+
+static void
+handler (int unused)
+{
+  _Exit (EXPECTED_STATUS);
+}
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+  struct sigaction sa;
+
+  sa.sa_handler = handler;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_RET ("sigaction (SIGUSR1, handler): %m\n");
+
+  raise (SIGUSR1);
+
+  xfree_sigstack (sstk);
+  FAIL_RET ("test process was not terminated by _Exit in signal handler");
+}
+
+#include <support/test-driver.c>
diff --git a/signal/tst-minsigstksz-3a.c b/signal/tst-minsigstksz-3a.c
new file mode 100644
index 0000000000..b58b8d01ba
--- /dev/null
+++ b/signal/tst-minsigstksz-3a.c
@@ -0,0 +1,69 @@
+/* Tests of signal delivery on an alternate stack (_exit).
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/check.h>
+#include <unistd.h>
+
+/* C2011 7.4.1.1p5 specifies that only the following operations are
+   guaranteed to be well-defined inside an asynchronous signal handler:
+     * any operation on a lock-free atomic object
+     * assigning a value to an object declared as volatile sig_atomic_t
+     * calling abort, _Exit, quick_exit, or signal
+       * signal may only be called with its first argument equal to the
+         number of the signal that caused the handler to be called
+
+   We use this list as a guideline for the set of operations that ought
+   also to be safe in a _synchronous_ signal delivered on an alternate
+   signal stack with only MINSIGSTKSZ bytes of space.
+
+   This test program tests calls to _exit, which is the same function
+   as _Exit, but specified by POSIX rather than ISO C.  For reasons
+   unknown to the author of this program, the C committee did not
+   think it could standardize _exit under that name; regardless, in a
+   POSIX-conformant environment, they should be completely
+   interchangeable.  */
+
+#define EXPECTED_STATUS 3
+
+static void
+handler (int unused)
+{
+  _exit (EXPECTED_STATUS);
+}
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+  struct sigaction sa;
+
+  sa.sa_handler = handler;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_RET ("sigaction (SIGUSR1, handler): %m\n");
+
+  raise (SIGUSR1);
+
+  xfree_sigstack (sstk);
+  FAIL_RET ("test process was not terminated by _exit in signal handler");
+}
+
+#include <support/test-driver.c>
diff --git a/signal/tst-minsigstksz-4.c b/signal/tst-minsigstksz-4.c
new file mode 100644
index 0000000000..0dc63b4dd4
--- /dev/null
+++ b/signal/tst-minsigstksz-4.c
@@ -0,0 +1,65 @@
+/* Tests of signal delivery on an alternate stack (quick_exit).
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/check.h>
+#include <stdlib.h>
+
+/* C2011 7.4.1.1p5 specifies that only the following operations are
+   guaranteed to be well-defined inside an asynchronous signal handler:
+     * any operation on a lock-free atomic object
+     * assigning a value to an object declared as volatile sig_atomic_t
+     * calling abort, _Exit, quick_exit, or signal
+       * signal may only be called with its first argument equal to the
+         number of the signal that caused the handler to be called
+
+   We use this list as a guideline for the set of operations that ought
+   also to be safe in a _synchronous_ signal delivered on an alternate
+   signal stack with only MINSIGSTKSZ bytes of space.
+
+   This test program tests calls to quick_exit.  Note that this is only
+   safe when there are no at_quick_exit callbacks.  */
+
+#define EXPECTED_STATUS 3
+
+static void
+handler (int unused)
+{
+  quick_exit (EXPECTED_STATUS);
+}
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+  struct sigaction sa;
+
+  sa.sa_handler = handler;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_RET ("sigaction (SIGUSR1, handler): %m\n");
+
+  raise (SIGUSR1);
+
+  xfree_sigstack (sstk);
+  FAIL_RET ("test process was not terminated by quick_exit in signal handler");
+}
+
+#include <support/test-driver.c>
diff --git a/support/Makefile b/support/Makefile
index 6ac4447def..432cf2fe6c 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -145,6 +145,7 @@ libsupport-routines = \
   xsetsockopt \
   xsigaction \
   xsignal \
+  xsigstack \
   xsocket \
   xstrdup \
   xstrndup \
@@ -205,6 +206,7 @@ tests = \
   tst-test_compare_blob \
   tst-test_compare_string \
   tst-xreadlink \
+  tst-xsigstack \
 
 ifeq ($(run-built-tests),yes)
 tests-special = \
diff --git a/support/tst-xsigstack.c b/support/tst-xsigstack.c
new file mode 100644
index 0000000000..42859c79e9
--- /dev/null
+++ b/support/tst-xsigstack.c
@@ -0,0 +1,64 @@
+/* Test of sigaltstack wrappers.
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/xunistd.h>
+#include <support/check.h>
+
+#include <stdint.h>
+#include <stdio.h>
+
+static volatile uintptr_t handler_stackaddr;
+
+static void
+handler (int unused)
+{
+  int var;
+  handler_stackaddr = (uintptr_t) &var;
+}
+
+int
+do_test (void)
+{
+  void *sstk = xalloc_sigstack (0);
+
+  unsigned char *sp;
+  size_t size;
+  xget_sigstack_location (sstk, &sp, &size);
+  printf ("signal stack installed: sp=%p size=%zu\n", sp, size);
+
+  struct sigaction sa;
+  sa.sa_handler = handler;
+  sa.sa_flags   = SA_RESTART | SA_ONSTACK;
+  sigfillset (&sa.sa_mask);
+  if (sigaction (SIGUSR1, &sa, 0))
+    FAIL_RET ("sigaction (SIGUSR1, handler): %m\n");
+
+  raise (SIGUSR1);
+
+  uintptr_t haddr = handler_stackaddr;
+  printf ("address of handler local variable: %p\n", (void *)haddr);
+  TEST_VERIFY ((uintptr_t)sp < haddr);
+  TEST_VERIFY (haddr < (uintptr_t)sp + size);
+
+  xfree_sigstack (sstk);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/support/xsignal.h b/support/xsignal.h
index f3308883a4..d89e2bc575 100644
--- a/support/xsignal.h
+++ b/support/xsignal.h
@@ -37,6 +37,23 @@ void xsigaction (int sig, const struct sigaction *newact,
 
 void xpthread_sigmask (int how, const sigset_t *set, sigset_t *oldset);
 
+/* Allocate and activate an alternate signal stack.  This stack will
+   have SIZE + MINSIGSTKSZ bytes of space, rounded up to a whole
+   number of pages.  There will be large (at least 1 MiB) inaccessible
+   guard bands on either side of it.  The return value is a cookie
+   that can be passed to xfree_sigstack to deactivate and deallocate
+   the stack again.  It is not necessary to call sigaltstack after
+   calling this function.  Terminates the process on error.  */
+void *xalloc_sigstack (size_t size);
+
+/* Deactivate and deallocate a signal stack created by xalloc_sigstack.  */
+void xfree_sigstack (void *stack);
+
+/* Extract the actual address and size of the alternate signal stack from
+   the cookie returned by xalloc_sigstack.  */
+void xget_sigstack_location (const void *stack, unsigned char **addrp,
+                             size_t *sizep);
+
 __END_DECLS
 
 #endif /* SUPPORT_SIGNAL_H */
diff --git a/support/xsigstack.c b/support/xsigstack.c
new file mode 100644
index 0000000000..cebfa19aa5
--- /dev/null
+++ b/support/xsigstack.c
@@ -0,0 +1,107 @@
+/* sigaltstack wrappers.
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsignal.h>
+#include <support/support.h>
+#include <support/xunistd.h>
+#include <support/check.h>
+
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/param.h> /* roundup, MAX */
+
+/* The "cookie" returned by xalloc_sigstack points to one of these
+   structures.  */
+struct sigstack_desc
+{
+  void *alloc_base;  /* Base address of the complete allocation.  */
+  size_t alloc_size; /* Size of the complete allocation.  */
+  stack_t alt_stack; /* The address and size of the stack itself.  */
+  stack_t old_stack; /* The previous signal stack.  */
+};
+
+void *
+xalloc_sigstack (size_t size)
+{
+  size_t pagesize = sysconf (_SC_PAGESIZE);
+  if (pagesize == -1)
+    FAIL_EXIT1 ("sysconf (_SC_PAGESIZE): %m\n");
+
+  /* Always supply at least MINSIGSTKSZ space; passing 0 as size means
+     only that much space.  No matter what the number is, round it up
+     to a whole number of pages.  */
+  size_t stacksize = roundup (size + MINSIGSTKSZ, pagesize);
+
+  /* The guard bands need to be large enough to intercept offset
+     accesses from a stack address that might otherwise hit another
+     mapping.  Make them at least twice as big as the stack itself, to
+     defend against an offset by the entire size of a large
+     stack-allocated array.  The minimum is 1MiB, which is arbitrarily
+     chosen to be larger than any "typical" wild pointer offset.
+     Again, no matter what the number is, round it up to a whole
+     number of pages.  */
+  size_t guardsize = roundup (MAX (2 * stacksize, 1024 * 1024), pagesize);
+
+  struct sigstack_desc *desc = xmalloc (sizeof (struct sigstack_desc));
+  desc->alloc_size = guardsize + stacksize + guardsize;
+  /* Use MAP_NORESERVE so that RAM will not be wasted on the guard
+     bands; touch all the pages of the actual stack before returning,
+     so we know they are allocated.  */
+  desc->alloc_base = xmmap (0,
+                            desc->alloc_size,
+                            PROT_READ|PROT_WRITE,
+                            MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|MAP_STACK,
+                            -1);
+
+  xmprotect (desc->alloc_base, guardsize, PROT_NONE);
+  xmprotect (desc->alloc_base + guardsize + stacksize, guardsize, PROT_NONE);
+  memset (desc->alloc_base + guardsize, 0xA5, stacksize);
+
+  desc->alt_stack.ss_sp    = desc->alloc_base + guardsize;
+  desc->alt_stack.ss_flags = 0;
+  desc->alt_stack.ss_size  = stacksize;
+
+  if (sigaltstack (&desc->alt_stack, &desc->old_stack))
+    FAIL_EXIT1 ("sigaltstack (new stack: sp=%p, size=%zu, flags=%u): %m\n",
+                desc->alt_stack.ss_sp, desc->alt_stack.ss_size,
+                desc->alt_stack.ss_flags);
+
+  return desc;
+}
+
+void
+xfree_sigstack (void *stack)
+{
+  struct sigstack_desc *desc = stack;
+
+  if (sigaltstack (&desc->old_stack, 0))
+    FAIL_EXIT1 ("sigaltstack (restore old stack: sp=%p, size=%zu, flags=%u): "
+                "%m\n", desc->old_stack.ss_sp, desc->old_stack.ss_size,
+                desc->old_stack.ss_flags);
+  xmunmap (desc->alloc_base, desc->alloc_size);
+  free (desc);
+}
+
+void
+xget_sigstack_location (const void *stack, unsigned char **addrp, size_t *sizep)
+{
+  const struct sigstack_desc *desc = stack;
+  *addrp = desc->alt_stack.ss_sp;
+  *sizep = desc->alt_stack.ss_size;
+}