[PATCH] Tests for minimal signal handler functionality in MINSIGSTKSZ space.

[Zack Weinberg <zackw at panix dot com>]

We don’t have a full conclusion about the way forward for alternate
signal stacks yet, but the notion that the very short list of things
that ISO C says you can do in an async signal handler should all work
in MINSIGSTKSZ space seemed to be well-received.  As a first step,
therefore, I propose to add 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.

All of these tests pass on x86-64.  build-many-glibcs won’t tell us
anything additional; the tests need to be run.

         * 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.
---
 signal/Makefile             |   2 +
 signal/tst-minsigstksz-1.c  | 131 ++++++++++++++++++++++++++++++++++++
 signal/tst-minsigstksz-2.c  |  66 ++++++++++++++++++
 signal/tst-minsigstksz-3.c  |  64 ++++++++++++++++++
 signal/tst-minsigstksz-3a.c |  69 +++++++++++++++++++
 signal/tst-minsigstksz-4.c  |  65 ++++++++++++++++++
 support/Makefile            |   2 +
 support/tst-xsigstack.c     |  64 ++++++++++++++++++
 support/xsignal.h           |  18 +++++
 support/xsigstack.c         |  95 ++++++++++++++++++++++++++
 10 files changed, 576 insertions(+)
 create mode 100644 signal/tst-minsigstksz-1.c
 create mode 100644 signal/tst-minsigstksz-2.c
 create mode 100644 signal/tst-minsigstksz-3.c
 create mode 100644 signal/tst-minsigstksz-3a.c
 create mode 100644 signal/tst-minsigstksz-4.c
 create mode 100644 support/tst-xsigstack.c
 create mode 100644 support/xsigstack.c

diff --git a/signal/Makefile b/signal/Makefile
index aa63434f47..d5a10d590d 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 93a5143016..7fb2de8cee 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 9ab8d1bfdd..3cf2a6e434 100644
--- a/support/xsignal.h
+++ b/support/xsignal.h
@@ -37,6 +37,24 @@ 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 at least MINSIGSTKSZ + size 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..74b459e2e6
--- /dev/null
+++ b/support/xsigstack.c
@@ -0,0 +1,95 @@
+/* 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");
+
+  size_t stacksize = roundup (size + MINSIGSTKSZ, pagesize);
+  size_t guardsize = MAX (stacksize, roundup (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;
+}
-- 
2.20.1