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.

1 files changed, 107 insertions, 0 deletions

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;
+}