Account for the extra stack size when rlimit is small enough

When rlimit is small enough to be used as the stacksize to be returned
in pthread_getattr_np, cases where a stack is made executable due to a
DSO load get stack size that is larger than what the kernel
allows. This is because in such a case the stack size does not account
for the pages that have auxv and program arguments.

Additionally, the stacksize for the process derived from this should
be truncated to align to page size to avoid going beyond rlimit.

nptl/ChangeLog

@@ -1,3 +1,13 @@
+2012-06-20  Siddhesh Poyarekar  <siddhesh@redhat.com>
+
+	[BZ #12416]
+	* Makefile (tests): Add test case.
+	* pthread_getattr_np.c (pthread_getattr_np): Deduct pages below
+	the __libc_stack_end page from stacksize.  Truncate stacksize to
+	make it page aligned when it is computed from RLIMIT_STACK.
+	* tst-pthread-getattr.c: New test case. Verify that stackaddr is
+	accessible.
+
 2012-06-07  Carlos Sánchez de La Lama  <csanchezdll@gmail.com>
 
 	[BZ #14205]

nptl/Makefile

@@ -245,7 +245,7 @@ tests = tst-typesizes \
 	tst-exec1 tst-exec2 tst-exec3 tst-exec4 \
 	tst-exit1 tst-exit2 tst-exit3 \
 	tst-stdio1 tst-stdio2 \
-	tst-stack1 tst-stack2 tst-stack3 \
+	tst-stack1 tst-stack2 tst-stack3 tst-pthread-getattr \
 	tst-unload \
 	tst-dlsym1 \
 	tst-sysconf \

nptl/pthread_getattr_np.c

@@ -120,8 +120,15 @@ pthread_getattr_np (thread_id, attr)
 		      && (uintptr_t) __libc_stack_end < to)
 		    {
 		      /* Found the entry.  Now we have the info we need.  */
-		      iattr->stacksize = rl.rlim_cur;
 		      iattr->stackaddr = stack_end;
+		      iattr->stacksize =
+		        rl.rlim_cur - (size_t) (to - (uintptr_t) stack_end);
+
+		      /* Cut it down to align it to page size since otherwise we
+		         risk going beyond rlimit when the kernel rounds up the
+		         stack extension request.  */
+		      iattr->stacksize = (iattr->stacksize
+					  & -(intptr_t) GLRO(dl_pagesize));
 
 		      /* The limit might be too high.  */
 		      if ((size_t) iattr->stacksize

nptl/tst-pthread-getattr.c

@@ -0,0 +1,120 @@
+/* Make sure that the stackaddr returned by pthread_getattr_np is
+   reachable.
+
+   Copyright (C) 2012 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 <stdio.h>
+#include <string.h>
+#include <sys/resource.h>
+#include <pthread.h>
+#include <alloca.h>
+
+/* Move the stack pointer so that stackaddr is accessible and then check if it
+   really is accessible.  This will segfault if it fails.  */
+static void
+allocate_and_test (void *stackaddr)
+{
+  void *mem = &mem;
+  /* FIXME:  The difference will be negative for _STACK_GROWSUP.  */
+  mem = alloca ((size_t) (mem - stackaddr));
+  *(int *)(mem) = 0;
+}
+
+static int
+get_self_pthread_attr (const char *id, void **stackaddr, size_t *stacksize)
+{
+  pthread_attr_t attr;
+  int ret;
+  pthread_t me = pthread_self ();
+
+  if ((ret = pthread_getattr_np (me, &attr)))
+    {
+      printf ("%s: pthread_getattr_np failed: %s\n", id, strerror (ret));
+      return 1;
+    }
+
+  if ((ret = pthread_attr_getstack (&attr, stackaddr, stacksize)))
+    {
+      printf ("%s: pthread_attr_getstack returned error: %s\n", id,
+	      strerror (ret));
+      return 1;
+    }
+
+  return 0;
+}
+
+/* Verify that the stack size returned by pthread_getattr_np is usable when
+   the returned value is subject to rlimit.  */
+static int
+check_stack_top (void)
+{
+  struct rlimit stack_limit;
+  void *stackaddr;
+  size_t stacksize = 0;
+  int ret;
+
+  puts ("Verifying that stack top is accessible");
+
+  ret = getrlimit (RLIMIT_STACK, &stack_limit);
+  if (ret)
+    {
+      perror ("getrlimit failed");
+      return 1;
+    }
+
+  if (get_self_pthread_attr ("check_stack_top", &stackaddr, &stacksize))
+    return 1;
+
+  /* Reduce the rlimit to a page less that what is currently being returned so
+     that we ensure that pthread_getattr_np uses rlimit.  The figure is
+     intentionally unaligned so to verify that pthread_getattr_np returns an
+     aligned stacksize that correctly fits into the rlimit.  We don't bother
+     about the case where the stack is limited by the vma below it and not by
+     the rlimit because the stacksize returned in that case is computed from
+     the end of that vma and is hence safe.  */
+  stack_limit.rlim_cur = stacksize - 4095;
+  printf ("Adjusting RLIMIT_STACK to %zu\n", stack_limit.rlim_cur);
+  if ((ret = setrlimit (RLIMIT_STACK, &stack_limit)))
+    {
+      perror ("setrlimit failed");
+      return 1;
+    }
+
+  if (get_self_pthread_attr ("check_stack_top2", &stackaddr, &stacksize))
+    return 1;
+
+  printf ("Adjusted rlimit: stacksize=%zu, stackaddr=%p\n", stacksize,
+          stackaddr);
+  allocate_and_test (stackaddr);
+
+  puts ("Stack top tests done");
+
+  return 0;
+}
+
+/* TODO: Similar check for thread stacks once the thread stack sizes are
+   fixed.  */
+static int
+do_test (void)
+{
+  return check_stack_top ();
+}
+
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"