Distinguish ELOOP diagnosis threshold from SYMLOOP_MAX.

1 files changed, 72 insertions, 0 deletions

diff --git a/sysdeps/generic/eloop-threshold.h b/sysdeps/generic/eloop-threshold.h
new file mode 100644
index 0000000000..2342d29a23
--- /dev/null
+++ b/sysdeps/generic/eloop-threshold.h
@@ -0,0 +1,72 @@
+/* Threshold at which to diagnose ELOOP.  Generic version.
+   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/>.  */
+
+#ifndef _ELOOP_THRESHOLD_H
+#define _ELOOP_THRESHOLD_H      1
+
+#include <limits.h>
+#include <sys/param.h>
+
+/* POSIX specifies SYMLOOP_MAX as the "Maximum number of symbolic
+   links that can be reliably traversed in the resolution of a
+   pathname in the absence of a loop."  This makes it a minimum that
+   we should certainly accept.  But it leaves open the possibility
+   that more might sometimes work--just not "reliably".
+
+   For example, Linux implements a complex policy whereby there is a
+   small limit on the number of direct symlink traversals (a symlink
+   to a symlink to a symlink), but larger limit on the total number of
+   symlink traversals overall.  Hence the SYMLOOP_MAX number should be
+   the small one, but the limit library functions enforce on users
+   should be the larger one.
+
+   So, we use the larger of the reported SYMLOOP_MAX (if any) and our
+   own constant MIN_ELOOP_THRESHOLD, below.  This constant should be
+   large enough that it never rules out a file name and directory tree
+   that the underlying system (i.e. calls to 'open' et al) would
+   resolve successfully.  It should be small enough that actual loops
+   are detected without a huge number of iterations.  */
+
+#ifndef MIN_ELOOP_THRESHOLD
+# define MIN_ELOOP_THRESHOLD    40
+#endif
+
+/* Return the maximum number of symlink traversals to permit
+   before diagnosing ELOOP.  */
+static inline unsigned int __attribute__ ((const))
+__eloop_threshold (void)
+{
+#ifdef SYMLOOP_MAX
+  const int symloop_max = SYMLOOP_MAX;
+#else
+  /* The function is marked 'const' even though we use memory and
+     call a function, because sysconf is required to return the
+     same value in every call and so it must always be safe to
+     call __eloop_threshold exactly once and reuse the value.  */
+  static long int sysconf_symloop_max;
+  if (sysconf_symloop_max == 0)
+    sysconf_symloop_max = __sysconf (_SC_SYMLOOP_MAX);
+  const unsigned int symloop_max = (sysconf_symloop_max <= 0
+                                    ? _POSIX_SYMLOOP_MAX
+                                    : sysconf_symloop_max);
+#endif
+
+  return MAX (symloop_max, MIN_ELOOP_THRESHOLD);
+}
+
+#endif  /* eloop-threshold.h */