Do not use HP_TIMING_NOW for random bits

This patch removes the HP_TIMING_BITS usage for fast random bits and replace
with clock_gettime (CLOCK_MONOTONIC).  It has unspecified starting time and
nano-second accuracy, so its randomness is significantly better than
gettimeofday.

Althoug it should incur in more overhead (specially for architecture that
support hp-timing), the symbol is also common implemented as a vDSO.

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu. I also
checked on a i686-gnu build.

	* include/random-bits.h: New file.
	* resolv/res_mkquery.c [HP_TIMING_AVAIL] (RANDOM_BITS,
	(__res_context_mkquery): Remove usage hp-timing usage and replace with
	random_bits.
	* resolv/res_send.c [HP_TIMING_AVAIL] (nameserver_offset): Likewise.
	* sysdeps/posix/tempname.c [HP_TIMING_AVAIL] (__gen_tempname):
	Likewise.

1 files changed, 41 insertions, 0 deletions

diff --git a/include/random-bits.h b/include/random-bits.h
new file mode 100644
index 0000000000..a0651a5a34
--- /dev/null
+++ b/include/random-bits.h
@@ -0,0 +1,41 @@
+/* Fast pseudo-random bits based on clock_gettime.
+   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/>.  */
+
+#ifndef _RANDOM_BITS_H
+# define _RANDOM_BITS_H
+
+#include <time.h>
+#include <stdint.h>
+
+/* Provides fast pseudo-random bits through clock_gettime.  It has unspecified
+   starting time, nano-second accuracy, its randomness is significantly better
+   than gettimeofday, and for mostly architectures it is implemented through
+   vDSO instead of a syscall.  Since the source is a system clock, the upper
+   bits will have less entropy. */
+static inline uint32_t
+random_bits (void)
+{
+  struct timespec tv;
+  __clock_gettime (CLOCK_MONOTONIC, &tv);
+  /* Shuffle the lower bits to minimize the clock bias.  */
+  uint32_t ret = tv.tv_nsec ^ tv.tv_sec;
+  ret ^= (ret << 24) | (ret >> 8);
+  return ret;
+}
+
+#endif