[PATCH v1 1/2] random-bits: Factor out entropy generating function

[Noah Goldstein]

On some architectures `clock_gettime` is undesirable as
it may use a syscall or there may be a faster alternative.
Future architecture specific functions can be added in
sysdeps/<arch>/random-bits-entropy.h to provide a version of
'random_bits_entropy' that doesn't use 'clock_gettime'.
---
 include/random-bits.h                 | 16 ++++++--------
 sysdeps/generic/random-bits-entropy.h | 31 +++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 10 deletions(-)
 create mode 100644 sysdeps/generic/random-bits-entropy.h

diff --git a/include/random-bits.h b/include/random-bits.h
index 17665b479a..016b87576c 100644
--- a/include/random-bits.h
+++ b/include/random-bits.h
@@ -19,21 +19,17 @@
 #ifndef _RANDOM_BITS_H
 # define _RANDOM_BITS_H
 
-#include <time.h>
-#include <stdint.h>
+# include <random-bits-entropy.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. */
+/* Provides fast pseudo-random bits through architecture specific
+   random_bits_entropy.  Expectation is source is some timing function so
+   the upper bits have less entropy.  */
 static inline uint32_t
 random_bits (void)
 {
-  struct __timespec64 tv;
-  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
+  uint32_t ret = random_bits_entropy ();
   /* 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;
 }
diff --git a/sysdeps/generic/random-bits-entropy.h b/sysdeps/generic/random-bits-entropy.h
new file mode 100644
index 0000000000..53290c7f7a
--- /dev/null
+++ b/sysdeps/generic/random-bits-entropy.h
@@ -0,0 +1,31 @@
+/* Fast function for generating entropy of random_bits.
+   Copyright (C) 2022 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdint.h>
+#include <time.h>
+
+/* Generically use 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.  */
+static inline uint32_t
+random_bits_entropy (void)
+{
+  struct __timespec64 tv;
+  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
+  return tv.tv_nsec ^ tv.tv_sec;
+}
-- 
2.25.1