stdlib: Add arc4random, arc4random_buf, and arc4random_uniform (BZ #4417)

The implementation is based on scalar Chacha20 with per-thread cache.
It uses getrandom or /dev/urandom as fallback to get the initial entropy,
and reseeds the internal state on every 16MB of consumed buffer.

To improve performance and lower memory consumption the per-thread cache
is allocated lazily on first arc4random functions call, and if the
memory allocation fails getentropy or /dev/urandom is used as fallback.
The cache is also cleared on thread exit iff it was initialized (so if
arc4random is not called it is not touched).

Although it is lock-free, arc4random is still not async-signal-safe
(the per thread state is not updated atomically).

The ChaCha20 implementation is based on RFC8439 [1], omitting the final
XOR of the keystream with the plaintext because the plaintext is a
stream of zeros.  This strategy is similar to what OpenBSD arc4random
does.

The arc4random_uniform is based on previous work by Florian Weimer,
where the algorithm is based on Jérémie Lumbroso paper Optimal Discrete
Uniform Generation from Coin Flips, and Applications (2013) [2], who
credits Donald E. Knuth and Andrew C. Yao, The complexity of nonuniform
random number generation (1976), for solving the general case.

The main advantage of this method is the that the unit of randomness is not
the uniform random variable (uint32_t), but a random bit.  It optimizes the
internal buffer sampling by initially consuming a 32-bit random variable
and then sampling byte per byte.  Depending of the upper bound requested,
it might lead to better CPU utilization.

Checked on x86_64-linux-gnu, aarch64-linux, and powerpc64le-linux-gnu.

Co-authored-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>

[1] https://datatracker.ietf.org/doc/html/rfc8439
[2] https://arxiv.org/pdf/1304.1916.pdf

7 files changed, 603 insertions, 0 deletions

diff --git a/stdlib/Makefile b/stdlib/Makefile
index d4a4d5679a..62f8253225 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -53,6 +53,8 @@ routines := \
   a64l \
   abort \
   abs \
+  arc4random \
+  arc4random_uniform \
   at_quick_exit \
   atof \
   atoi \
diff --git a/stdlib/Versions b/stdlib/Versions
index 5e9099a153..d09a308fb5 100644
--- a/stdlib/Versions
+++ b/stdlib/Versions
@@ -136,6 +136,11 @@ libc {
     strtof32; strtof64; strtof32x;
     strtof32_l; strtof64_l; strtof32x_l;
   }
+  GLIBC_2.36 {
+    arc4random;
+    arc4random_buf;
+    arc4random_uniform;
+  }
   GLIBC_PRIVATE {
     # functions which have an additional interface since they are
     # are cancelable.
diff --git a/stdlib/arc4random.c b/stdlib/arc4random.c
new file mode 100644
index 0000000000..65547e79aa
--- /dev/null
+++ b/stdlib/arc4random.c
@@ -0,0 +1,208 @@
+/* Pseudo Random Number Generator based on ChaCha20.
+   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 <arc4random.h>
+#include <errno.h>
+#include <not-cancel.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/param.h>
+#include <sys/random.h>
+#include <tls-internal.h>
+
+/* arc4random keeps two counters: 'have' is the current valid bytes not yet
+   consumed in 'buf' while 'count' is the maximum number of bytes until a
+   reseed.
+
+   Both the initial seed and reseed try to obtain entropy from the kernel
+   and abort the process if none could be obtained.
+
+   The state 'buf' improves the usage of the cipher calls, allowing to call
+   optimized implementations (if the architecture provides it) and minimize
+   function call overhead.  */
+
+#include <chacha20.c>
+
+/* Called from the fork function to reset the state.  */
+void
+__arc4random_fork_subprocess (void)
+{
+  struct arc4random_state_t *state = __glibc_tls_internal ()->rand_state;
+  if (state != NULL)
+    {
+      explicit_bzero (state, sizeof (*state));
+      /* Force key init.  */
+      state->count = -1;
+    }
+}
+
+/* Return the current thread random state or try to create one if there is
+   none available.  In the case malloc can not allocate a state, arc4random
+   will try to get entropy with arc4random_getentropy.  */
+static struct arc4random_state_t *
+arc4random_get_state (void)
+{
+  struct arc4random_state_t *state = __glibc_tls_internal ()->rand_state;
+  if (state == NULL)
+    {
+      state = malloc (sizeof (struct arc4random_state_t));
+      if (state != NULL)
+	{
+	  /* Force key initialization on first call.  */
+	  state->count = -1;
+	  __glibc_tls_internal ()->rand_state = state;
+	}
+    }
+  return state;
+}
+
+static void
+arc4random_getrandom_failure (void)
+{
+  __libc_fatal ("Fatal glibc error: cannot get entropy for arc4random\n");
+}
+
+static void
+arc4random_rekey (struct arc4random_state_t *state, uint8_t *rnd, size_t rndlen)
+{
+  chacha20_crypt (state->ctx, state->buf, state->buf, sizeof state->buf);
+
+  /* Mix optional user provided data.  */
+  if (rnd != NULL)
+    {
+      size_t m = MIN (rndlen, CHACHA20_KEY_SIZE + CHACHA20_IV_SIZE);
+      for (size_t i = 0; i < m; i++)
+	state->buf[i] ^= rnd[i];
+    }
+
+  /* Immediately reinit for backtracking resistance.  */
+  chacha20_init (state->ctx, state->buf, state->buf + CHACHA20_KEY_SIZE);
+  explicit_bzero (state->buf, CHACHA20_KEY_SIZE + CHACHA20_IV_SIZE);
+  state->have = sizeof (state->buf) - (CHACHA20_KEY_SIZE + CHACHA20_IV_SIZE);
+}
+
+static void
+arc4random_getentropy (void *rnd, size_t len)
+{
+  if (__getrandom_nocancel (rnd, len, GRND_NONBLOCK) == len)
+    return;
+
+  int fd = TEMP_FAILURE_RETRY (__open64_nocancel ("/dev/urandom",
+						  O_RDONLY | O_CLOEXEC));
+  if (fd != -1)
+    {
+      uint8_t *p = rnd;
+      uint8_t *end = p + len;
+      do
+	{
+	  ssize_t ret = TEMP_FAILURE_RETRY (__read_nocancel (fd, p, end - p));
+	  if (ret <= 0)
+	    arc4random_getrandom_failure ();
+	  p += ret;
+	}
+      while (p < end);
+
+      if (__close_nocancel (fd) == 0)
+	return;
+    }
+  arc4random_getrandom_failure ();
+}
+
+/* Check if the thread context STATE should be reseed with kernel entropy
+   depending of requested LEN bytes.  If there is less than requested,
+   the state is either initialized or reseeded, otherwise the internal
+   counter subtract the requested length.  */
+static void
+arc4random_check_stir (struct arc4random_state_t *state, size_t len)
+{
+  if (state->count <= len || state->count == -1)
+    {
+      uint8_t rnd[CHACHA20_KEY_SIZE + CHACHA20_IV_SIZE];
+      arc4random_getentropy (rnd, sizeof rnd);
+
+      if (state->count == -1)
+	chacha20_init (state->ctx, rnd, rnd + CHACHA20_KEY_SIZE);
+      else
+	arc4random_rekey (state, rnd, sizeof rnd);
+
+      explicit_bzero (rnd, sizeof rnd);
+
+      /* Invalidate the buf.  */
+      state->have = 0;
+      memset (state->buf, 0, sizeof state->buf);
+      state->count = CHACHA20_RESEED_SIZE;
+    }
+  else
+    state->count -= len;
+}
+
+void
+__arc4random_buf (void *buffer, size_t len)
+{
+  struct arc4random_state_t *state = arc4random_get_state ();
+  if (__glibc_unlikely (state == NULL))
+    {
+      arc4random_getentropy (buffer, len);
+      return;
+    }
+
+  arc4random_check_stir (state, len);
+  while (len > 0)
+    {
+      if (state->have > 0)
+	{
+	  size_t m = MIN (len, state->have);
+	  uint8_t *ks = state->buf + sizeof (state->buf) - state->have;
+	  memcpy (buffer, ks, m);
+	  explicit_bzero (ks, m);
+	  buffer += m;
+	  len -= m;
+	  state->have -= m;
+	}
+      if (state->have == 0)
+	arc4random_rekey (state, NULL, 0);
+    }
+}
+libc_hidden_def (__arc4random_buf)
+weak_alias (__arc4random_buf, arc4random_buf)
+
+uint32_t
+__arc4random (void)
+{
+  uint32_t r;
+
+  struct arc4random_state_t *state = arc4random_get_state ();
+  if (__glibc_unlikely (state == NULL))
+    {
+      arc4random_getentropy (&r, sizeof (uint32_t));
+      return r;
+    }
+
+  arc4random_check_stir (state, sizeof (uint32_t));
+  if (state->have < sizeof (uint32_t))
+    arc4random_rekey (state, NULL, 0);
+  uint8_t *ks = state->buf + sizeof (state->buf) - state->have;
+  memcpy (&r, ks, sizeof (uint32_t));
+  memset (ks, 0, sizeof (uint32_t));
+  state->have -= sizeof (uint32_t);
+
+  return r;
+}
+libc_hidden_def (__arc4random)
+weak_alias (__arc4random, arc4random)
diff --git a/stdlib/arc4random.h b/stdlib/arc4random.h
new file mode 100644
index 0000000000..cd39389c19
--- /dev/null
+++ b/stdlib/arc4random.h
@@ -0,0 +1,48 @@
+/* Arc4random definition used on TLS.
+   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/>.  */
+
+#ifndef _CHACHA20_H
+#define _CHACHA20_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+/* Internal ChaCha20 state.  */
+#define CHACHA20_STATE_LEN	16
+#define CHACHA20_BLOCK_SIZE	64
+
+/* Maximum number bytes until reseed (16 MB).  */
+#define CHACHA20_RESEED_SIZE	(16 * 1024 * 1024)
+
+/* Internal arc4random buffer, used on each feedback step so offer some
+   backtracking protection and to allow better used of vectorized
+   chacha20 implementations.  */
+#define CHACHA20_BUFSIZE        (8 * CHACHA20_BLOCK_SIZE)
+
+_Static_assert (CHACHA20_BUFSIZE >= CHACHA20_BLOCK_SIZE + CHACHA20_BLOCK_SIZE,
+		"CHACHA20_BUFSIZE < CHACHA20_BLOCK_SIZE + CHACHA20_BLOCK_SIZE");
+
+struct arc4random_state_t
+{
+  uint32_t ctx[CHACHA20_STATE_LEN];
+  size_t have;
+  size_t count;
+  uint8_t buf[CHACHA20_BUFSIZE];
+};
+
+#endif
diff --git a/stdlib/arc4random_uniform.c b/stdlib/arc4random_uniform.c
new file mode 100644
index 0000000000..1326dfa593
--- /dev/null
+++ b/stdlib/arc4random_uniform.c
@@ -0,0 +1,140 @@
+/* Random pseudo generator number which returns a single 32 bit value
+   uniformly distributed but with an upper_bound.
+   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 <endian.h>
+#include <libc-lock.h>
+#include <stdlib.h>
+#include <sys/param.h>
+
+/* Return the number of bytes which cover values up to the limit.  */
+__attribute__ ((const))
+static uint32_t
+byte_count (uint32_t n)
+{
+  if (n < (1U << 8))
+    return 1;
+  else if (n < (1U << 16))
+    return 2;
+  else if (n < (1U << 24))
+    return 3;
+  else
+    return 4;
+}
+
+/* Fill the lower bits of the result with randomness, according to the
+   number of bytes requested.  */
+static void
+random_bytes (uint32_t *result, uint32_t byte_count)
+{
+  *result = 0;
+  unsigned char *ptr = (unsigned char *) result;
+  if (__BYTE_ORDER == __BIG_ENDIAN)
+    ptr += 4 - byte_count;
+  __arc4random_buf (ptr, byte_count);
+}
+
+uint32_t
+__arc4random_uniform (uint32_t n)
+{
+  if (n <= 1)
+    /* There is no valid return value for a zero limit, and 0 is the
+       only possible result for limit 1.  */
+    return 0;
+
+  /* The bits variable serves as a source for bits.  Prefetch the
+     minimum number of bytes needed.  */
+  uint32_t count = byte_count (n);
+  uint32_t bits_length = count * CHAR_BIT;
+  uint32_t bits;
+  random_bytes (&bits, count);
+
+  /* Powers of two are easy.  */
+  if (powerof2 (n))
+    return bits & (n - 1);
+
+  /* The general case.  This algorithm follows Jérémie Lumbroso,
+     Optimal Discrete Uniform Generation from Coin Flips, and
+     Applications (2013), who credits Donald E. Knuth and Andrew
+     C. Yao, The complexity of nonuniform random number generation
+     (1976), for solving the general case.
+
+     The implementation below unrolls the initialization stage of the
+     loop, where v is less than n.  */
+
+  /* Use 64-bit variables even though the intermediate results are
+     never larger than 33 bits.  This ensures the code is easier to
+     compile on 64-bit architectures.  */
+  uint64_t v;
+  uint64_t c;
+
+  /* Initialize v and c.  v is the smallest power of 2 which is larger
+     than n.*/
+  {
+    uint32_t log2p1 = 32 - __builtin_clz (n);
+    v = 1ULL << log2p1;
+    c = bits & (v - 1);
+    bits >>= log2p1;
+    bits_length -= log2p1;
+  }
+
+  /* At the start of the loop, c is uniformly distributed within the
+     half-open interval [0, v), and v < 2n < 2**33.  */
+  while (true)
+    {
+      if (v >= n)
+        {
+          /* If the candidate is less than n, accept it.  */
+          if (c < n)
+            /* c is uniformly distributed on [0, n).  */
+            return c;
+          else
+            {
+              /* c is uniformly distributed on [n, v).  */
+              v -= n;
+              c -= n;
+              /* The distribution was shifted, so c is uniformly
+                 distributed on [0, v) again.  */
+            }
+        }
+      /* v < n here.  */
+
+      /* Replenish the bit source if necessary.  */
+      if (bits_length == 0)
+        {
+          /* Overwrite the least significant byte.  */
+	  random_bytes (&bits, 1);
+	  bits_length = CHAR_BIT;
+        }
+
+      /* Double the range.  No overflow because v < n < 2**32.  */
+      v *= 2;
+      /* v < 2n here.  */
+
+      /* Extract a bit and append it to c.  c remains less than v and
+         thus 2**33.  */
+      c = (c << 1) | (bits & 1);
+      bits >>= 1;
+      --bits_length;
+
+      /* At this point, c is uniformly distributed on [0, v) again,
+         and v < 2n < 2**33.  */
+    }
+}
+libc_hidden_def (__arc4random_uniform)
+weak_alias (__arc4random_uniform, arc4random_uniform)
diff --git a/stdlib/chacha20.c b/stdlib/chacha20.c
new file mode 100644
index 0000000000..c47b8418f2
--- /dev/null
+++ b/stdlib/chacha20.c
@@ -0,0 +1,187 @@
+/* Generic ChaCha20 implementation (used on arc4random).
+   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 <array_length.h>
+#include <endian.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+/* 32-bit stream position, then 96-bit nonce.  */
+#define CHACHA20_IV_SIZE	16
+#define CHACHA20_KEY_SIZE	32
+
+#define CHACHA20_STATE_LEN	16
+
+/* The ChaCha20 implementation is based on RFC8439 [1], omitting the final
+   XOR of the keystream with the plaintext because the plaintext is a
+   stream of zeros.  */
+
+enum chacha20_constants
+{
+  CHACHA20_CONSTANT_EXPA = 0x61707865U,
+  CHACHA20_CONSTANT_ND_3 = 0x3320646eU,
+  CHACHA20_CONSTANT_2_BY = 0x79622d32U,
+  CHACHA20_CONSTANT_TE_K = 0x6b206574U
+};
+
+static inline uint32_t
+read_unaligned_32 (const uint8_t *p)
+{
+  uint32_t r;
+  memcpy (&r, p, sizeof (r));
+  return r;
+}
+
+static inline void
+write_unaligned_32 (uint8_t *p, uint32_t v)
+{
+  memcpy (p, &v, sizeof (v));
+}
+
+#if __BYTE_ORDER == __BIG_ENDIAN
+# define read_unaligned_le32(p) __builtin_bswap32 (read_unaligned_32 (p))
+# define set_state(v)		__builtin_bswap32 ((v))
+#else
+# define read_unaligned_le32(p) read_unaligned_32 ((p))
+# define set_state(v)		(v)
+#endif
+
+static inline void
+chacha20_init (uint32_t *state, const uint8_t *key, const uint8_t *iv)
+{
+  state[0]  = CHACHA20_CONSTANT_EXPA;
+  state[1]  = CHACHA20_CONSTANT_ND_3;
+  state[2]  = CHACHA20_CONSTANT_2_BY;
+  state[3]  = CHACHA20_CONSTANT_TE_K;
+
+  state[4]  = read_unaligned_le32 (key + 0 * sizeof (uint32_t));
+  state[5]  = read_unaligned_le32 (key + 1 * sizeof (uint32_t));
+  state[6]  = read_unaligned_le32 (key + 2 * sizeof (uint32_t));
+  state[7]  = read_unaligned_le32 (key + 3 * sizeof (uint32_t));
+  state[8]  = read_unaligned_le32 (key + 4 * sizeof (uint32_t));
+  state[9]  = read_unaligned_le32 (key + 5 * sizeof (uint32_t));
+  state[10] = read_unaligned_le32 (key + 6 * sizeof (uint32_t));
+  state[11] = read_unaligned_le32 (key + 7 * sizeof (uint32_t));
+
+  state[12] = read_unaligned_le32 (iv + 0 * sizeof (uint32_t));
+  state[13] = read_unaligned_le32 (iv + 1 * sizeof (uint32_t));
+  state[14] = read_unaligned_le32 (iv + 2 * sizeof (uint32_t));
+  state[15] = read_unaligned_le32 (iv + 3 * sizeof (uint32_t));
+}
+
+static inline uint32_t
+rotl32 (unsigned int shift, uint32_t word)
+{
+  return (word << (shift & 31)) | (word >> ((-shift) & 31));
+}
+
+static void
+state_final (const uint8_t *src, uint8_t *dst, uint32_t v)
+{
+#ifdef CHACHA20_XOR_FINAL
+  v ^= read_unaligned_32 (src);
+#endif
+  write_unaligned_32 (dst, v);
+}
+
+static inline void
+chacha20_block (uint32_t *state, uint8_t *dst, const uint8_t *src)
+{
+  uint32_t x0, x1, x2, x3, x4, x5, x6, x7;
+  uint32_t x8, x9, x10, x11, x12, x13, x14, x15;
+
+  x0 = state[0];
+  x1 = state[1];
+  x2 = state[2];
+  x3 = state[3];
+  x4 = state[4];
+  x5 = state[5];
+  x6 = state[6];
+  x7 = state[7];
+  x8 = state[8];
+  x9 = state[9];
+  x10 = state[10];
+  x11 = state[11];
+  x12 = state[12];
+  x13 = state[13];
+  x14 = state[14];
+  x15 = state[15];
+
+  for (int i = 0; i < 20; i += 2)
+    {
+#define QROUND(_x0, _x1, _x2, _x3) 			\
+  do {							\
+   _x0 = _x0 + _x1; _x3 = rotl32 (16, (_x0 ^ _x3)); 	\
+   _x2 = _x2 + _x3; _x1 = rotl32 (12, (_x1 ^ _x2)); 	\
+   _x0 = _x0 + _x1; _x3 = rotl32 (8,  (_x0 ^ _x3));	\
+   _x2 = _x2 + _x3; _x1 = rotl32 (7,  (_x1 ^ _x2));	\
+  } while(0)
+
+      QROUND (x0, x4, x8,  x12);
+      QROUND (x1, x5, x9,  x13);
+      QROUND (x2, x6, x10, x14);
+      QROUND (x3, x7, x11, x15);
+
+      QROUND (x0, x5, x10, x15);
+      QROUND (x1, x6, x11, x12);
+      QROUND (x2, x7, x8,  x13);
+      QROUND (x3, x4, x9,  x14);
+    }
+
+  state_final (&src[0], &dst[0], set_state (x0 + state[0]));
+  state_final (&src[4], &dst[4], set_state (x1 + state[1]));
+  state_final (&src[8], &dst[8], set_state (x2 + state[2]));
+  state_final (&src[12], &dst[12], set_state (x3 + state[3]));
+  state_final (&src[16], &dst[16], set_state (x4 + state[4]));
+  state_final (&src[20], &dst[20], set_state (x5 + state[5]));
+  state_final (&src[24], &dst[24], set_state (x6 + state[6]));
+  state_final (&src[28], &dst[28], set_state (x7 + state[7]));
+  state_final (&src[32], &dst[32], set_state (x8 + state[8]));
+  state_final (&src[36], &dst[36], set_state (x9 + state[9]));
+  state_final (&src[40], &dst[40], set_state (x10 + state[10]));
+  state_final (&src[44], &dst[44], set_state (x11 + state[11]));
+  state_final (&src[48], &dst[48], set_state (x12 + state[12]));
+  state_final (&src[52], &dst[52], set_state (x13 + state[13]));
+  state_final (&src[56], &dst[56], set_state (x14 + state[14]));
+  state_final (&src[60], &dst[60], set_state (x15 + state[15]));
+
+  state[12]++;
+}
+
+static void
+chacha20_crypt (uint32_t *state, uint8_t *dst, const uint8_t *src,
+		size_t bytes)
+{
+  while (bytes >= CHACHA20_BLOCK_SIZE)
+    {
+      chacha20_block (state, dst, src);
+
+      bytes -= CHACHA20_BLOCK_SIZE;
+      dst += CHACHA20_BLOCK_SIZE;
+      src += CHACHA20_BLOCK_SIZE;
+    }
+
+  if (__glibc_unlikely (bytes != 0))
+    {
+      uint8_t stream[CHACHA20_BLOCK_SIZE];
+      chacha20_block (state, stream, src);
+      memcpy (dst, stream, bytes);
+      explicit_bzero (stream, sizeof stream);
+    }
+}
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index bf7cd438e1..3a630a0ce8 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -533,6 +533,19 @@ extern int seed48_r (unsigned short int __seed16v[3],
 extern int lcong48_r (unsigned short int __param[7],
 		      struct drand48_data *__buffer)
      __THROW __nonnull ((1, 2));
+
+/* Return a random integer between zero and 2**32-1 (inclusive).  */
+extern __uint32_t arc4random (void)
+     __THROW __wur;
+
+/* Fill the buffer with random data.  */
+extern void arc4random_buf (void *__buf, size_t __size)
+     __THROW __nonnull ((1));
+
+/* Return a random number between zero (inclusive) and the specified
+   limit (exclusive).  */
+extern __uint32_t arc4random_uniform (__uint32_t __upper_bound)
+     __THROW __wur;
 # endif	/* Use misc.  */
 #endif	/* Use misc or X/Open.  */