stdlib/arc4random.c

   1 /* Pseudo Random Number Generator
   2    Copyright (C) 2022-2023 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include <not-cancel.h>
  21 #include <stdio.h>
  22 #include <stdlib.h>
  23 #include <sys/mman.h>
  24 #include <sys/param.h>
  25 #include <sys/random.h>
  26
  27 static void
  28 arc4random_getrandom_failure (void)
  29 {
  30   __libc_fatal ("Fatal glibc error: cannot get entropy for arc4random\n");
  31 }
  32
  33 void
  34 __arc4random_buf (void *p, size_t n)
  35 {
  36   static int seen_initialized;
  37   ssize_t l;
  38   int fd;
  39
  40   if (n == 0)
  41     return;
  42
  43   for (;;)
  44     {
  45       l = TEMP_FAILURE_RETRY (__getrandom_nocancel (p, n, 0));
  46       if (l > 0)
  47         {
  48           if ((size_t) l == n)
  49             return; /* Done reading, success.  */
  50           p = (uint8_t *) p + l;
  51           n -= l;
  52           continue; /* Interrupted by a signal; keep going.  */
  53         }
  54       else if (l == -ENOSYS)
  55         break; /* No syscall, so fallback to /dev/urandom.  */
  56       arc4random_getrandom_failure ();
  57     }
  58
  59   if (atomic_load_relaxed (&seen_initialized) == 0)
  60     {
  61       /* Poll /dev/random as an approximation of RNG initialization.  */
  62       struct pollfd pfd = { .events = POLLIN };
  63       pfd.fd = TEMP_FAILURE_RETRY (
  64           __open64_nocancel ("/dev/random", O_RDONLY | O_CLOEXEC | O_NOCTTY));
  65       if (pfd.fd < 0)
  66         arc4random_getrandom_failure ();
  67       if (TEMP_FAILURE_RETRY (__poll_infinity_nocancel (&pfd, 1)) < 0)
  68         arc4random_getrandom_failure ();
  69       if (__close_nocancel (pfd.fd) < 0)
  70         arc4random_getrandom_failure ();
  71       atomic_store_relaxed (&seen_initialized, 1);
  72     }
  73
  74   fd = TEMP_FAILURE_RETRY (
  75       __open64_nocancel ("/dev/urandom", O_RDONLY | O_CLOEXEC | O_NOCTTY));
  76   if (fd < 0)
  77     arc4random_getrandom_failure ();
  78   for (;;)
  79     {
  80       l = TEMP_FAILURE_RETRY (__read_nocancel (fd, p, n));
  81       if (l <= 0)
  82         arc4random_getrandom_failure ();
  83       if ((size_t) l == n)
  84         break; /* Done reading, success.  */
  85       p = (uint8_t *) p + l;
  86       n -= l;
  87     }
  88   if (__close_nocancel (fd) < 0)
  89     arc4random_getrandom_failure ();
  90 }
  91 libc_hidden_def (__arc4random_buf)
  92 weak_alias (__arc4random_buf, arc4random_buf)
  93
  94 uint32_t
  95 __arc4random (void)
  96 {
  97   uint32_t r;
  98   __arc4random_buf (&r, sizeof (r));
  99   return r;
 100 }
 101 libc_hidden_def (__arc4random)
 102 weak_alias (__arc4random, arc4random)