sysdeps/unix/sysv/linux/getentropy.c

   1 /* Implementation of getentropy based on the getrandom system call.
   2    Copyright (C) 2016-2024 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 <sys/random.h>
  20 #include <assert.h>
  21 #include <errno.h>
  22 #include <unistd.h>
  23 #include <sysdep.h>
  24
  25 /* Write LENGTH bytes of randomness starting at BUFFER.  Return 0 on
  26    success and -1 on failure.  */
  27 int
  28 getentropy (void *buffer, size_t length)
  29 {
  30   /* The interface is documented to return EIO for buffer lengths
  31      longer than 256 bytes.  */
  32   if (length > 256)
  33     {
  34       __set_errno (EIO);
  35       return -1;
  36     }
  37
  38   /* Try to fill the buffer completely.  Even with the 256 byte limit
  39      above, we might still receive an EINTR error (when blocking
  40      during boot).  */
  41   void *end = buffer + length;
  42   while (buffer < end)
  43     {
  44       /* NB: No cancellation point.  */
  45       ssize_t bytes = INLINE_SYSCALL_CALL (getrandom, buffer, end - buffer, 0);
  46       if (bytes < 0)
  47         {
  48           if (errno == EINTR)
  49             /* Try again if interrupted by a signal.  */
  50             continue;
  51           else
  52             return -1;
  53         }
  54       if (bytes == 0)
  55         {
  56           /* No more bytes available.  This should not happen under
  57              normal circumstances.  */
  58           __set_errno (EIO);
  59           return -1;
  60         }
  61       /* Try again in case of a short read.  */
  62       buffer += bytes;
  63     }
  64   return 0;
  65 }