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1 /*
2  * Copyright (C) 2016 The Android Open Source Project
3  * All rights reserved.
4  *
5  * Redistribution and use in source and binary forms, with or without
6  * modification, are permitted provided that the following conditions
7  * are met:
8  *  * Redistributions of source code must retain the above copyright
9  *    notice, this list of conditions and the following disclaimer.
10  *  * Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in
12  *    the documentation and/or other materials provided with the
13  *    distribution.
14  *
15  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
18  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
19  * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
21  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
22  * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
23  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
24  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
25  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26  * SUCH DAMAGE.
27  */
28 
29 #include "private/bionic_arc4random.h"
30 
31 #include <stdlib.h>
32 #include <string.h>
33 #include <sys/auxv.h>
34 #include <unistd.h>
35 
36 #include <async_safe/log.h>
37 
38 #include "private/KernelArgumentBlock.h"
39 
__libc_safe_arc4random_buf(void * buf,size_t n,KernelArgumentBlock & args)40 void __libc_safe_arc4random_buf(void* buf, size_t n, KernelArgumentBlock& args) {
41   // Only call arc4random_buf once we have `/dev/urandom` because getentropy(3)
42   // will fall back to using `/dev/urandom` if getrandom(2) fails, and abort if
43   // if can't use `/dev/urandom`.
44   static bool have_urandom = access("/dev/urandom", R_OK) == 0;
45   if (have_urandom) {
46     arc4random_buf(buf, n);
47     return;
48   }
49 
50   static size_t at_random_bytes_consumed = 0;
51   if (at_random_bytes_consumed + n > 16) {
52     async_safe_fatal("ran out of AT_RANDOM bytes, have %zu, requested %zu",
53                      16 - at_random_bytes_consumed, n);
54   }
55 
56   memcpy(buf, reinterpret_cast<char*>(args.getauxval(AT_RANDOM)) + at_random_bytes_consumed, n);
57   at_random_bytes_consumed += n;
58   return;
59 }
60