[libcxx] Support getentropy as a source of randomness for std::random_device

[phosek]

Summary

Use this source use on Fuchsia where this is the officially way
to obtain randomness. This could be also used on other platforms
that already support getentropy such as *BSD or Linux.

[mcgrathr]

LGTM

[jwakely]

libcxx/trunk/src/random.cpp

29

musl only declares getentropy in <unistd.h> not <sys/random.h>. Glibc declares it in both. Should <unistd.h> also be included when _LIBCPP_USING_GETENTROPY is defined, so it can work more portably?

[jwakely]

libcxx/trunk/src/random.cpp

29

I suppose it doesn't really matter, since _LIBCPP_USING_GETENTROPY is currently only defined for fuchsia and wasi, which both declare getentropy in <sys/random.h>.

[mcgrathr]

libcxx/trunk/src/random.cpp

29

FWIW, if there is an emerging norm to declare getentropy in <unistd.h> then Fuchsia's libc can add it to our <unistd.h> and there's no real problem relying on that in libc++ sources within about a week of when we land that change in Fuchsia's trunk.

[jwakely]

libcxx/trunk/src/random.cpp

29

@fweimer added it to glibc's unistd.h for compatibility with existing BSD code:
https://sourceware.org/bugzilla/show_bug.cgi?id=17252#c9

Maybe @dalias can confirm whether musl did it for the same reason?

Does it make sense to "standardize" on declaring getentropy in unistd.h?

[fweimer]

getentropy comes from OpenBSD (among others), and it's in <unistd.h> there: https://man.openbsd.org/getentropy.2

libcxx/trunk/include/__config

 #endif // __sun__
 
 #if defined(__CloudABI__)
   // Certain architectures provide arc4random(). Prefer using
   // arc4random() over /dev/{u,}random to make it possible to obtain
   // random data even when using sandboxing mechanisms such as chroots,
   // Capsicum, etc.
 # define _LIBCPP_USING_ARC4_RANDOM
+#elif defined(__Fuchsia__)
+# define _LIBCPP_USING_GETENTROPY
 #elif defined(__native_client__)
   // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
   // including accesses to the special files under /dev. C++11's
   // std::random_device is instead exposed through a NaCl syscall.
 # define _LIBCPP_USING_NACL_RANDOM
 #elif defined(_LIBCPP_WIN32API)
 # define _LIBCPP_USING_WIN32_RANDOM
 #else

libcxx/trunk/src/random.cpp

 #if defined(__sun__)
 #define rename solaris_headers_are_broken
 #endif // defined(__sun__)
 
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 
-#if defined(_LIBCPP_USING_DEV_RANDOM)
+#if defined(_LIBCPP_USING_GETENTROPY)
+#include <sys/random.h>
+#elif defined(_LIBCPP_USING_DEV_RANDOM)
 #include <fcntl.h>
 #include <unistd.h>
 #elif defined(_LIBCPP_USING_NACL_RANDOM)
 #include <nacl/nacl_random.h>
 #endif
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(_LIBCPP_USING_ARC4_RANDOM)
+#if defined(_LIBCPP_USING_GETENTROPY)
+
+random_device::random_device(const string& __token)
+{
+    if (__token != "/dev/urandom")
+        __throw_system_error(ENOENT, ("random device not supported " + __token).c_str());
+}
+
+random_device::~random_device()
+{
+}
+
+unsigned
+random_device::operator()()
+{
+    unsigned r;
+    size_t n = sizeof(r);
+    int err = getentropy(&r, n);
+    if (err)
+        __throw_system_error(errno, "random_device getentropy failed");
+    return r;
+}
+
+#elif defined(_LIBCPP_USING_ARC4_RANDOM)
 
 random_device::random_device(const string& __token)
 {
     if (__token != "/dev/urandom")
         __throw_system_error(ENOENT, ("random device not supported " + __token).c_str());
 }
 
 random_device::~random_device()