Use getentropy() when available

When available, use getentropy() in preference to SYS_getrandom or
/dev/urandom.  This binding is present in glibc 2.25 and the BSDs.

ticket: 9149 (new)

src/configure.ac

@@ -466,7 +466,7 @@ AC_CONFIG_HEADERS(include/autoconf.h, [echo timestamp > include/autoconf.stamp])
 AC_C_CONST
 AC_HEADER_DIRENT
 AC_FUNC_STRERROR_R
-AC_CHECK_FUNCS(strdup setvbuf seteuid setresuid setreuid setegid setresgid setregid setsid flock fchmod chmod strptime geteuid setenv unsetenv getenv gmtime_r localtime_r bswap16 bswap64 mkstemp getusershell access getcwd srand48 srand srandom stat strchr strerror timegm explicit_bzero explicit_memset getresuid getresgid)
+AC_CHECK_FUNCS(strdup setvbuf seteuid setresuid setreuid setegid setresgid setregid setsid flock fchmod chmod strptime geteuid setenv unsetenv getenv gmtime_r localtime_r bswap16 bswap64 mkstemp getusershell access getcwd srand48 srand srandom stat strchr strerror timegm explicit_bzero explicit_memset getresuid getresgid getentropy)
 
 AC_CHECK_FUNC(mkstemp,
 [MKSTEMP_ST_OBJ=

src/lib/crypto/krb/prng.c

@@ -96,17 +96,34 @@ read_entropy_from_device(const char *device, unsigned char *buf, size_t len)
 static krb5_boolean
 get_os_entropy(unsigned char *buf, size_t len)
 {
-#if defined(__linux__) && defined(SYS_getrandom)
+#if defined(HAVE_GETENTROPY)
+    int r;
+    size_t seg;
+
+    /* getentropy() has a maximum length of 256. */
+    while (len > 0) {
+        seg = (len > 256) ? 256 : len;
+        r = getentropy(buf, len);
+        if (r != 0)
+            break;
+        len -= seg;
+        buf += seg;
+    }
+    if (len == 0)
+        return TRUE;
+#elif defined(__linux__) && defined(SYS_getrandom)
+    /*
+     * Linux added SYS_getrandom in 3.17 (2014), but glibc did not have a
+     * wrapper until 2.25 (2017).  This block can be deleted when that interval
+     * is far in the past.
+     */
     int r;
 
     while (len > 0) {
         /*
          * Pull from the /dev/urandom pool, but require it to have been seeded.
          * This ensures strong randomness while only blocking during first
          * system boot.
-         *
-         * glibc does not currently provide a binding for getrandom:
-         * https://sourceware.org/bugzilla/show_bug.cgi?id=17252
          */
         errno = 0;
         r = syscall(SYS_getrandom, buf, len, 0);