use getrandom() on recent linux distros

Instead of using the internal DRBG-based RNG, use the `getrandom()` system
call where available.
Keep the existing version as fall-back for other systems that don't provide
that system call.

... and for cases where auto-detection fails or the user simply wants to
use the internal RNG a new configure-switch is provided.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

configure.ac

@@ -41,6 +41,8 @@ AC_ARG_ENABLE([tls],
     [AS_HELP_STRING([--disable-tls], [disable TLS support])])
 AC_ARG_ENABLE([cares],
     [AS_HELP_STRING([--enable-cares], [use c-ares for DNS resolution])])
+AC_ARG_ENABLE([getrandom],
+    [AS_HELP_STRING([--disable-getrandom], [disable usage of the getrandom() system call])])
 
 AC_ARG_ENABLE([fuzzing],
     [AS_HELP_STRING([--enable-fuzzing], [turn on fuzzing test])],
@@ -96,6 +98,12 @@ elif test "x$enable_tls" != xno; then
         )])
 fi
 
+AC_CHECK_FUNCS([getrandom], [], [enable_getrandom=no])
+if test "$enable_getrandom" = "no"; then
+    AC_DEFINE(DONT_USE_GETRANDOM)
+    AC_MSG_NOTICE([libstrophe will not use the getrandom() system call])
+fi
+
 with_parser=""
 if test "x$with_libxml2" != xyes; then
     PKG_CHECK_MODULES([expat], [expat >= 2.0.0],

src/rand.c

@@ -23,13 +23,23 @@
 #include <string.h> /* memeset */
 #include <time.h>   /* clock, time */
 
+#if !defined(DONT_USE_GETRANDOM) && defined(__linux__) && \
+    defined(__GLIBC_PREREQ)
+#if __GLIBC_PREREQ(2, 25)
+#define USE_GETRANDOM
+#include <sys/random.h>
+#include <errno.h>
+#endif
+#endif
+
 #include "common.h"  /* xmpp_alloc, xmpp_free */
 #include "ostypes.h" /* uint8_t, uint32_t, size_t */
-#include "sha1.h"
-#include "snprintf.h" /* xmpp_snprintf */
 
 #include "rand.h" /* xmpp_rand_t */
 
+#ifndef USE_GETRANDOM
+#include "sha1.h"
+
 #define outlen SHA1_DIGEST_SIZE
 #define seedlen (440 / 8)
 #define reseed_interval 0x7fffffff
@@ -285,6 +295,51 @@ void xmpp_rand_bytes(xmpp_rand_t *rand, unsigned char *output, size_t len)
     }
 }
 
+#else
+
+static int _read_getrandom(void *p, size_t n)
+{
+    unsigned char *q = (unsigned char *)p;
+    while (n > 0u) {
+        ssize_t ret = getrandom(q, n, 0);
+        if (ret < 0) {
+            if (errno == EINTR) {
+                continue;
+            }
+            return 1;
+        }
+        q += ret;
+        n -= (size_t)ret;
+    }
+    return 0;
+}
+
+struct _xmpp_rand_t {
+    char nothing;
+};
+
+static xmpp_rand_t _xmpp_rand;
+
+xmpp_rand_t *xmpp_rand_new(xmpp_ctx_t *ctx)
+{
+    UNUSED(ctx);
+    return &_xmpp_rand;
+}
+
+void xmpp_rand_free(xmpp_ctx_t *ctx, xmpp_rand_t *rand)
+{
+    UNUSED(ctx);
+    assert(rand == &_xmpp_rand);
+}
+
+void xmpp_rand_bytes(xmpp_rand_t *rand, unsigned char *output, size_t len)
+{
+    assert(rand == &_xmpp_rand);
+    assert(_read_getrandom(output, len) == 0);
+}
+
+#endif
+
 int xmpp_rand(xmpp_rand_t *rand)
 {
     int result;

tests/test_rand.c

@@ -20,6 +20,8 @@
 /* include rand.c to access private structures and functions */
 #include "rand.c"
 
+#ifndef USE_GETRANDOM
+
 /* stubs to build test without whole libstrophe */
 void *xmpp_alloc(const xmpp_ctx_t *ctx, size_t size)
 {
@@ -160,3 +162,19 @@ int main()
 
     return 0;
 }
+
+#else
+
+int main()
+{
+    uint8_t output[1024];
+    xmpp_rand_t *rand = xmpp_rand_new(NULL);
+
+    assert(rand != NULL);
+    /* this would assert if it failed */
+    xmpp_rand_bytes(rand, output, sizeof(output));
+
+    return 0;
+}
+
+#endif