libmisc: use /dev/urandom as a generic fallback for read_random_bytes()

On systems with Linux kernel < 3.17, getentropy() and getrandom() may
exist but return ENOSYS.  Use /dev/urandom as a fallback to avoid a hard
requirement on Linux kernel version.

Fixes #512.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>

diff --git a/libmisc/salt.c b/libmisc/salt.c
index 450293d70b64dc83aaee549cdd2c5f8c6235ebeb..1279ca09fdd44c59f14ca7c75346238ec0484d60 100644 (file)
--- a/libmisc/salt.c
+++ b/libmisc/salt.c
@@ -156,24 +156,23 @@ static long read_random_bytes (void)
        /* arc4random_buf, if it exists, can never fail.  */
        arc4random_buf (&randval, sizeof (randval));
        goto end;
+#endif
 
-#elif defined(HAVE_GETENTROPY)
+#ifdef HAVE_GETENTROPY
        /* getentropy may exist but lack kernel support.  */
-       if (getentropy (&randval, sizeof (randval))) {
-               goto fail;
+       if (getentropy (&randval, sizeof (randval)) == 0) {
+               goto end;
        }
+#endif
 
-       goto end;
-
-#elif defined(HAVE_GETRANDOM)
+#ifdef HAVE_GETRANDOM
        /* Likewise getrandom.  */
-       if ((size_t) getrandom (&randval, sizeof (randval), 0) != sizeof (randval)) {
-               goto fail;
+       if ((size_t) getrandom (&randval, sizeof (randval), 0) == sizeof (randval)) {
+               goto end;
        }
+#endif
 
-       goto end;
-
-#else
+       /* Use /dev/urandom as a last resort.  */
        FILE *f = fopen ("/dev/urandom", "r");
        if (NULL == f) {
                goto fail;
@@ -186,7 +185,6 @@ static long read_random_bytes (void)
 
        fclose(f);
        goto end;
-#endif
 
 fail:
        fprintf (log_get_logfd(),