Add the --threshold option

Add an option to feed the kernel entropy pool with random data
only when the available entropy falls below a given threshold.

src/scdrand.c

@@ -64,6 +64,12 @@ specified, the default is %d bytes.\n\n", DEFAULT_RANDOM_BYTES);
                         (default: %d seconds).\n\
 \n", DEFAULT_LOOP_ITERATIONS, DEFAULT_LOOP_INTERVAL);
 
+    puts("\
+  -t, --threshold N     Do nothing if there is already N bits\n\
+                        of entropy available in the kernel pool.\n\
+                        Set to 0 (default) to always add entropy.\n\
+");
+
     printf("Report bugs to <%s>.\n", PACKAGE_BUGREPORT);
 
     exit(status);
@@ -246,6 +252,22 @@ connect_to_scdaemon(void)
     return ctx;
 }
 
+static int
+get_available_entropy(void)
+{
+    int random_fd, entropy;
+
+    if ( (random_fd = open("/dev/random", O_RDONLY)) == -1 )
+        err(EXIT_FAILURE, "Cannot open /dev/random");
+
+    if ( ioctl(random_fd, RNDGETENTCNT, &entropy) == -1 )
+        err(EXIT_FAILURE, "Cannot get available entropy");
+
+    close(random_fd);
+
+    return entropy;
+}
+
 /*
  * Fork a new process solely charged with the task of adding entropy
  * to the kernel pool (the only task that requires root privileges).
@@ -330,7 +352,7 @@ int
 main(int argc, char **argv)
 {
     int c, fd, n, loop;
-    unsigned nbytes, interval;
+    unsigned nbytes, interval, threshold;
     assuan_context_t ctx;
     unsigned char random_buffer[MAX_RANDOM_BYTES];
 
@@ -340,6 +362,7 @@ main(int argc, char **argv)
         { "loop",       0, NULL, 'l' },
         { "max-loop",   1, NULL, 'L' },
         { "interval",   1, NULL, 'i' },
+        { "threshold",  1, NULL, 't' },
         { NULL,         0, NULL, 0 }
     };
 
@@ -347,8 +370,9 @@ main(int argc, char **argv)
     nbytes = DEFAULT_RANDOM_BYTES;
     loop = DEFAULT_LOOP_ITERATIONS;
     interval = DEFAULT_LOOP_INTERVAL;
+    threshold = 0;
 
-    while ( (c = getopt_long(argc, argv, "hvlL:i:", options, NULL)) != -1 ) {
+    while ( (c = getopt_long(argc, argv, "hvlL:i:t:", options, NULL)) != -1 ) {
         switch ( c ) {
         case 'h':
             usage(EXIT_SUCCESS);
@@ -373,6 +397,10 @@ main(int argc, char **argv)
         case 'i':
             interval = get_uinteger_or_die(optarg);
             break;
+
+        case 't':
+            threshold = get_uinteger_or_die(optarg);
+            break;
         }
     }
 
@@ -395,8 +423,11 @@ main(int argc, char **argv)
 
     ctx = connect_to_scdaemon();
     while ( loop == -1 || loop-- > 0 ) {
-        n = get_challenge(ctx, random_buffer, nbytes);
-        write(fd, random_buffer, n);
+
+        if ( threshold == 0 || get_available_entropy() < threshold ) {
+            n = get_challenge(ctx, random_buffer, nbytes);
+            write(fd, random_buffer, n);
+        }
 
         if ( loop != 0 )
             sleep(interval);