Increase entropy of generated pad files, patch from Toby Speight <T.M.Speight.90@cantab.net>.

- Seeding the random number generator from PID and current time
provides very little entropy, as these can be guessed quite closely
by an attacker, so use the kernel's random number generator instead.

src/pad.c

@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 #include <pwd.h>
 #include <time.h>
 #include "conf.h"
@@ -181,6 +182,8 @@ static void pusb_pad_update(t_pusb_options *opts,
 	FILE	*f_system = NULL;
 	char	magic[1024];
 	int		i;
+	unsigned int seed;
+	int devrandom;
 
 	if (!pusb_pad_should_update(opts, user))
 		return ;
@@ -201,7 +204,14 @@ static void pusb_pad_update(t_pusb_options *opts,
 	pusb_pad_protect(user, fileno(f_system));
 
 	log_debug("Generating %d bytes unique pad...\n", sizeof(magic));
-	srand(getpid() * time(NULL));
+	devrandom = open("/dev/random", O_RDONLY);
+	if (devrandom < 0 || read(devrandom, &seed, sizeof seed) != sizeof seed) {
+		log_debug("/dev/random seeding failed...\n");
+		seed = getpid() * time(NULL); /* low-entropy fallback */
+	}
+	if (devrandom > 0)
+		close(devrandom);
+	srand(seed);
 	for (i = 0; i < sizeof(magic); ++i)
 		magic[i] = (char)rand();
 	log_debug("Writing pad to the device...\n");