libspl/random: add switch to force pseudo-random numbers for all calls

ztest wants to force all kernel random calls to use the pseudo-random
generator (/dev/urandom), to avoid depleting the system entropy pool
just for testing.

Up until the previous commit, it did this by switching the path that the
libzpool (now libspl) random API would use to get random data from; that
is, it took advantage of an implementation detail.

Now that that hole is closed to it, we need another method. This commit
introduces that; a simple API call to enable/disable "force pseudo"
mode.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #17861

Author: robn

cmd/ztest.c

@@ -8952,6 +8952,12 @@ main(int argc, char **argv)
 
 	libspl_init();
 
+	/*
+	 * Force random_get_bytes() to use /dev/urandom in order to prevent
+	 * ztest from needlessly depleting the system entropy pool.
+	 */
+	random_force_pseudo(B_TRUE);
+
 	if (!fd_data_str) {
 		process_options(argc, argv);
 

lib/libspl/include/sys/random.h

@@ -32,6 +32,8 @@
 extern int random_get_bytes(uint8_t *ptr, size_t len);
 extern int random_get_pseudo_bytes(uint8_t *ptr, size_t len);
 
+extern void random_force_pseudo(boolean_t onoff);
+
 static __inline__ uint32_t
 random_in_range(uint32_t range)
 {

lib/libspl/random.c

@@ -37,6 +37,8 @@
 
 static int random_fd = -1, urandom_fd = -1;
 
+static boolean_t force_pseudo = B_FALSE;
+
 void
 random_init(void)
 {
@@ -60,6 +62,12 @@ random_fini(void)
 	urandom_fd = -1;
 }
 
+void
+random_force_pseudo(boolean_t onoff)
+{
+	force_pseudo = onoff;
+}
+
 static int
 random_get_bytes_common(uint8_t *ptr, size_t len, int fd)
 {
@@ -81,6 +89,8 @@ random_get_bytes_common(uint8_t *ptr, size_t len, int fd)
 int
 random_get_bytes(uint8_t *ptr, size_t len)
 {
+	if (force_pseudo)
+		return (random_get_pseudo_bytes(ptr, len));
 	return (random_get_bytes_common(ptr, len, random_fd));
 }