userfaultfd/selftests: don't rely on GNU extensions for random numbers

Patch series "Small userfaultfd selftest fixups", v2.

This patch (of 3):

Two arguments for doing this:

First, and maybe most importantly, the resulting code is significantly
shorter / simpler.

Then, we avoid using GNU libc extensions.  Why does this matter? It
makes testing userfaultfd with the selftest easier e.g.  on distros
which use something other than glibc (e.g., Alpine, which uses musl);
basically, it makes the test more portable.

Link: https://lkml.kernel.org/r/20210930212309.4001967-2-axelrasmussen@google.com
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -57,6 +57,7 @@
 #include <assert.h>
 #include <inttypes.h>
 #include <stdint.h>
+#include <sys/random.h>
 
 #include "../kselftest.h"
 
@@ -518,22 +519,10 @@ static void continue_range(int ufd, __u64 start, __u64 len)
 static void *locking_thread(void *arg)
 {
 	unsigned long cpu = (unsigned long) arg;
-	struct random_data rand;
 	unsigned long page_nr = *(&(page_nr)); /* uninitialized warning */
-	int32_t rand_nr;
 	unsigned long long count;
-	char randstate[64];
-	unsigned int seed;
 
-	if (bounces & BOUNCE_RANDOM) {
-		seed = (unsigned int) time(NULL) - bounces;
-		if (!(bounces & BOUNCE_RACINGFAULTS))
-			seed += cpu;
-		bzero(&rand, sizeof(rand));
-		bzero(&randstate, sizeof(randstate));
-		if (initstate_r(seed, randstate, sizeof(randstate), &rand))
-			err("initstate_r failed");
-	} else {
+	if (!(bounces & BOUNCE_RANDOM)) {
 		page_nr = -bounces;
 		if (!(bounces & BOUNCE_RACINGFAULTS))
 			page_nr += cpu * nr_pages_per_cpu;
@@ -541,15 +530,8 @@ static void *locking_thread(void *arg)
 
 	while (!finished) {
 		if (bounces & BOUNCE_RANDOM) {
-			if (random_r(&rand, &rand_nr))
-				err("random_r failed");
-			page_nr = rand_nr;
-			if (sizeof(page_nr) > sizeof(rand_nr)) {
-				if (random_r(&rand, &rand_nr))
-					err("random_r failed");
-				page_nr |= (((unsigned long) rand_nr) << 16) <<
-					   16;
-			}
+			if (getrandom(&page_nr, sizeof(page_nr), 0) != sizeof(page_nr))
+				err("getrandom failed");
 		} else
 			page_nr += 1;
 		page_nr %= nr_pages;