linux-kernel - [PATCH v2 1/3] userfaultfd/selftests: don't rely on GNU extensions for random numbers

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Message-Id: <20210930212309.4001967-2-axelrasmussen@google.com>
Date:   Thu, 30 Sep 2021 14:23:07 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>
Cc:     linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: [PATCH v2 1/3] userfaultfd/selftests: don't rely on GNU extensions
 for random numbers

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.

Reviewed-by: Peter Xu <peterx@...hat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 26 ++++--------------------
 1 file changed, 4 insertions(+), 22 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 10ab56c2484a..2a71a91559a7 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/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"
 
@@ -501,22 +502,10 @@ 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;
@@ -524,15 +513,8 @@ 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;
-- 
2.33.0.800.g4c38ced690-goog