linux-kernel - [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random

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Date:   Fri, 11 Jun 2021 09:41:55 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>, tglx@...utronix.de,
        linuxram@...ibm.com, sandipan@...ux.ibm.com,
        akpm@...ux-foundation.org, fweimer@...hat.com,
        desnesn@...ux.vnet.ibm.com, mingo@...nel.org,
        bauerman@...ux.ibm.com, aneesh.kumar@...ux.ibm.com,
        mpe@...erman.id.au, mhocko@...nel.org, msuchanek@...e.de,
        shuah@...nel.org, x86@...nel.org
Subject: [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random


From: Dave Hansen <dave.hansen@...ux.intel.com>

The "random" pkey allocation code currently does the good old:

	srand((unsigned int)time(NULL));

*But*, it unfortunately does this on every random pkey allocation.

There may be thousands of these a second.  time() has a one second
resolution.  So, each time alloc_random_pkey() is called, the PRNG is
*RESET* to time().  This is nasty.  Normally, if you do:

	srand(<ANYTHING>);
	foo = rand();
	bar = rand();

You'll be quite guaranteed that 'foo' and 'bar' are different.
But, if you do:

	srand(1);
	foo = rand();
	srand(1);
	bar = rand();

You are quite guaranteed that 'foo' and 'bar' are the *SAME*.
The recent "fix" effectively forced the test case to use the
same "random" pkey for the whole test, unless the test run
crossed a second boundary.

Only run srand() once at program startup.

This explains some very odd and persistent test failures I've been
seeing.

Fixes: 6e373263ce07 ("selftests/vm/pkeys: fix alloc_random_pkey() to make it really random")
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Ram Pai <linuxram@...ibm.com>
Cc: Sandipan Das <sandipan@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Florian Weimer <fweimer@...hat.com>
Cc: "Desnes A. Nunes do Rosario" <desnesn@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Michal Suchanek <msuchanek@...e.de>
Cc: Shuah Khan <shuah@...nel.org>
Cc: x86@...nel.org
---

 b/tools/testing/selftests/vm/protection_keys.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff -puN tools/testing/selftests/vm/protection_keys.c~selftests_vm_pkeys_Fix_alloc_random_pkey_to_make_it_really_really_random-1 tools/testing/selftests/vm/protection_keys.c
--- a/tools/testing/selftests/vm/protection_keys.c~selftests_vm_pkeys_Fix_alloc_random_pkey_to_make_it_really_really_random-1	2021-06-11 09:41:31.385468066 -0700
+++ b/tools/testing/selftests/vm/protection_keys.c	2021-06-11 09:41:31.389468066 -0700
@@ -561,7 +561,6 @@ int alloc_random_pkey(void)
 	int nr_alloced = 0;
 	int random_index;
 	memset(alloced_pkeys, 0, sizeof(alloced_pkeys));
-	srand((unsigned int)time(NULL));
 
 	/* allocate every possible key and make a note of which ones we got */
 	max_nr_pkey_allocs = NR_PKEYS;
@@ -1552,6 +1551,8 @@ int main(void)
 	int nr_iterations = 22;
 	int pkeys_supported = is_pkeys_supported();
 
+	srand((unsigned int)time(NULL));
+
 	setup_handlers();
 
 	printf("has pkeys: %d\n", pkeys_supported);
_