linux-kernel - [PATCH 5.10 574/593] selftests/vm/pkeys: fix alloc_random

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Message-Id: <20210712060958.184692348@linuxfoundation.org>
Date:   Mon, 12 Jul 2021 08:12:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        Florian Weimer <fweimer@...hat.com>,
        "Desnes A. Nunes do Rosario" <desnesn@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Suchanek <msuchanek@...e.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.10 574/593] selftests/vm/pkeys: fix alloc_random_pkey() to make it really, really random

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

[ Upstream commit f36ef407628835a7d7fb3d235b1f1aac7022d9a3 ]

Patch series "selftests/vm/pkeys: Bug fixes and a new test".

There has been a lot of activity on the x86 front around the XSAVE
architecture which is used to context-switch processor state (among other
things).  In addition, AMD has recently joined the protection keys club by
adding processor support for PKU.

The AMD implementation helped uncover a kernel bug around the PKRU "init
state", which actually applied to Intel's implementation but was just
harder to hit.  This series adds a test which is expected to help find
this class of bug both on AMD and Intel.  All the work around pkeys on x86
also uncovered a few bugs in the selftest.

This patch (of 4):

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.

Link: https://lkml.kernel.org/r/20210611164153.91B76FB8@viggo.jf.intel.com
Link: https://lkml.kernel.org/r/20210611164155.192D00FF@viggo.jf.intel.com
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>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
Cc: Ram Pai <linuxram@...ibm.com>
Cc: Sandipan Das <sandipan@...ux.ibm.com>
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: Michael Ellerman <mpe@...erman.id.au>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Michal Suchanek <msuchanek@...e.de>
Cc: Shuah Khan <shuah@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/testing/selftests/vm/protection_keys.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/vm/protection_keys.c b/tools/testing/selftests/vm/protection_keys.c
index fdbb602ecf32..9ee0ae5d3e06 100644
--- a/tools/testing/selftests/vm/protection_keys.c
+++ b/tools/testing/selftests/vm/protection_keys.c
@@ -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);
-- 
2.30.2