[PATCH 4.19 223/421] selftests/vm/pkeys: fix alloc_random_pkey() to make it really, really random

Greg Kroah-Hartman
2021-07-19T16:24:11Z

From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: "Desnes A. Nunes do Rosario" <desnesn@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Suchanek <msuchanek@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/x86/protection_keys.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/x86/protection_keys.c b/tools/testing/selftests/x86/protection_keys.c
index b8778960da10..27661302a698 100644
--- a/tools/testing/selftests/x86/protection_keys.c
+++ b/tools/testing/selftests/x86/protection_keys.c
@@ -613,7 +613,6 @@ 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;
@@ -1479,6 +1478,8 @@ int main(void)
{
int nr_iterations = 22;

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

printf("has pku: %d\n", cpu_has_pku());
--
2.30.2