[PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random

Dave Hansen
2021-06-11T16:42:00Z

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@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
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: "Aneesh Kumar K.V" <aneesh.kumar@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>
Cc: x86@kernel.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);
_