Messages in this thread Patch in this message | | | Subject | [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random | From | Dave Hansen <> | Date | Fri, 11 Jun 2021 09:41:55 -0700 |
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From: Dave Hansen <dave.hansen@linux.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@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); _
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