From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Subject: selftests/vm/pkeys: fix alloc_random_pkey() to make it really, really random 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@xxxxxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20210611164155.192D00FF@xxxxxxxxxxxxxxxxxx Fixes: 6e373263ce07 ("selftests/vm/pkeys: fix alloc_random_pkey() to make it really random") Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> Cc: Ram Pai <linuxram@xxxxxxxxxx> Cc: Sandipan Das <sandipan@xxxxxxxxxxxxx> Cc: Florian Weimer <fweimer@xxxxxxxxxx> Cc: "Desnes A. Nunes do Rosario" <desnesn@xxxxxxxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Michal Suchanek <msuchanek@xxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- tools/testing/selftests/vm/protection_keys.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/tools/testing/selftests/vm/protection_keys.c~selftests-vm-pkeys-fix-alloc_random_pkey-to-make-it-really-really-random +++ a/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); _
