From: Dave Hansen <dave.hansen@linux.intel.com>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org,
Dave Hansen <dave.hansen@linux.intel.com>,
tglx@linutronix.de, linuxram@us.ibm.com, sandipan@linux.ibm.com,
akpm@linux-foundation.org, fweimer@redhat.com,
desnesn@linux.vnet.ibm.com, mingo@kernel.org,
bauerman@linux.ibm.com, aneesh.kumar@linux.ibm.com,
mpe@ellerman.id.au, mhocko@kernel.org, msuchanek@suse.de,
shuah@kernel.org, x86@kernel.org
Subject: [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random
Date: Fri, 11 Jun 2021 09:41:55 -0700 [thread overview]
Message-ID: <20210611164155.192D00FF@viggo.jf.intel.com> (raw)
In-Reply-To: <20210611164153.91B76FB8@viggo.jf.intel.com>
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);
_
WARNING: multiple messages have this Message-ID (diff)
From: Dave Hansen <dave.hansen@linux.intel.com>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org,Dave Hansen
<dave.hansen@linux.intel.com>,tglx@linutronix.de,linuxram@us.ibm.com,sandipan@linux.ibm.com,akpm@linux-foundation.org,fweimer@redhat.com,desnesn@linux.vnet.ibm.com,mingo@kernel.org,bauerman@linux.ibm.com,aneesh.kumar@linux.ibm.com,mpe@ellerman.id.au,mhocko@kernel.org,msuchanek@suse.de,shuah@kernel.org,x86@kernel.org
Subject: [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random
Date: Fri, 11 Jun 2021 09:41:55 -0700 [thread overview]
Message-ID: <20210611164155.192D00FF@viggo.jf.intel.com> (raw)
In-Reply-To: <20210611164153.91B76FB8@viggo.jf.intel.com>
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);
_
next prev parent reply other threads:[~2021-06-11 16:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-11 16:41 [PATCH 0/4] selftests/vm/pkeys: Bug fixes and a new test Dave Hansen
2021-06-11 16:41 ` Dave Hansen
2021-06-11 16:41 ` Dave Hansen [this message]
2021-06-11 16:41 ` [PATCH 1/4] selftests/vm/pkeys: Fix alloc_random_pkey() to make it really, really random Dave Hansen
2021-06-11 16:41 ` [PATCH 2/4] selftests/vm/pkeys: Handle negative sys_pkey_alloc() return code Dave Hansen
2021-06-11 16:41 ` Dave Hansen
2021-06-11 16:42 ` [PATCH 3/4] selftests/vm/pkeys: Refill shadow register after implicit kernel write Dave Hansen
2021-06-11 16:42 ` Dave Hansen
2021-06-11 16:42 ` [PATCH 4/4] selftests/vm/pkeys: Exercise x86 XSAVE init state Dave Hansen
2021-06-11 16:42 ` Dave Hansen
2021-06-13 8:54 ` [PATCH 0/4] selftests/vm/pkeys: Bug fixes and a new test Aneesh Kumar K.V
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