git: 28d70deaafa6 - main - Fix race between first rand(3) calls.

[Andriy Gapon]

On 20/07/2021 22:40, Konstantin Belousov wrote:
> On Tue, Jul 20, 2021 at 05:30:33PM +0000, Alexander Motin wrote:
>> The branch main has been updated by mav:
>>
>> URL: https://cgit.FreeBSD.org/src/commit/?id=28d70deaafa62c5d1602de5272c0aad0fcca8aff
>>
>> commit 28d70deaafa62c5d1602de5272c0aad0fcca8aff
>> Author:     Alexander Motin <mav at FreeBSD.org>
>> AuthorDate: 2021-07-20 17:15:08 +0000
>> Commit:     Alexander Motin <mav at FreeBSD.org>
>> CommitDate: 2021-07-20 17:30:28 +0000
>>
>>      Fix race between first rand(3) calls.
>>      
>>      Before this patch there was a chance for thread that called rand(3)
>>      slightly later to see rand3_state already allocated, but not yet
>>      initialized.  While this API is not expected to be thread-safe, it
>>      is not expected to crash.  ztest on 64-thread system reproduced it
>>      reliably for me.
>>      
>>      MFC after:      1 month
>> ---
>>   lib/libc/stdlib/rand.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c
>> index bddb0f040302..353f59349e1d 100644
>> --- a/lib/libc/stdlib/rand.c
>> +++ b/lib/libc/stdlib/rand.c
>> @@ -44,6 +44,7 @@ __FBSDID("$FreeBSD$");
>>   #include <stdbool.h>
>>   #include <stdlib.h>
>>   #include <syslog.h>
>> +#include <machine/atomic.h>
>>   #include "un-namespace.h"
>>   
>>   #include "random.h"
>> @@ -68,11 +69,15 @@ static struct __random_state *rand3_state;
>>   static void
>>   initialize_rand3(void)
>>   {
>> +	struct __random_state *state;
>>   	int error;
>>   
>> -	rand3_state = allocatestate(TYPE_3);
>> -	error = initstate_r(rand3_state, 1, rand3_state->rst_randtbl, BREAK_3);
>> +	state = allocatestate(TYPE_3);
>> +	error = initstate_r(state, 1, state->rst_randtbl, BREAK_3);
>>   	assert(error == 0);
>> +	if (!atomic_cmpset_rel_ptr((volatile uintptr_t *)&rand3_state,
>> +	    (uintptr_t)NULL, (uintptr_t)state))
>> +		free(state);
> For this to have effect on less ordered architectures (AKA non-x86), at
> least reads of rand3_state should be atomic_load_acq().
> 
> But consider using _once() in libc, which takes care about ordering/threads.

FWIW, I've been using the following patch locally.
And the reason for the patched is discussed here: 
https://www.mail-archive.com/freebsd-current@freebsd.org/msg182365.html

commit cc38290d14a5bbab64e419a2d6d8c34f61840829
Author:     Andriy Gapon <avg at FreeBSD.org>
AuthorDate: Mon Apr 5 17:33:07 2021 +0300
Commit:     Andriy Gapon <avg at FreeBSD.org>
CommitDate: Tue Jun 8 10:34:28 2021 +0300

     [tmp?] make rand/srand initialization thread safe

     But rand(3) still remains non-thread-safe in general.
     This is only to prevent an initialization time crash in ztest.
     Probably should be fixed there, e.g. by using a different random number
     API or by adding some locking.

diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c
index bddb0f040302..0d3ed2230e10 100644
--- a/lib/libc/stdlib/rand.c
+++ b/lib/libc/stdlib/rand.c
@@ -41,11 +41,13 @@ __FBSDID("$FreeBSD$");
  #include <sys/param.h>
  #include <sys/sysctl.h>
  #include <assert.h>
+#include <pthread.h>
  #include <stdbool.h>
  #include <stdlib.h>
  #include <syslog.h>
  #include "un-namespace.h"

+#include "libc_private.h"
  #include "random.h"

  /*
@@ -64,6 +66,7 @@ __FBSDID("$FreeBSD$");
   * the advantage of being the one already in the tree.
   */
  static struct __random_state *rand3_state;
+static pthread_once_t  __random_state_once = PTHREAD_ONCE_INIT;

  static void
  initialize_rand3(void)
@@ -78,16 +81,14 @@ initialize_rand3(void)
  int
  rand(void)
  {
-	if (rand3_state == NULL)
-		initialize_rand3();
+	_once(&__random_state_once, initialize_rand3);
  	return ((int)random_r(rand3_state));
  }

  void
  srand(unsigned seed)
  {
-	if (rand3_state == NULL)
-		initialize_rand3();
+	_once(&__random_state_once, initialize_rand3);
  	srandom_r(rand3_state, seed);
  }



-- 
Andriy Gapon