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

[Konstantin Belousov]

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.
>  }
>  
>  int