Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()

[Joe Perches]

On Mon, 2020-03-02 at 14:04 +0100, glider@xxxxxxxxxx wrote:
> Certain copy_from_user() invocations in binder.c are known to
> unconditionally initialize locals before their first use, like e.g. in
> the following case:
[]
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
[]
> @@ -3788,7 +3788,7 @@ static int binder_thread_write(struct binder_proc *proc,
>  
>  		case BC_TRANSACTION_SG:
>  		case BC_REPLY_SG: {
> -			struct binder_transaction_data_sg tr;
> +			struct binder_transaction_data_sg tr __no_initialize;
>  
>  			if (copy_from_user(&tr, ptr, sizeof(tr)))

I fail to see any value in marking tr with __no_initialize
when it's immediately written to by copy_from_user.

>  				return -EFAULT;
> @@ -3799,7 +3799,7 @@ static int binder_thread_write(struct binder_proc *proc,
>  		}
>  		case BC_TRANSACTION:
>  		case BC_REPLY: {
> -			struct binder_transaction_data tr;
> +			struct binder_transaction_data tr __no_initialize;
>  
>  			if (copy_from_user(&tr, ptr, sizeof(tr)))

etc...