linux-kernel - Re: [PATCH RESEND] lib/test_printf.c: call wait_for_random

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Date:   Thu, 7 Jun 2018 20:47:25 +0200
From:   Thierry Escande <thierry.escande@...aro.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, linux-kernel@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH RESEND] lib/test_printf.c: call wait_for_random_bytes()
 before plain %p tests

Hi Petr,

On 07/06/2018 14:24, Petr Mladek wrote:
> On Mon 2018-06-04 13:37:08, Thierry Escande wrote:
>> If the test_printf module is loaded before the crng is initialized, the
>> plain 'p' tests will fail because the printed address will not be hashed
>> and the buffer will contain '(ptrval)' instead.
>> This patch adds a call to wait_for_random_bytes() before plain 'p' tests
>> to make sure the crng is initialized.
> 
> Hmm, my system did not boot with this patch and
> CONFIG_TEST_PRINTF=y
> 
> I guess that there was not enough entropy. It is basically a deadlock.
> The build-in module init calls are called when SMP is enabled but
> there is no real activity and the init calls are called sequentially.
> Therefore there is nothing that could produce some entropy, ...
> 
> I suggest to skip the test when crng is not ready.

Thanks for pointing this out.

The issue is that the crng_ready() is not available outside of random.c. 
I'll leave the call to wait_for_random_bytes() if test_printf is 
compiled as a module and use add_random_ready_callback() to check if 
crng is initialized otherwise. Does that sound ok to you?

Regards,
Thierry

> 
> Best Regards,
> Petr
> 
>> Signed-off-by: Thierry Escande <thierry.escande@...aro.org>
>> Acked-by: Tobin C. Harding <me@...in.cc>
>> ---
>>   lib/test_printf.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/lib/test_printf.c b/lib/test_printf.c
>> index 71ebfa43ad05..839be9385a8a 100644
>> --- a/lib/test_printf.c
>> +++ b/lib/test_printf.c
>> @@ -260,6 +260,13 @@ plain(void)
>>   {
>>   	int err;
>>   
>> +	/*
>> +	 * Make sure crng is ready. Otherwise we get "(ptrval)" instead
>> +	 * of a hashed address when printing '%p' in plain_hash() and
>> +	 * plain_format().
>> +	 */
>> +	wait_for_random_bytes();
>> +
>>   	err = plain_hash();
>>   	if (err) {
>>   		pr_warn("plain 'p' does not appear to be hashed\n");
>> -- 
>> 2.14.1
>>