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Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID
To:     Oleg Nesterov <oleg@redhat.com>, Adrian Reber <areber@redhat.com>
Cc:     Christian Brauner <christian@brauner.io>,
        Eric Biederman <ebiederm@xmission.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Jann Horn <jannh@google.com>, linux-kernel@vger.kernel.org,
        Andrei Vagin <avagin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
References: <20190806191551.22192-1-areber@redhat.com>
 <20190807154828.GD24112@redhat.com>
From:   Dmitry Safonov <0x7f454c46@gmail.com>
Message-ID: <b57e809d-e5fa-bda2-ee81-e86116bb2856@gmail.com>
Date:   Wed, 7 Aug 2019 16:57:08 +0100
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On 8/7/19 4:48 PM, Oleg Nesterov wrote:
> On 08/06, Adrian Reber wrote:
>>
>> @@ -2530,12 +2530,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>>  					      struct clone_args __user *uargs,
>>  					      size_t size)
>>  {
>> +	struct pid_namespace *pid_ns = task_active_pid_ns(current);
>>  	struct clone_args args;
>>  
>>  	if (unlikely(size > PAGE_SIZE))
>>  		return -E2BIG;
>>  
>> -	if (unlikely(size < sizeof(struct clone_args)))
>> +	/* The struct needs to be at least the size of the original struct. */
>> +	if (size < (sizeof(struct clone_args) - sizeof(__aligned_u64)))
>>  		return -EINVAL;
> 
> slightly off-topic, but with or without this patch I do not understand
> -EINVAL. Can't we replace this check with
> 
> 	if (size < sizeof(struct clone_args))
> 		memset((void*)&args + size, sizeof(struct clone_args) - size, 0);
> 
> ?
> 
> this way we can new members at the end of clone_args and this matches
> the "if (size > sizeof(struct clone_args))" block below which promises
> that whatever we add into clone_args a zero value should work.

What if the size is lesser than offsetof(struct clone_args, stack_size)?
Probably, there should be still a check that it's not lesser than what's
the required minimum..

Also note, that (kargs) and (args) are a bit different beasts in this
context..
kargs lies on the stack and might want to be with zero-initializer
:	struct kernel_clone_args kargs = {};

-- 
          Dmitry