linux-kernel - Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET

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Message-ID: <20190807162112.GF24112@redhat.com>
Date:   Wed, 7 Aug 2019 18:21:12 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Dmitry Safonov <0x7f454c46@...il.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID

On 08/07, Dmitry Safonov wrote:
> 
> 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..

Not sure I understand... I mean, this doesn't differ from the case when
size == sizeof(clone_args) but uargs->stack == NULL ?

> 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 = {};

I don't think so. Lets consider this patch which adds the new set_tid
into clone_args and kernel_clone_args. copy_clone_args_from_user() does

	*kargs = (struct kernel_clone_args){
		.flags		= args.flags,
		.pidfd		= u64_to_user_ptr(args.pidfd),
		.child_tid	= u64_to_user_ptr(args.child_tid),
		.parent_tid	= u64_to_user_ptr(args.parent_tid),
		.exit_signal	= args.exit_signal,
		.stack		= args.stack,
		.stack_size	= args.stack_size,
		.tls		= args.tls,
	};

so this patch should simply add

		.set_tid	= args.set_tid;

at the end. No?

Oleg.