Re: [PATCH v3 2/5] coredump: Let dump_emit() bail out on short writes

[Al Viro]

On Tue, Aug 18, 2020 at 03:40:28PM +0200, Oleg Nesterov wrote:
> On 08/18, Jann Horn wrote:
> >
> > +	if (dump_interrupted())
> > +		return 0;
> > +	n = __kernel_write(file, addr, nr, &pos);
> > +	if (n != nr)
> > +		return 0;
> > +	file->f_pos = pos;
> 
> Just curious, can't we simply do
> 
> 	__kernel_write(file, addr, nr, &file->f_pos);
> 
> and avoid "loff_t pos" ?

	Bloody bad pattern; it would be (probably) safe in this case,
but in general ->f_pos is shared data.  Exposing it to fuckloads of
->write() instances is a bad idea - we had bugs like that.

	General rule: never pass an address of ->f_pos to anything,
and limit access to it as much as possible.