Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

[Alexey Dobriyan <adobriyan@gmail.com>]

On Thu, May 28, 2015 at 01:48:25AM +0300, Cyrill Gorcunov wrote:
> On Thu, May 28, 2015 at 01:29:42AM +0300, Alexey Dobriyan wrote:
> > > > +
> > > > +	page = (char *)__get_free_page(GFP_TEMPORARY);
> > > > +	if (!page) {
> > > > +		rv = -ENOMEM;
> > > > +		goto out_mmput;
> > > > +	}
> > > > +
> > > > +	down_read(&mm->mmap_sem);
> > > > +	arg_start = mm->arg_start;
> > > > +	arg_end = mm->arg_end;
> > > > +	env_start = mm->env_start;
> > > > +	env_end = mm->env_end;
> > > > +	up_read(&mm->mmap_sem);
> > > 
> > > Could you please explain why this down/up is needed?
> > 
> > Code is written this way to get constistent snapshot of data.
> 
> it does not. you fetch data into local variables which is the
> same as simply read them locklessly in general (because later
> you refer to local vars).

It is snapshot w.r.t getting both pairs not snapshot w.r.t atomicity or
something (unsigned long access is atomic after all). Once down_write()
is used in the other place, it even becomes obviously correct code!

> > If you look at PR_SET_MM_* code, you'll notice down_read(&mm->mmap_sem)
> > as well which is a separate bug because you're _writing_ those fields
> > eventually in prctl_set_mm(), yuck!
> 
> yes, there members are modified under read-lock and initially
> i didn't see any problem with that except one can have inconsistent
> statistics output because another process modified these fields
> (we validate that new members are having sane values at least
>  in new interface and after your first patch). But now I think
> that down_write may be more suitable here.
> 
> 	Cyrill