Re: brk checks in PR_SET_MM code

From: Cyrill Gorcunov
Date: Thu Dec 17 2020 - 16:28:54 EST


On Thu, Dec 17, 2020 at 10:42:02AM +0300, Cyrill Gorcunov wrote:
> On Wed, Dec 16, 2020 at 08:29:30PM -0500, Keno Fischer wrote:
> > Hi all,
> >
> > The code in prctl(PR_SET_MM, ...) performs a number of sanity checks,
> > among them
> >
> > ```
> > /*
> > * @brk should be after @end_data in traditional maps.
> > */
> > if (prctl_map->start_brk <= prctl_map->end_data ||
> > prctl_map->brk <= prctl_map->end_data)
> > goto out;
> > ```
> >
>
> Thanks for pointing, Keno! I don't remember the details right now,
> gimme some time and once I refresh my memory I'll reply with
> details.

Indeed, when loaded via ld directly we've got a different layout:

# /lib64/ld-linux-x86-64.so.2 ~/t

start_code 7fc25b0a4000
end_code 7fc25b0c4524
start_stack 7fffcc6b2400
start_data 7fc25b0ce4c0
end_data 7fc25b0cff98
start_brk 55555710c000
sbrk(0) 55555710c000

Note though that as far as I understand the layout is provided by
ld loader. I contrast the regular load

# ~/t

start_code 401000
end_code 401a15
start_stack 7ffce4577dd0
start_data 403e10
end_data 40408c
start_brk b5b000
sbrk(0) b5b000

I fear we've not been using ld's loaded programs in c/r procedure much
that's why it has not been noted earlier. Need to think how to fix it.
Using the whole memory map for verification procedure is a correct way
thus the commit you mention is doing exactly what it should but we need
to figure out how to deal with fdpic loaded files... I'll back once I
figure it out (hopefully more-less soon). Thanks a huge for report!