linux-kernel - Re: [PATCHv2 3/6] x86/arch_prctl/vdso: add ARCH_MAP_VDSO

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Date:	Thu, 7 Jul 2016 14:11:58 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Michal Hocko <mhocko@...e.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Dmitry Safonov" <0x7f454c46@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Ingo Molnar <mingo@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>, <xemul@...tuozzo.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCHv2 3/6] x86/arch_prctl/vdso: add ARCH_MAP_VDSO_*

On 07/06/2016 05:30 PM, Andy Lutomirski wrote:
> On Wed, Jun 29, 2016 at 3:57 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
>> Add API to change vdso blob type with arch_prctl.
>> As this is usefull only by needs of CRIU, expose
>> this interface under CONFIG_CHECKPOINT_RESTORE.
>
>> +#ifdef CONFIG_CHECKPOINT_RESTORE
>> +       case ARCH_MAP_VDSO_X32:
>> +               return do_map_vdso(VDSO_X32, addr, false);
>> +       case ARCH_MAP_VDSO_32:
>> +               return do_map_vdso(VDSO_32, addr, false);
>> +       case ARCH_MAP_VDSO_64:
>> +               return do_map_vdso(VDSO_64, addr, false);
>> +#endif
>> +
>
> This will have an odd side effect: if the old mapping is still around,
> its .fault will start behaving erratically.  I wonder if we can either
> reliably zap the old vma (or check that it's not there any more)
> before mapping a new one or whether we can associate the vdso image
> with the vma (possibly by having a separate vm_special_mapping for
> each vdso_image.  The latter is quite easy: change vdso_image to embed
> vm_special_mapping and use container_of in vdso_fault to fish the
> vdso_image back out.  But we'd have to embed another
> vm_special_mapping for the vvar mapping as well for the same reason.
>
> I'm also a bit concerned that __install_special_mapping might not get
> all the cgroup and rlimit stuff right.  If we ensure that any old
> mappings are gone, then the damage is bounded, but otherwise someone
> might call this in a loop and fill their address space with arbitrary
> numbers of special mappings.

Well, I have deleted code that unmaps old vdso because I didn't saw
a reason why it's bad and wanted to reduce code. But well, now I do see
reasons, thanks.

Hmm, what do you think if I do it a little different way then embedding
vm_special_mapping: just that old hack with vma_ops. If I add a close()
hook there and make there context.vdso = NULL pointer, then I can test
it on remap. This can also have nice feature as restricting partial
munmap of vdso blob. Is this sounds sane?