Re: [PATCHv2] ARM32: Support mremap() for sigpage/vDSO

[Dmitry Safonov <dsafonov@virtuozzo.com>]

+ Adding back to CC x86 guys - as I've removed in ping messages.

On 05/23/2017 11:09 PM, Russell King - ARM Linux wrote:
> On Thu, May 18, 2017 at 02:13:29PM +0300, Dmitry Safonov wrote:
>> On 04/25/2017 08:18 PM, Dmitry Safonov wrote:
>>> On 04/14/2017 04:25 PM, Dmitry Safonov wrote:
>>>> CRIU restores application mappings on the same place where they
>>>> were before Checkpoint. That means, that we need to move vDSO
>>>> and sigpage during restore on exactly the same place where
>>>> they were before C/R.
>>>>
>>>> Make mremap() code update mm->context.{sigpage,vdso} pointers
>>>> during VMA move. Sigpage is used for landing after handling
>>>> a signal - if the pointer is not updated during moving, the
>>>> application might crash on any signal after mremap().
>>>>
>>>> vDSO pointer on ARM32 is used only for setting auxv at this moment,
>>>> update it during mremap() in case of future usage.
>>>>
>>>> Without those updates, current work of CRIU on ARM32 is not reliable.
>>>> Historically, we error Checkpointing if we find vDSO page on ARM32
>>>> and suggest user to disable CONFIG_VDSO.
>>>> But that's not correct - it goes from x86 where signal processing
>>>> is ended in vDSO blob. For arm32 it's sigpage, which is not disabled
>>>> with `CONFIG_VDSO=n'.
>>>>
>>>> Looks like C/R was working by luck - because userspace on ARM32 at
>>>> this moment always sets SA_RESTORER.
>>>>
>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>> Cc: Russell King <linux@armlinux.org.uk>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Cc: Andy Lutomirski <luto@amacapital.net>
>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>> Cc: Cyrill Gorcunov <gorcunov@openvz.org>
>>>> Cc: Pavel Emelyanov <xemul@virtuozzo.com>
>>>> Cc: Christopher Covington <cov@codeaurora.org>
>>>> Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
>>>> ---
>>>> v2: (buildbot) Fix (unsinged long) to (void*) cast warning.
>>>>
>>>>   arch/arm/kernel/process.c |  8 ++++++++
>>>>   arch/arm/kernel/vdso.c    | 18 ++++++++++++++++++
>>>>   arch/x86/entry/vdso/vma.c |  3 ---
>>>>   mm/mmap.c                 |  4 ++++
>>>>   4 files changed, 30 insertions(+), 3 deletions(-)
>>>
>>> Ping?
>>
>> Ping?
> 
> I'm mostly happy with the ARM bits, but I can't take the patch without
> acks from others because it touches other architectures/generic code.
> 

Fair enough, thanks.

Andy, does moving of this WARN_ON() looks good to you?
I've done it to reuse it over arches and between vdso/sigpage mappings,
reducing code duplication.

Here is the patch, so you don't need to search it in the mailbox:
https://patchwork.kernel.org/patch/9681273/

-- 
              Dmitry