Subject: Re: [PATCH] arm/vdso: introduce vdso_mremap hook
To: Christopher Covington <cov@codeaurora.org>,
        <linux-kernel@vger.kernel.org>
References: <20161101172214.2938-1-dsafonov@virtuozzo.com>
 <0b41c28b-20ef-332f-d8d6-e381e05b8252@codeaurora.org>
 <714d2aea-ed4c-6272-89c1-e1d0e037855e@virtuozzo.com>
 <d1aa8bec-a53e-cd30-e66a-39bebb6a400a@codeaurora.org>
CC: <0x7f454c46@gmail.com>, Kevin Brodsky <kevin.brodsky@arm.com>,
        "Andy Lutomirski" <luto@amacapital.net>,
        Oleg Nesterov <oleg@redhat.com>,
        "Russell King" <linux@armlinux.org.uk>,
        Will Deacon <will.deacon@arm.com>,
        <linux-arm-kernel@lists.infradead.org>, <linux-mm@kvack.org>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Nathan Lynch <nathan_lynch@mentor.com>,
        Michael Ellerman <mpe@ellerman.id.au>
From: Dmitry Safonov <dsafonov@virtuozzo.com>
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Date: Mon, 7 Nov 2016 21:13:22 +0300
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On 11/07/2016 09:08 PM, Christopher Covington wrote:
> On 11/07/2016 12:16 PM, Dmitry Safonov wrote:
>> On 11/07/2016 08:00 PM, Christopher Covington wrote:
>>> Hi Dmitry,
>>>
>>> On 11/01/2016 01:22 PM, Dmitry Safonov wrote:
>>>>   Add vdso_mremap hook which will fix context.vdso pointer after mremap()
>>>> on vDSO vma. This is needed for correct landing after syscall execution.
>>>> Primary goal of this is for CRIU on arm - we need to restore vDSO image
>>>> at the exactly same place where the vma was in dumped application. With
>>>> the help of this hook we'll move vDSO at the new position.
>>>>   The CRIU code handles situations like when vDSO of dumped application
>>>> was different from vDSO on restoring system. This usally happens when
>>>> some new symbols are being added to vDSO. In these situations CRIU
>>>> inserts jump trampolines from old vDSO blob to new vDSO on restore.
>>>> By that reason even if on restore vDSO blob lies on the same address as
>>>> blob in dumped application - we still need to move it if it differs.
>>>>
>>>>   There was previously attempt to add this functionality for arm64 by
>>>> arch_mremap hook [1], while this patch introduces this with minimal
>>>> effort - the same way I've added it to x86:
>>>> commit b059a453b1cf ("x86/vdso: Add mremap hook to vm_special_mapping")
>>>>
>>>>   At this moment, vdso restoring code is disabled for arm/arm64 arch
>>>> in CRIU [2], so C/R is only working for !CONFIG_VDSO kernels. This patch
>>>> is aimed to fix that.
>>>>   The same hook may be introduced for arm64 kernel, but at this moment
>>>> arm64 vdso code is actively reworked by Kevin, so we can do it on top.
>>>>   Separately, I've refactored arch_remap hook out from ppc64 [3].
>>>>
>>>> [1]: https://marc.info/?i=1448455781-26660-1-git-send-email-cov@codeaurora.org
>>>> [2]: https://github.com/xemul/criu/blob/master/Makefile#L39
>>>> [3]: https://marc.info/?i=20161027170948.8279-1-dsafonov@virtuozzo.com
>>>>
>>>> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
>>>> Cc: Christopher Covington <cov@codeaurora.org>
>>>> Cc: Andy Lutomirski <luto@amacapital.net>
>>>> Cc: Oleg Nesterov <oleg@redhat.com>
>>>> Cc: Russell King <linux@armlinux.org.uk>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>> Cc: linux-mm@kvack.org
>>>> Cc: Cyrill Gorcunov <gorcunov@openvz.org>
>>>> Cc: Pavel Emelyanov <xemul@virtuozzo.com>
>>>> Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
>>>> ---
>>>>  arch/arm/kernel/vdso.c | 21 +++++++++++++++++++++
>>>>  1 file changed, 21 insertions(+)
>>>>
>>>> diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
>>>> index 53cf86cf2d1a..d1001f87c2f6 100644
>>>> --- a/arch/arm/kernel/vdso.c
>>>> +++ b/arch/arm/kernel/vdso.c
>>>> @@ -54,8 +54,11 @@ static const struct vm_special_mapping vdso_data_mapping = {
>>>>      .pages = &vdso_data_page,
>>>>  };
>>>>
>>>> +static int vdso_mremap(const struct vm_special_mapping *sm,
>>>> +        struct vm_area_struct *new_vma);
>>>>  static struct vm_special_mapping vdso_text_mapping __ro_after_init = {
>>>>      .name = "[vdso]",
>>>> +    .mremap = vdso_mremap,
>>>>  };
>>>>
>>>>  struct elfinfo {
>>>> @@ -254,6 +257,24 @@ void arm_install_vdso(struct mm_struct *mm, unsigned long addr)
>>>>          mm->context.vdso = addr;
>>>>  }
>>>>
>>>> +static int vdso_mremap(const struct vm_special_mapping *sm,
>>>> +        struct vm_area_struct *new_vma)
>>>> +{
>>>> +    unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
>>>> +    unsigned long vdso_size = (vdso_total_pages - 1) << PAGE_SHIFT;
>>>> +
>>>> +    /* Disallow partial vDSO blob remap */
>>>> +    if (vdso_size != new_size)
>>>> +        return -EINVAL;
>>>> +
>>>> +    if (WARN_ON_ONCE(current->mm != new_vma->vm_mm))
>>>> +        return -EFAULT;
>>>> +
>>>> +    current->mm->context.vdso = new_vma->vm_start;
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>>  static void vdso_write_begin(struct vdso_data *vdata)
>>>>  {
>>>>      ++vdso_data->seq_count;
>>>>
>>>
>>> What do you think about putting this code somewhere generic (not under
>>> arch/*), so that powerpc and arm64 can reuse it once the cosmetic changes
>>> to make them compatible are made? My thought was that it could be defined
>>> underneath CONFIG_GENERIC_VDSO, which architectures could select as they
>>> became compatible.
>>
>> Hi Chistopher,
>>
>> Well, I don't think we won something out of generalization of simple assignment for context.vdso pointer accross arches. And a need to rename
>> vdso over arches for saving one single line?
>
> I count 17 lines, which duplicated across 3 architectures becomes 51 lines.
> Presumable in the future other architectures will want CRIU support as well.
> Additionally, should fixes ever be required, fixing one implementation instead
> of 3+ is preferred.
>
>> Also I don't like a bit this arch_mremap hook and need to nullify
>> vdso pointer.
>
> I'm sorry for the confusion but I in no way meant to imply that the
> arch_mremap hook should be carried forward. I fully  agree that the function
> pointer in struct vm_special_mapping is the better way to go.
>
> If you don't want to implement a version with vdso_mremap defined in a
> generic location (using it from struct vm_special_mapping), do you mind if I
> propose such a version?

Sure, do it, no objections.

-- 
              Dmitry