Re: [PATCH] arm64/vdso: Support mremap() for vDSO

[Dmitry Safonov]

2017-07-28 19:48 GMT+03:00 Will Deacon <will.deacon@xxxxxxx>:
> On Wed, Jul 26, 2017 at 08:07:37PM +0300, Dmitry Safonov wrote:
>> vDSO VMA address is saved in mm_context for the purpose of using
>> restorer from vDSO page to return to userspace after signal handling.
>>
>> In Checkpoint Restore in Userspace (CRIU) project we place vDSO VMA
>> on restore back to the place where it was on the dump.
>> With the exception for x86 (where there is API to map vDSO with
>> arch_prctl()), we move vDSO inherited from CRIU task to restoree
>> position by mremap().
>>
>> CRIU does support arm64 architecture, but kernel doesn't update
>> context.vdso pointer after mremap(). Which results in translation
>> fault after signal handling on restored application:
>> https://github.com/xemul/criu/issues/288
>>
>> Make vDSO code track the VMA address by supplying .mremap() fops
>> the same way it's done for x86 and arm32 by:
>> commit b059a453b1cf ("x86/vdso: Add mremap hook to vm_special_mapping")
>> commit 280e87e98c09 ("ARM: 8683/1: ARM32: Support mremap() for sigpage/vDSO").
>>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
>> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
>> Cc: Christopher Covington <cov@xxxxxxxxxxxxxx>
>> Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
>> ---
>>  arch/arm64/kernel/vdso.c | 15 +++++++++++++++
>>  1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
>> index e8f759f764f2..2d419006ad43 100644
>> --- a/arch/arm64/kernel/vdso.c
>> +++ b/arch/arm64/kernel/vdso.c
>> @@ -110,12 +110,27 @@ int aarch32_setup_vectors_page(struct linux_binprm *bprm, int uses_interp)
>>  }
>>  #endif /* CONFIG_COMPAT */
>>
>> +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_end - vdso_start;
>
> You might be able to use vdso_pages here, but it depends on my question
> below.

Yes, shifting with PAGE_SHIFT.
Is it just a preference?

>
>> +
>> +     if (vdso_size != new_size)
>> +             return -EINVAL;
>> +
>> +     current->mm->context.vdso = (void *)new_vma->vm_start;
>> +
>> +     return 0;
>> +}
>> +
>>  static struct vm_special_mapping vdso_spec[2] __ro_after_init = {
>>       {
>>               .name   = "[vvar]",
>>       },
>>       {
>>               .name   = "[vdso]",
>> +             .mremap = vdso_mremap,
>
> Does this mean we move the vdso text, but not the data page? How does that
> work?

Well, the kernel tracks only vdso pages - to find restorer addr after a signal.
In userspace one needs to move vvar and vdso vma pair accordingly,
with the same order and offset of course.

-- 
             Dmitry