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

[Dmitry Safonov]

2017-08-08 12:44 GMT+03:00 Will Deacon <will.deacon@xxxxxxx>:
> On Tue, Aug 08, 2017 at 12:29:50PM +0300, Dmitry Safonov wrote:
>> 2017-08-02 19:04 GMT+03:00 Will Deacon <will.deacon@xxxxxxx>:
>> > On Fri, Jul 28, 2017 at 10:06:20PM +0300, Dmitry Safonov wrote:
>> >> 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?
>> >
>> > Yeah, just a minor thing, although thinking about it again, I don't know
>> > what you're trying to achieve with the size check anyway. Userspace is only
>> > going to hurt itself if it screws up the layout, so why police this?
>>
>> Well, it's for keeping the same semantics as on x86.
>> The idea of restriction to partial mremap() is suggested by Andy
>> so that userspace won't be allowed to hurt itself and to simplify
>> kernel code on x86.
>
> I still don't see why that's a useful thing for us to be doing on arm64, but
> ok.
>
> Either way:
>
> Reviewed-by: Will Deacon <will.deacon@xxxxxxx>

Thanks!

-- 
             Dmitry