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

[Will Deacon]

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>

Will