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From:   Thomas Garnier <thgarnie@chromium.org>
Date:   Tue, 9 Jul 2019 11:47:48 -0700
Message-ID: <CAJcbSZFt6Xu=r+bvhPVoGnRL4Y2iRhnFo7_oBmwzW_LzxYO47w@mail.gmail.com>
Subject: Re: [PATCH v8 06/11] x86/CPU: Adapt assembly for PIE support
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>
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On Tue, Jul 9, 2019 at 11:39 AM Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> On Mon, Jul 08, 2019 at 12:35:13PM -0700, Thomas Garnier wrote:
> > On Mon, Jul 8, 2019 at 12:09 PM Alexey Dobriyan <adobriyan@gmail.com> wrote:
> > >
> > > Thomas Garnier wrote:
> > > > -             "pushq $1f\n\t"
> > > > +             "movabsq $1f, %q0\n\t"
> > > > +             "pushq %q0\n\t"
> > > >               "iretq\n\t"
> > > >               UNWIND_HINT_RESTORE
> > > >               "1:"
> > >
> > > Fake PIE. True PIE looks like this:
> >
> > I used movabsq in couple assembly changes where the memory context is
> > unclear and relative reference might lead to issues. It happened on
> > early boot and hibernation save/restore paths. Do you think a relative
> > reference in this function will always be accurate?
>
> As long as iretq target is not too far it should be OK.
>
> I'm not really sure which issues can pop up.
>
> IRETQ is 64-bit only, RIP-relative addressing is 64-bit only.
> Assembler (hopefully) will error compilation if target is too far.
>
> And it is shorter than movabsq.

Agree, I will change it and run some tests for the next iteration.

>
> > > ffffffff81022d70 <do_sync_core>:
> > > ffffffff81022d70:       8c d0                   mov    eax,ss
> > > ffffffff81022d72:       50                      push   rax
> > > ffffffff81022d73:       54                      push   rsp
> > > ffffffff81022d74:       48 83 04 24 08          add    QWORD PTR [rsp],0x8
> > > ffffffff81022d79:       9c                      pushf
> > > ffffffff81022d7a:       8c c8                   mov    eax,cs
> > > ffffffff81022d7c:       50                      push   rax
> > > ffffffff81022d7d:  ===> 48 8d 05 03 00 00 00    lea    rax,[rip+0x3]        # ffffffff81022d87 <do_sync_core+0x17>
> > > ffffffff81022d84:       50                      push   rax
> > > ffffffff81022d85:       48 cf                   iretq
> > > ffffffff81022d87:       c3                      ret
> > >
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > >
> > > --- a/arch/x86/include/asm/processor.h
> > > +++ b/arch/x86/include/asm/processor.h
> > > @@ -710,7 +710,8 @@ static inline void sync_core(void)
> > >                 "pushfq\n\t"
> > >                 "mov %%cs, %0\n\t"
> > >                 "pushq %q0\n\t"
> > > -               "pushq $1f\n\t"
> > > +               "leaq 1f(%%rip), %q0\n\t"
> > > +               "pushq %q0\n\t"
> > >                 "iretq\n\t"
> > >                 UNWIND_HINT_RESTORE
> > >                 "1:"