linux-kernel - Re: [PATCH v8 06/11] x86/CPU: Adapt assembly for PIE support

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Message-ID: <20190709183921.GA27282@avx2>
Date:   Tue, 9 Jul 2019 21:39:21 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Thomas Garnier <thgarnie@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 06/11] x86/CPU: Adapt assembly for PIE support

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@...il.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.

> > 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@...il.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:"