linux-kernel - Re: [PATCH 3/5] efi/x86: Make efi32_pe

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Message-ID: <CAKv+Gu-8HeNaRYZNtTHr1_VF1aH=BRKF4CaeyP-PPfHNQN2paA@mail.gmail.com>
Date:   Mon, 2 Mar 2020 17:57:04 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] efi/x86: Make efi32_pe_entry more readable

On Mon, 2 Mar 2020 at 17:54, Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Mon, Mar 02, 2020 at 08:49:17AM +0100, Ard Biesheuvel wrote:
> > On Mon, 2 Mar 2020 at 00:04, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> ...
> > >         call    1f
> > > -1:     pop     %ebp
> > > -       subl    $1b, %ebp
> > > +1:     pop     %ebx
> > > +       subl    $1b, %ebx
> ...
> > >
> > > +       movl    %ebx, %ebp                      // startup_32 for efi32_pe_stub_entry
> >
> > The code that follows efi32_pe_stub_entry still expects the runtime
> > displacement in %ebp, so we'll need to pass that in another way here.
> >
> > >         jmp     efi32_pe_stub_entry
>
> Didn't follow -- what do you mean by runtime displacement?
>
> efi32_pe_stub_entry expects the runtime address of startup_32 to be in
> %ebp, but with the changes for keeping the frame pointer in %ebp, I
> changed the runtime address to be in %ebx instead. Hence I added that
> movl %ebx, %ebp to put it in %ebp just before calling efi32_pe_stub_entry.
> That should be fine, no?

But how does that work with:

SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
    movl %ecx, efi32_boot_args(%ebp)
    movl %edx, efi32_boot_args+4(%ebp)
    movb $0, efi_is64(%ebp)


?