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

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Message-ID: <20200302165359.GA2599505@rani.riverdale.lan>
Date:   Mon, 2 Mar 2020 11:54:17 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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, 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?