linux-kernel - Re: [PATCH 3/9] efi/x86: Move efi stub globals from .bss to .data

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Message-ID: <20200410190356.GA1213308@rani.riverdale.lan>
Date:   Fri, 10 Apr 2020 15:03:56 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Brian Gerst <brgerst@...il.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...e.de>,
        Colin Ian King <colin.king@...onical.com>,
        Gary Lin <glin@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Sergey Shatunov <me@...k.pw>, Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 3/9] efi/x86: Move efi stub globals from .bss to .data

On Fri, Apr 10, 2020 at 08:03:15PM +0200, Ard Biesheuvel wrote:
> On Fri, 10 Apr 2020 at 20:01, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > On Fri, Apr 10, 2020 at 06:03:38PM +0200, Ard Biesheuvel wrote:
> > > On Fri, 10 Apr 2020 at 17:16, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> > > >
> > > > On Fri, Apr 10, 2020 at 10:20:42AM +0200, Ard Biesheuvel wrote:
> > > > > On Thu, 9 Apr 2020 at 23:08, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> > > > > >
> > > > > > On Thu, Apr 09, 2020 at 04:53:07PM -0400, Brian Gerst wrote:
> > > > > > > > Can we use the -fno-zero-initialized-in-bss compiler flag instead of
> > > > > > > > explicitly marking global variables?
> > > > > > >
> > > > > > > Scratch that.  Apparently it only works when a variable is explicitly
> > > > > > > initialized to zero.
> > > > > > >
> > > > > > > --
> > > > > > > Brian Gerst
> > > > > >
> > > > > > Right, there doesn't seem to be a compiler option to turn off the use of
> > > > > > .bss altogether.
> > > > >
> > > > > Yeah. I'll try to come up with a way to consolidate this a bit across
> > > > > architectures (which is a bit easier now that all of the EFI stub C
> > > > > code lives in the same place). It is probably easiest to use a section
> > > > > renaming trick similar to the one I added for ARM (as Arvind suggested
> > > > > as well, IIRC), and get rid of the per-symbol annotations altogether.
> > > >
> > > > Does that work for 32-bit ARM, or does it need to be .data to tell the
> > > > compiler to avoid generating GOT references? If that's fine, we don't
> > > > actually need to rename sections -- linker script magic is enough. For
> > > > eg, the below pulls the EFI stub bss into .data for x86 without the need
> > > > for the annotations.
> > > >
> > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > > > index 508cfa6828c5..e324819c95bc 100644
> > > > --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > > > @@ -52,6 +52,7 @@ SECTIONS
> > > >                 _data = . ;
> > > >                 *(.data)
> > > >                 *(.data.*)
> > > > +               drivers/firmware/efi/libstub/lib.a:(.bss .bss.*)
> > > >                 _edata = . ;
> > > >         }
> > > >         . = ALIGN(L1_CACHE_BYTES);
> > >
> > > No, we can add this to ARM as well, and get rid of the
> > > __efistub_global annotations entirely.
> >
> > Cool.
> >
> > >
> > > We'll still need .data.efistub for the .data pieces, but that is a
> > > separate issue.
> >
> > You can avoid that by using an archive specification like above. i.e.
> > adding
> >         drivers/firmware/efi/libstub/lib.a:(.data .data.*)
> > to the .init.data output section will pull in just the .data input
> > sections from the EFI stub into the .init.data section.
> 
> Sure. But the ARM decompressor linker script currently discards .data
> before this point in the linker script, and relies on this as a safety
> net to ensure that no new .data items get added to the decompressor
> binary (which runs after the stub)

Ah.