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

Arvind Sankar
2020-04-10T15:16:18Z

On Fri, Apr 10, 2020 at 10:20:42AM +0200, Ard Biesheuvel wrote:
> On Thu, 9 Apr 2020 at 23:08, Arvind Sankar <nivedita@alum.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);