linux-kernel - Re: [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from bzImage

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Date:   Mon, 6 Apr 2020 12:52:43 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...en8.de>, Sergey Shatunov <me@...k.pw>,
        hpa@...or.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
        x86@...nel.org, linux-efi <linux-efi@...r.kernel.org>,
        initramfs@...r.kernel.org,
        Donovan Tremura <neurognostic@...tonmail.ch>,
        Harald Hoyer <harald@...er.xyz>
Subject: Re: [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from
 bzImage

On Mon, Apr 06, 2020 at 06:22:33PM +0200, Ard Biesheuvel wrote:
> 
> > We could just pull the stub's bss section all into data with objcopy
> > similar to what ARM64 does [1]? i.e. rename .bss to .bss.efistub and
> > then pull that into .data in the linker script for the compressed
> > kernel?
> >
> 
> I don't follow. I'm not aware of arm64 doing anything out of the
> ordinary with .bss? Note that arm64 does not have a separate
> decompressor, so the EFI stub is part of the kernel proper. This is
> why sections are renamed to start with .init

ARM64 renames the stub sections prefixing them with .init, and that
includes .bss. The ARM64 linker script then puts .init.bss into the
.init.data section. So I was suggesting doing something similar with the
x86 stub, renaming .bss to .bss.efistub, and then putting that into
.data via linker script.

Anyway, I'll do the section annotation for now as that will be less
churn and we can revisit this after that.

> 
> > There is also this scary looking comment in gnu-efi's linker script:
> >         /* the EFI loader doesn't seem to like a .bss section, so we stick
> >            it all into .data: */
> > I don't know what the history of that is.
> >
> 
> I don't remember, to be honest, but I'm pretty sure I copy-pasted that
> from elsewhere at the time.
> 
> > [1] As an aside, why doesn't ARM do this as well rather than using the
> > section(.data) annotation?
> >
> 
> The ARM decompressor has this hideous hack, where text [and rodata]
> executes straight from flash, and BSS is relocated into DRAM. In order
> for this to work, it actually *requires* GOT indirections for BSS
> items, to ensure that all BSS references use absolute addresses, which
> can be relocated independently from the rest of the kernel image. This
> is the reason ARM does not permit a .data section in the decompressor.
> However, the EFI stub does not support execute in place from flash,
> and so we can permit a .data section there. At the same time, we don't
> support GOT indirections in the EFI stub, so we cannot use the BSS
> section. So instead, we just put the few BSS pieces we have into .data
> instead.
> 

I see, thanks.