linux-kernel - Re: [PATCH 2/2] x86/efi: Implement support for embedding SBAT data for x86

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Message-ID: <CAMj1kXHTtGP-UcJLut7OVUuULzg59npyoQhb+kuCZO5x-2NYWA@mail.gmail.com>
Date: Tue, 29 Apr 2025 12:08:41 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: x86@...nel.org, linux-efi@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Peter Jones <pjones@...hat.com>, Daniel Berrange <berrange@...hat.com>, 
	Emanuele Giuseppe Esposito <eesposit@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>, 
	Greg KH <gregkh@...uxfoundation.org>, Luca Boccassi <bluca@...ian.org>, 
	Peter Zijlstra <peterz@...radead.org>, Matthew Garrett <mjg59@...f.ucam.org>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, 
	Eric Snowberg <eric.snowberg@...cle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/efi: Implement support for embedding SBAT data
 for x86

On Tue, 29 Apr 2025 at 11:55, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Ard Biesheuvel <ardb@...nel.org> writes:
>
> > On Thu, 24 Apr 2025 at 10:10, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> ...
>
> >> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> >> index fdbce022db55..b9b80eccdc02 100644
> >> --- a/arch/x86/boot/compressed/Makefile
> >> +++ b/arch/x86/boot/compressed/Makefile
> >> @@ -107,6 +107,8 @@ vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/mem.o
> >>  vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
> >>  vmlinux-libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
> >>
> >> +vmlinux-objs-$(CONFIG_EFI_SBAT) += $(objtree)/drivers/firmware/efi/libstub/sbat.o
> >> +
> >
> > Please drop this, and put the .incbin directly into header.S
> >
>
> I'm sorry I'm probably missing something important but my understanding
> is that that header.S is compiled into setup.elf:
>
>  ld -m elf_x86_64 -z noexecstack --no-warn-rwx-segments  -m elf_i386 -z
>  noexecstack -T arch/x86/boot/setup.ld  ... arch/x86/boot/header.o ...  -o arch/x86/boot/setup.elf
>
> and then the result gets concatenated with vmlinux.bin to get bzImage:
>
>  objcopy  -O binary arch/x86/boot/setup.elf arch/x86/boot/setup.bin
>  cp arch/x86/boot/setup.bin arch/x86/boot/bzImage; truncate -s %4K arch/x86/boot/bzImage; cat arch/x86/boot/vmlinux.bin >>arch/x86/boot/bzImage
>
> so if we want to have SBAT at the very end of bzImage without dirty
> tricks it must be at the very end of vmlinux.bin, not setup.bin. I can,
> of course, use some existing compilation unit but to be honest I can't
> find anything suitable.
>

Yeah, you're right. I keep forgetting the insane way the bzImage is
put together.

So you'll need to incorporate $(CONFIG_EFI_SBAT_FILE) into
arch/x86/boot/vmlinux. But that does not mean it needs to be
constructed under drivers/firmware/efi/libstub, and it also doesn't
mean you need filechk and a separate .o file, right?