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

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Message-ID: <87ikmn9tri.fsf@redhat.com>
Date: Tue, 29 Apr 2025 11:55:29 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Ard Biesheuvel <ardb@...nel.org>
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

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.

-- 
Vitaly