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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: x86@kernel.org, linux-efi@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Peter Jones <pjones@redhat.com>,
	Daniel Berrange <berrange@redhat.com>,
	Emanuele Giuseppe Esposito <eesposit@redhat.com>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Luca Boccassi <bluca@debian.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	Eric Snowberg <eric.snowberg@oracle.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] x86/efi: Implement support for embedding SBAT data for x86
Date: Tue, 29 Apr 2025 11:55:29 +0200	[thread overview]
Message-ID: <87ikmn9tri.fsf@redhat.com> (raw)
In-Reply-To: <CAMj1kXFMmhROmaDZ0gsw+ozG5iSkMvSXb15qexToUSAFyBn5hQ@mail.gmail.com>

Ard Biesheuvel <ardb@kernel.org> writes:

> On Thu, 24 Apr 2025 at 10:10, Vitaly Kuznetsov <vkuznets@redhat.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