Re: [RFC PATCH] efi: Put Linux specific magic number in the DOS header

[Heinrich Schuchardt]

On 11/9/22 15:16, Ard Biesheuvel wrote:
> GRUB currently relies on the magic number in the image header of ARM and
> arm64 EFI kernel images to decide whether or not the image in question
> is a bootable kernel.
>
> However, the purpose of the magic number is to identify the image as one
> that implements the bare metal boot protocol, and so GRUB, which only
> does EFI boot, can only boot images that could potentially be booted in
> a non-EFI manner as well.
>
> This is problematic for the new zboot decompressor image format, as it
> can only boot in EFI mode, and must therefore not use the bare metal
> boot magic number in its header.
>
> For this reason, the strict magic number was dropped from GRUB, to
> permit essentially any kind of EFI executable to be booted via the
> 'linux' command, blurring the line between the linux loader and the
> chainloader.
>
> So let's use the same field in the DOS header that RISC-V and arm64
> already use for their 'bare metal' magic numbers to store a 'generic
> Linux kernel' magic number, which can be used to identify bootable
> kernel images in PE format which don't necessarily implement a bare
> metal boot protocol in the same binary.
>
> Let's set the generic magic number for x86 images as well: existing
> bootloaders already have their own methods to identify x86 Linux images
> that can be booted in a non-EFI manner, and having the magic number in
> place there will ease any future transitions in loader implementations
> to merge the x86 and non-x86 EFI boot paths.
>
> Note that 32-bit ARM already uses the same location in the header for a
> different purpose, but the ARM support is already widely implemented and
> the EFI zboot decompressor is not available on ARM anyway, so we just
> disregard it here.
>
> Cc: Huacai Chen <chenhuacai@kernel.org>
> Cc: Atish Patra <atishp@rivosinc.com>
> Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Cc: Daniel Kiper <daniel.kiper@oracle.com>
> Cc: Leif Lindholm <quic_llindhol@quicinc.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> The idea is that, with this in place, the existing arm64 support in GRUB
> can be made generic, with the arm64 variant of the arch image header
> typedef being made generic as well.
>
> Any code that attempts to identify EFI images as kernel images should
> check for the arm64, RISC-V and generic values, and if the latter, look
> at the PE machine type if it wants to know the architecture.
>
>   arch/loongarch/kernel/head.S                | 3 ++-
>   arch/x86/boot/header.S                      | 3 ++-
>   drivers/firmware/efi/libstub/zboot-header.S | 3 ++-
>   include/linux/pe.h                          | 7 +++++++
>   4 files changed, 13 insertions(+), 3 deletions(-)

We need files in Documentation/ that describes the meaning and value of 
the field per architecture.

We already have:

riscv: Documentation/riscv/boot-image-header.rst
arm64: Documentation/arm64/booting.rst

But other UEFI architectures are missing.

Otherwise I am fine with the change.

Best regards

Heinrich

> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index 97425779ce9f3499..e1deea93aaafa069 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -25,7 +25,8 @@ _head:
>         .dword  kernel_entry            /* Kernel entry point */
>         .dword  _end - _text            /* Kernel image effective size */
>         .quad   0                       /* Kernel image load offset from start 
> of RAM */
> -       .org    0x3c                    /* 0x20 ~ 0x3b reserved */
> +       .org    0x38                    /* 0x20 ~ 0x38 reserved */
> +       .long   LINUX_PE_MAGIC
>         .long   pe_header - _head       /* Offset to the PE header */
>   
>   pe_header:
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index d31982509654dcb1..9338c68e7413d6e6 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -80,10 +80,11 @@ bs_die:
>         ljmp    $0xf000,$0xfff0
>   
>   #ifdef CONFIG_EFI_STUB
> -       .org    0x3c
> +       .org    0x38
>         #
>         # Offset to the PE header.
>         #
> +       .long   LINUX_PE_MAGIC
>         .long   pe_header
>   #endif /* CONFIG_EFI_STUB */
>   
> diff --git a/drivers/firmware/efi/libstub/zboot-header.S b/drivers/firmware/efi/libstub/zboot-header.S
> index 9e6fe061ab07a008..97c2637337d79913 100644
> --- a/drivers/firmware/efi/libstub/zboot-header.S
> +++ b/drivers/firmware/efi/libstub/zboot-header.S
> @@ -20,7 +20,8 @@ __efistub_efi_zboot_header:
>         .long           __efistub__gzdata_size - ZBOOT_SIZE_LEN // payload size
>         .long           0, 0                                    // reserved
>         .asciz          COMP_TYPE                               // compression 
> type
> -       .org            .Ldoshdr + 0x3c
> +       .org            .Ldoshdr + 0x38
> +       .long           LINUX_PE_MAGIC
>         .long           .Lpehdr - .Ldoshdr                      // PE header 
> offset
>   
>   .Lpehdr:
> diff --git a/include/linux/pe.h b/include/linux/pe.h
> index 1d3836ef9d92dcd8..fa176c24167c301c 100644
> --- a/include/linux/pe.h
> +++ b/include/linux/pe.h
> @@ -31,6 +31,13 @@
>   #define LINUX_EFISTUB_MAJOR_VERSION           0x1
>   #define LINUX_EFISTUB_MINOR_VERSION           0x0
>   
> +/*
> + * LINUX_PE_MAGIC appears at offset 0x30 into the MSDOS header of EFI bootable
> + * Linux kernel images that target the architecture as specified by the PE/COFF
> + * header machine type field.
> + */
> +#define LINUX_PE_MAGIC 0x818223cd
> +
>   #define MZ_MAGIC      0x5a4d  /* "MZ" */
>   
>   #define PE_MAGIC		0x00004550	/* "PE\0\0" */