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From: Arvind Sankar <>
To: Ard Biesheuvel <>
Subject: Re: [RFC PATCH 0/3] efi/x86: add support for generic EFI mixed mode boot
Date: Thu, 13 Feb 2020 12:53:18 -0500
Message-ID: <20200213175317.GC1400002@rani.riverdale.lan> (raw)
In-Reply-To: <>

On Thu, Feb 13, 2020 at 03:59:25PM +0100, Ard Biesheuvel wrote:
> This series is another part of my effort to reduce the level of knowledge
> on the part of the bootloader or firmware of internal per-architecture
> details regarding where/how the kernel is loaded and where its initrd and
> other context data are passed.
> The x86 architecture has a so-called 'EFI handover protocol', which defines
> how the bootparams struct should be populated, and how it should be
> interpreted to figure out where to load the kernel, and at which offset in
> the binary the entrypoint is located. This scheme allows the initrd to be
> loaded beforehand, and allows 32-bit firmware to invoke a 64-bit kernel
> via a special entrypoint that manages the state transitions between the
> two execution modes.
> Due to this, x86 loaders currently do not rely on LoadImage and StartImage,
> and therefore, are forced to re-implement things like image authentication
> for secure boot and taking the measurements for measured boot in their open
> coded clones of these routines.
> My previous series on this topic [0] implements a generic way to load the
> initrd from any source supported by the loader without relying on something
> like device trees or bootparams structures, and so native boot should not
> need the EFI handover protocol anymore after those change are merged.
> What remains is mixed mode boot, which also needs the EFI handover protocol
> regardless of whether an initrd is loaded or not. So let's get rid of that
> requirement, and take advantage of the fact that EDK2 based firmware does
> support LoadImage() for X64 binaries on IA32 firmware, which means we can
> rely on the secure boot and measured boot checks being performed by the
> firmware. The only thing we need to put on top is a way to discover the
> non-native entrypoint into the binary in a way that does not rely on x86
> specific headers and data structures.
> So let's introduce a new .compat header in the PE/COFF metadata of the
> bzImage, and populate it with a <machine type, entrypoint> tuple, allowing
> a generic EFI loader to decide whether the entrypoint supports its native
> machine type, and invoke it as an ordinary EFI application entrypoint.
> Since we will not be passing a bootparams structure, we need to discover
> the base of the image (which contains the setup header) via the loaded
> image protocol before we can enter the kernel in 32-bit mode at startup_32()
> A loader implementation for OVMF can be found at [1]. Note that this loader
> code is fully generic, and could be used without modifications if other
> architectures ever emerge that support kernels that can be invoked from a
> non-native (but cross-type supported) loader.
> [0]
> [1]

As an alternative to the new section, how about having a CONFIG option
to emit the 64-bit kernel with a 32-bit PE header instead, which would
point to efi32_pe_entry? In that case it could be directly loaded by
existing firmware already. You could even have a tool that can mangle an
existing bzImage's header from 64-bit to 32-bit, say using the newly
added kernel_info structure to record the existence and location of

Also, the PE header can live anywhere inside the image, right? Is there
any reason to struggle to shoehorn it into the "boot sector"?

  parent reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-13 14:59 Ard Biesheuvel
2020-02-13 14:59 ` [RFC PATCH 1/3] efi/x86: drop redundant .bss section Ard Biesheuvel
2020-02-13 14:59 ` [RFC PATCH 2/3] efi/x86: add true mixed mode entry point into .compat section Ard Biesheuvel
2020-02-13 16:59   ` Arvind Sankar
2020-02-13 17:13     ` Ard Biesheuvel
2020-02-13 14:59 ` [RFC PATCH 3/3] efi/x86: implement mixed mode boot without the handover protocol Ard Biesheuvel
2020-02-13 17:23   ` Arvind Sankar
2020-02-13 17:42     ` Ard Biesheuvel
2020-02-13 17:53 ` Arvind Sankar [this message]
2020-02-13 17:55   ` [RFC PATCH 0/3] efi/x86: add support for generic EFI mixed mode boot Ard Biesheuvel
2020-02-13 18:47     ` Arvind Sankar
2020-02-13 22:36       ` Ard Biesheuvel
2020-02-14  0:10         ` Arvind Sankar
2020-02-14  0:12           ` Arvind Sankar
2020-02-14  0:21             ` Ard Biesheuvel
2020-02-14  0:38               ` Arvind Sankar

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