linux-kernel - Re: [PATCH V3] LoongArch: Add efistub booting support

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Date:   Mon, 5 Sep 2022 09:02:15 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     Xi Ruoyao <xry111@...111.site>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>, loongarch@...ts.linux.dev,
        linux-arch <linux-arch@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3] LoongArch: Add efistub booting support

On Mon, 5 Sept 2022 at 05:51, Huacai Chen <chenhuacai@...nel.org> wrote:
>
> Hi, Ard,
>
> On Mon, Sep 5, 2022 at 5:59 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > On Sun, 4 Sept 2022 at 15:24, Huacai Chen <chenhuacai@...nel.org> wrote:
> > >
> > > Hi, Ard,
> > >
> > > On Thu, Sep 1, 2022 at 6:40 PM Huacai Chen <chenhuacai@...nel.org> wrote:
> > > >
> > > > Hi, Ard,
> > > >
> > > > On Sat, Aug 27, 2022 at 3:14 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > > > >
> > > > > On Sat, 27 Aug 2022 at 06:41, Xi Ruoyao <xry111@...111.site> wrote:
> > > > > >
> > > > > > Tested V3 with the magic number check manually removed in my GRUB build.
> > > > > > The system boots successfully.  I've not tested Arnd's zBoot patch yet.
> > > > >
> > > > > I am Ard not Arnd :-)
> > > > >
> > > > > Please use this branch when testing the EFI decompressor:
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=efi-decompressor-v4
> > > > The root cause of LoongArch zboot boot failure has been found, it is a
> > > > binutils bug, latest toolchain with the below patch can solve the
> > > > problem.
> > > >
> > > > diff --git a/bfd/elfnn-loongarch.c b/bfd/elfnn-loongarch.c
> > > > index 5b44901b9e0..fafdc7c7458 100644
> > > > --- a/bfd/elfnn-loongarch.c
> > > > +++ b/bfd/elfnn-loongarch.c
> > > > @@ -2341,9 +2341,10 @@ loongarch_elf_relocate_section (bfd
> > > > *output_bfd, struct bfd_link_info *info,
> > > >      case R_LARCH_SOP_PUSH_PLT_PCREL:
> > > >        unresolved_reloc = false;
> > > >
> > > > -      if (resolved_to_const)
> > > > +      if (!is_undefweak && resolved_to_const)
> > > >          {
> > > >            relocation += rel->r_addend;
> > > > +          relocation -= pc;
> > > >            break;
> > > >          }
> > > >        else if (is_undefweak)
> > > >
> > > >
> > > > Huacai
> > > Now the patch is submitted here:
> > > https://sourceware.org/pipermail/binutils/2022-September/122713.html
> > >
> >
> > Great. Given the severity of this bug, I imagine that building the
> > LoongArch kernel will require a version of binutils that carries this
> > fix.
> >
> > Therefore, i will revert back to the original approach for accessing
> > uncompressed_size, using an extern declaration with an __aligned(1)
> > attribute.
> >
> > > And I have some other questions about kexec: kexec should jump to the
> > > elf entry or the pe entry? I think is the elf entry, because if we
> > > jump to the pe entry, then SVAM will be executed twice (but it should
> > > be executed only once). However, how can we jump to the elf entry if
> > > we use zboot? Maybe it is kexec-tool's responsibility to decompress
> > > the zboot kernel image?
> > >
> >
> > Yes, very good point. Kexec kernels cannot boot via the EFI entry
> > point, as the boot services will already be shutdown. So the kexec
> > kernel needs to boot via the same entrypoint in the core kernel that
> > the EFI stub calls when it hands over.
> >
> > For the EFI zboot image in particular, we will need to teach kexec how
> > to decompress them. The zboot image has a header that
> > a) describes it as a EFI linux zimg
> > b) describes the start and end offset of the compressed payload
> > c) describes which compression algorithm was used.
> >
> > This means that any non-EFI loader (including kexec) should be able to
> > extract the inner PE/COFF image and decompress it. For arm64 and
> > RISC-V, this is sufficient as the EFI and raw images are the same. For
> > LoongArch, I suppose it means we need a way to enter the core kernel
> > directly via the entrypoint that the EFI stub uses when handing over
> > (and pass the original DT argument so the kexec kernel has access to
> > the EFI and ACPI firmware tables)
> OK, then is this implementation [1] acceptable? I remember that you
> said the MS-DOS header shouldn't contain other information, so I guess
> this is unacceptable?
>

No, this looks reasonable to me. I objected to using magic numbers in
the 'pure PE' view of the image, as it does not make sense for a pure
PE loader such as GRUB to rely on such metadata.

In this case (like on arm64), we are dealing with something else: we
need to identify the image to the kernel itself, and here, using the
unused space in the MS-DOS header is fine.

> [1] https://lore.kernel.org/loongarch/c4dbb14a-5580-1e47-3d15-5d2079e88404@loongson.cn/T/#mb8c1dc44f7fa2d3ef638877f0cd3f958f0be96ad
>
> Huacai