Re: [PATCH v3 0/5] x86/xen: Drop absolute references from startup code

Ard Biesheuvel
2024-10-29T12:50:24Z

On Wed, 9 Oct 2024 at 18:09, Ard Biesheuvel <ardb+git@google.com> wrote:
>
> From: Ard Biesheuvel <ardb@kernel.org>
>
> This series was broken out of the series I sent recently [0], after
> Jason pointed out that my Xen startup code changes conflict with his
> changes to make the PVH startup code position independent.
>
> Jason's work reduces the delta of my changes, and given that my other
> series will likely advance at a much slower pace, the Xen changes are
> presented here so they can be merged independently.
>
> The end result after applying this series (see below) is that there are
> no longer any Xen-related absolute relocations that need to be applied
> to .head.text, a section which carries code that may be invoked from the
> 1:1 mapping of memory before the kernel virtual mapping is up. The use
> of absolute references in this code section has resulted in a few boot
> issues that were very hard to track down (Clang built kernels running
> under SEV-SNP in particular, which does not provide the best debug
> experience).
>
> Even though the occurrences in the Xen startup code were fine, there is
> now a lot of C code emitted into .head.text as well, and so it would be
> helpful to teach objtool to reject absolute references entirely in this
> section (or rely on the linker for that). Therefore, not relying on them
> in the first place is a step towards that goal.
>
> Changes since v2 [2]:
> - add Jason's Tested-by to patch #4
> - use a better name for the linker defined symbols used in the ELF notes
> (patch #4)
> - add a comment in the linker script explaining why the symbol values
> are constructed in the way they are
> - rebase onto v6.12-rc2
>
> Changes since v1 [1]:
> - add Jason's Rb to patches #2, #3 and #5
> - drop the use of a 32-bit field for the ELF note- QEMU reads a u64 and
> so the top word needs to remain 0x0
> - tweak #ifdefs in patch #4 so the hypercall_page linker symbol does not
> depend on CONFIG_XEN_PV
> - rebase onto v6.12-rc1
>
> Changes wrt [0]:
> - add Jason's Rb to patch #1
> - rebase onto xen/tip's linux-next branch
> - split out fix for GDT descriptor size field
> - add patch to remove the zeroing of phys_base, which is no longer
> needed
> - use a 32-bit field for XEN_ELFNOTE_PHYS32_ENTRY, and use its contents
> to obtain the build time physical address of pvh_startup_xen()
>
> [0] https://lore.kernel.org/all/20240925150059.3955569-30-ardb+git@google.com
> [1] https://lore.kernel.org/all/20240926104113.80146-7-ardb+git@google.com/
> [2] https://lore.kernel.org/all/20240930071513.909462-7-ardb+git@google.com/
>
> Relocation section '.rela.head.text' at offset 0xb428 contains 15 entries:
> Offset Info Type Sym. Value Sym. Name + Addend
> 000000000018 000800000002 R_X86_64_PC32 0000000000000000 .init.data + 18
> 00000000002f 000e00000002 R_X86_64_PC32 0000000000000000 pvh_start_info + 2f
> 000000000037 000f00000002 R_X86_64_PC32 0000000000000000 pvh_start_info_sz + 37
> 000000000042 000800000002 R_X86_64_PC32 0000000000000000 .init.data + 4092
> 000000000060 001000000002 R_X86_64_PC32 000000000000002c xen_elfnote_phys3[...] + 60
> 000000000068 001100000002 R_X86_64_PC32 0000000000000000 phys_base + 68
> 00000000006e 001200000002 R_X86_64_PC32 0000000000005000 pvh_init_top_pgt + 6e
> 000000000089 001300000002 R_X86_64_PC32 0000000000006000 pvh_level3_ident_pgt + 89
> 000000000091 001400000002 R_X86_64_PC32 0000000000008000 pvh_level3_kernel_pgt + 91
> 0000000000a3 001500000002 R_X86_64_PC32 0000000000009000 pvh_level2_kernel_pgt + a3
> 0000000000be 001200000002 R_X86_64_PC32 0000000000005000 pvh_init_top_pgt + be
> 0000000000de 000800000002 R_X86_64_PC32 0000000000000000 .init.data + 1c
> 0000000000e9 001600000002 R_X86_64_PC32 0000000000000000 xen_prepare_pvh - 4
> 0000000000f8 001700000002 R_X86_64_PC32 0000000000000000 pvh_bootparams - 4
> 0000000000fd 001800000004 R_X86_64_PLT32 0000000000000000 startup_64 - 4
>
> Relocation section '.rela.note.Xen' at offset 0xb668 contains 1 entry:
> Offset Info Type Sym. Value Sym. Name + Addend
> 00000000002c 001a00000002 R_X86_64_PC32 0000000000000000 xen_elfnote_phys3[...] + 0
>
> Cc: Jason Andryuk <jason.andryuk@amd.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: x86@kernel.org
> Cc: xen-devel@lists.xenproject.org
>

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