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

[Jürgen Groß <jgross@xxxxxxxx>]

On 29.10.24 13:50, Ard Biesheuvel wrote:
> On Wed, 9 Oct 2024 at 18:09, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx
> > [1] https://lore.kernel.org/all/20240926104113.80146-7-ardb+git@xxxxxxxxxx/
> > [2] https://lore.kernel.org/all/20240930071513.909462-7-ardb+git@xxxxxxxxxx/
> >
> > 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@xxxxxxx>
> > Cc: Juergen Gross <jgross@xxxxxxxx>
> > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Ping?
I have queued this series for 6.13.


Juergen

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