Re: [PATCH v4 0/7] x86/boot: Remove runtime relocations from compressed kernel

[Sedat Dilek]

On Tue, Jul 14, 2020 at 4:38 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> The compressed kernel currently contains bogus runtime relocations in
> the startup code in head_{32,64}.S, which are generated by the linker,
> but must not actually be processed at runtime.
>
> This generates warnings when linking with the BFD linker, and errors
> with LLD, which defaults to erroring on runtime relocations in read-only
> sections. It also requires the -z noreloc-overflow hack for the 64-bit
> kernel, which prevents us from linking it as -pie on an older BFD linker
> (<= 2.26) or on LLD, because the locations that are to be apparently
> relocated are only 32-bits in size and so cannot really have
> R_X86_64_RELATIVE relocations.
>
> This series aims to get rid of these relocations. I've build- and
> boot-tested with combinations of clang/gcc-10 with lld/bfd-2.34, and
> gcc-4.9.0 with bfd-2.24, skipping clang on 32-bit because it currently
> has other issues [0].
>
> The first three patches by Ard remove indirection via the GOT from the
> compressed kernel code.
>
> The next patch is an independent fix for LLD, to avoid an orphan
> section in arch/x86/boot/setup.elf.
>
> The fifth patch gets rid of almost all the relocations. It uses
> standard PIC addressing technique for 32-bit, i.e. loading a register
> with the address of _GLOBAL_OFFSET_TABLE_ and then using GOTOFF
> references to access variables. For 64-bit, there is 32-bit code that
> cannot use RIP-relative addressing, and also cannot use the 32-bit
> method, since GOTOFF references are 64-bit only. This is instead handled
> using a macro to replace a reference like gdt with (gdt-startup_32)
> instead. The assembler will generate a PC32 relocation entry, with
> addend set to (.-startup_32), and these will be replaced with constants
> at link time. This works as long as all the code using such references
> lives in the same section as startup_32, i.e. in .head.text.
>
> The sixth patch addresses a remaining issue with the BFD linker, which
> generates runtime relocations for absolute symbols. We use z_input_len
> and z_output_len, defined in the generated piggy.S file, as symbols
> whose absolute "addresses" are actually the size of the compressed
> payload and the size of the decompressed kernel image respectively. LLD
> does not generate relocations for these two symbols, but the BFD linker
> does, prior to the upcoming 2.35. To get around this, piggy.S is
> extended to also define two u32 variables (in .rodata) with the lengths,
> and the head code is modified to use those instead of the symbol
> addresses.
>
> An alternative way to handle z_input_len/z_output_len would be to just
> include piggy.S in head_{32,64}.S instead of as a separate object file,
> since the GNU assembler doesn't generate relocations for symbols set to
> constants.
>
> The last patch adds a check in the linker script to ensure that no
> runtime relocations get reintroduced.
>
> [0] https://lore.kernel.org/lkml/20200504230309.237398-1-ndesaulniers@xxxxxxxxxx/
>

How to test this series without building a full new kernel?

make $make_opts vmlinux

- Sedat -

> Changes from v3:
> - Move hidden.h to include/linux so the EFI stub and the compressed
>   kernel can share the same file
>
> Changes from v2:
> - Incorporate Ard's patches for eliminating GOT references into this
>   series
> - Rebase on v5.8-rc3
>
> v2: https://lore.kernel.org/lkml/20200525225918.1624470-1-nivedita@xxxxxxxxxxxx/
>
> Changes from v1:
> - Add .text.* to setup.ld instead of just .text.startup
> - Rename the la() macro introduced in the second patch for 64-bit to
>   rva(), and rework the explanatory comment.
> - In the last patch, check both .rel.dyn and .rela.dyn, instead of just
>   one per arch.
>
>
> Ard Biesheuvel (3):
>   x86/boot/compressed: Move .got.plt entries out of the .got section
>   x86/boot/compressed: Force hidden visibility for all symbol references
>   x86/boot/compressed: Get rid of GOT fixup code
>
> Arvind Sankar (4):
>   x86/boot: Add .text.* to setup.ld
>   x86/boot: Remove run-time relocations from .head.text code
>   x86/boot: Remove runtime relocations from head_{32,64}.S
>   x86/boot: Check that there are no runtime relocations
>
>  arch/x86/boot/compressed/Makefile      |  37 +-----
>  arch/x86/boot/compressed/head_32.S     |  99 +++++----------
>  arch/x86/boot/compressed/head_64.S     | 165 ++++++++++---------------
>  arch/x86/boot/compressed/mkpiggy.c     |   6 +
>  arch/x86/boot/compressed/vmlinux.lds.S |  24 +++-
>  arch/x86/boot/setup.ld                 |   2 +-
>  drivers/firmware/efi/libstub/Makefile  |   2 +-
>  drivers/firmware/efi/libstub/hidden.h  |   6 -
>  include/linux/hidden.h                 |  19 +++
>  9 files changed, 152 insertions(+), 208 deletions(-)
>  delete mode 100644 drivers/firmware/efi/libstub/hidden.h
>  create mode 100644 include/linux/hidden.h
>
>
> base-commit: 11ba468877bb23f28956a35e896356252d63c983
> --
> 2.26.2
>