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    SubjectRe: [PATCH v4 0/7] x86/boot: Remove runtime relocations from compressed kernel
    On Tue, Jul 14, 2020 at 4:38 AM Arvind Sankar <nivedita@alum.mit.edu> 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@google.com/
    >

    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@alum.mit.edu/
    >
    > 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
    >

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