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    SubjectRe: [PATCH v5 0/7] x86/boot: Remove run-time relocations from compressed kernel
    On Wed, Jul 15, 2020 at 2:41 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
    >
    > The compressed kernel currently contains bogus run-time relocations in
    > the startup code in head_{32,64}.S, which are generated by the linker,
    > but must not actually be processed at run-time.
    >
    > This generates warnings when linking with the BFD linker, and errors
    > with LLD, which defaults to erroring on run-time 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 run-time 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
    > run-time relocations get reintroduced.
    >
    > [0] https://lore.kernel.org/lkml/20200504230309.237398-1-ndesaulniers@google.com/
    >
    > Changes from v4:
    > - Move -pie --no-dynamic-linker from KBUILD_LDFLAGS to LDFLAGS_vmlinux
    > Sedat: I'm not clear on whether you tested with the final LDFLAGS,
    > could you confirm: i.e. if you tested with -pie passed to LLD?
    > - Replace runtime -> run-time to be consistent in wording
    >

    Hi Arvind,

    thanks for v5.

    With my diff applied against your patchset *v4*:

    diff --git a/arch/x86/boot/compressed/Makefile
    b/arch/x86/boot/compressed/Makefile
    index 789d5d14d8b0..d0aafcd8cf6c 100644
    --- a/arch/x86/boot/compressed/Makefile
    +++ b/arch/x86/boot/compressed/Makefile
    @@ -51,8 +51,8 @@ UBSAN_SANITIZE :=n
    KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
    # Compressed kernel should be built as PIE since it may be loaded at any
    # address by the bootloader.
    -KBUILD_LDFLAGS += -pie $(call ld-option, --no-dynamic-linker)
    -LDFLAGS_vmlinux := -T
    +LDFLAGS_vmlinux := -pie $(call ld-option, --no-dynamic-linker)
    +LDFLAGS_vmlinux += -T

    hostprogs := mkpiggy
    HOST_EXTRACFLAGS += -I$(srctree)/tools/include
    I was able to build/assemble with LLVM/Clang v11.0.0-git+ffee8040534
    and boot on bare metal.

    Note:
    I have applied some additional patches to be compliant with LLVM_IAS=1
    (Clang's Integrated Assembler) and LLVM=1 means LLVM utilities.
    ( As pointed out zstd-v7. )

    - Sedat -

    P.S.: Check my build-log

    $ grep 'arch/x86/boot/compressed/vmlinux'
    build-log_5.8.0-rc5-3-amd64-llvm11-ias.txt
    make -f ./scripts/Makefile.build obj=arch/x86/boot/compressed
    arch/x86/boot/compressed/vmlinux
    clang-11 -E -Wp,-MMD,arch/x86/boot/compressed/.vmlinux.lds.d
    -nostdinc -isystem /usr/lib/llvm-11/lib/clang/11.0.0/include
    -I./arch/x86/include -I./arch/x86/include/generated -I./include
    -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi
    -I./include/uapi -I./include/generated/uapi -include
    ./include/linux/kconfig.h -D__KERNEL__ -Qunused-arguments -P -Ux86
    -D__ASSEMBLY__ -DLINKER_SCRIPT -o arch/x86/boot/compressed/vmlinux.lds
    arch/x86/boot/compressed/vmlinux.lds.S
    llvm-objcopy-11 -R .comment -S vmlinux arch/x86/boot/compressed/vmlinux.bin
    arch/x86/tools/relocs vmlinux >
    arch/x86/boot/compressed/vmlinux.relocs;arch/x86/tools/relocs
    --abs-relocs vmlinux
    { cat arch/x86/boot/compressed/vmlinux.bin
    arch/x86/boot/compressed/vmlinux.relocs | zstd -22 --ultra; printf
    \114\015\315\001; } > arch/x86/boot/compressed/vmlinux.bin.zst
    arch/x86/boot/compressed/mkpiggy
    arch/x86/boot/compressed/vmlinux.bin.zst >
    arch/x86/boot/compressed/piggy.S
    ld.lld-11 -m elf_x86_64 -pie --no-dynamic-linker -T
    arch/x86/boot/compressed/vmlinux.lds
    arch/x86/boot/compressed/kernel_info.o
    arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o
    arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o
    arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o
    arch/x86/boot/compressed/cpuflags.o
    arch/x86/boot/compressed/early_serial_console.o
    arch/x86/boot/compressed/kaslr.o arch/x86/boot/compressed/kaslr_64.o
    arch/x86/boot/compressed/mem_encrypt.o
    arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o
    drivers/firmware/efi/libstub/lib.a
    arch/x86/boot/compressed/efi_thunk_64.o -o
    arch/x86/boot/compressed/vmlinux
    llvm-nm-11 arch/x86/boot/compressed/vmlinux | sed -n -e
    's/^\([0-9a-fA-F]*\) [a-zA-Z]
    \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|efi32_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|z_.*\)$/#define
    ZO_ 0x/p' > arch/x86/boot/zoffset.h
    llvm-objcopy-11 -O binary -R .note -R .comment -S
    arch/x86/boot/compressed/vmlinux arch/x86/boot/vmlinux.bin
    - EOT -

    > 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 run-time relocations from head_{32,64}.S
    > x86/boot: Check that there are no run-time relocations
    >
    > arch/x86/boot/compressed/Makefile | 39 +-----
    > 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, 153 insertions(+), 209 deletions(-)
    > delete mode 100644 drivers/firmware/efi/libstub/hidden.h
    > create mode 100644 include/linux/hidden.h
    >
    >
    > base-commit: e9919e11e219eaa5e8041b7b1a196839143e9125
    > --
    > 2.26.2
    >

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