Re: [PATCH v5 0/7] x86/boot: Remove run-time relocations from compressed kernel

Sedat Dilek
2020-07-15T07:11:21Z

On Wed, Jul 15, 2020 at 3:46 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> 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 -
>

Re-tested v5 of your patchset, feel free to add appropriate credits.

- 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 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
> >