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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 <> 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]
> 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
index 789d5d14d8b0..d0aafcd8cf6c 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -51,8 +51,8 @@ UBSAN_SANITIZE :=n
# 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.

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'
make -f ./scripts/ obj=arch/x86/boot/compressed
clang-11 -E -Wp,-MMD,arch/x86/boot/compressed/
-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/
llvm-objcopy-11 -R .comment -S vmlinux arch/x86/boot/compressed/vmlinux.bin
arch/x86/tools/relocs vmlinux >
--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/vmlinux.bin.zst >
ld.lld-11 -m elf_x86_64 -pie --no-dynamic-linker -T
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/kaslr.o arch/x86/boot/compressed/kaslr_64.o
arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o
arch/x86/boot/compressed/efi_thunk_64.o -o
llvm-nm-11 arch/x86/boot/compressed/vmlinux | sed -n -e
's/^\([0-9a-fA-F]*\) [a-zA-Z]
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:
> 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/ | 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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