Messages in this thread | | | From | Sedat Dilek <> | Date | Wed, 15 Jul 2020 09:11:05 +0200 | Subject | Re: [PATCH v5 0/7] x86/boot: Remove run-time relocations from compressed kernel |
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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 > >
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