Messages in this thread | | | From | Sedat Dilek <> | Date | Tue, 14 Jul 2020 15:15:49 +0200 | Subject | Re: [PATCH v4 0/7] x86/boot: Remove runtime relocations from compressed kernel |
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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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