Messages in this thread | | | From | Arvind Sankar <> | Subject | [PATCH v5 0/7] x86/boot: Remove run-time relocations from compressed kernel | Date | Tue, 14 Jul 2020 20:41:26 -0400 |
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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
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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