Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 25 May 2020 09:10:23 +0200 | Subject | Re: [PATCH 0/4] x86/boot: Remove runtime relocations from compressed kernel |
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On Sun, 24 May 2020 at 23:28, 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 normally have > R_X86_64_RELATIVE relocations. > > This series aims to get rid of these relocations. It is based on > efi/next (efi-changes-for-v5.8), where the latest patches touch the > head code to eliminate the global offset table. >
Note: I dropped my decompressor linker script changes from that tag, but they are still at the top of the efi/next branch.
Given these changes to go on top, I think it is better to merge all of them separately, and let the x86 maintainers decide how and when. (I can prepare a branch and a separate PR if desired)
For the series (modulo one nit in a separate reply)
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> The first patch is an independent fix for LLD, to avoid an orphan > section in arch/x86/boot/setup.elf [0]. > > The second 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 third patch addresses a remaining issue with the BFD linker, which > insists on generating 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. 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. Since the GOT has been eliminated > as well, the compressed kernel has no runtime relocations whatsoever any > more. > > [0] https://lore.kernel.org/lkml/20200521152459.558081-1-nivedita@alum.mit.edu/ > > Arvind Sankar (4): > x86/boot: Add .text.startup to setup.ld > x86/boot: Remove runtime 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 | 36 +--------- > arch/x86/boot/compressed/head_32.S | 59 +++++++-------- > arch/x86/boot/compressed/head_64.S | 99 +++++++++++++++----------- > arch/x86/boot/compressed/mkpiggy.c | 6 ++ > arch/x86/boot/compressed/vmlinux.lds.S | 11 +++ > arch/x86/boot/setup.ld | 2 +- > 6 files changed, 109 insertions(+), 104 deletions(-) > > -- > 2.26.2 >
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