Messages in this thread | | | From | Sedat Dilek <> | Date | Tue, 26 May 2020 14:29:27 +0200 | Subject | Re: [PATCH v2 0/4] x86/boot: Remove runtime relocations from compressed kernel |
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On Tue, May 26, 2020 at 12:59 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 normally have > R_X86_64_RELATIVE relocations. > > This series aims to get rid of these relocations. It is based on > efi/next, where the latest patches touch the head code to eliminate the > global offset table. > > The first patch is an independent fix for LLD, to avoid an orphan > section in arch/x86/boot/setup.elf. > > 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, 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. Since the GOT has been eliminated > as well, the compressed kernel has no runtime relocations whatsoever any > more. > > 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. >
Hi,
I would like to test this patchset v2 on top of Linux v5.7-rc7 together with:
[1] x86/boot: Discard .discard.unreachable for arch/x86/boot/compressed/vmlinux [2] x86/boot: Correct relocation destination on old linkers
I tried to pull efi/next on top of Linux v5.7-rc7 and cleaned up the merge problems, but I am not sure I did it correctly. So, which patches are really relevant from efi/next?
What's your suggestions?
Regards, - Sedat -
[1] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/patch/?id=d6ee6529436a15a0541aff6e1697989ee7dc2c44 [2] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/patch/?id=5214028dd89e49ba27007c3ee475279e584261f0
> 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 | 36 +-------- > arch/x86/boot/compressed/head_32.S | 59 +++++++------- > arch/x86/boot/compressed/head_64.S | 108 +++++++++++++++---------- > arch/x86/boot/compressed/mkpiggy.c | 6 ++ > arch/x86/boot/compressed/vmlinux.lds.S | 8 ++ > arch/x86/boot/setup.ld | 2 +- > 6 files changed, 115 insertions(+), 104 deletions(-) > > -- > 2.26.2 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200525225918.1624470-1-nivedita%40alum.mit.edu.
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