Messages in this thread | | | From | Ard Biesheuvel <> | Date | Tue, 26 May 2020 14:30:47 +0200 | Subject | Re: [PATCH v2 0/4] x86/boot: Remove runtime relocations from compressed kernel |
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On Tue, 26 May 2020 at 14:29, Sedat Dilek <sedat.dilek@gmail.com> wrote: > > 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? >
efi/next is here:
https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git/log/?h=next
You'll need the top 3 patches.
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