Re: [PATCH v2 0/4] x86/boot: Remove runtime relocations from compressed kernel

[Sedat Dilek <sedat.dilek@gmail.com>]

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
>
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