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

[Fangrui Song]

On 2020-05-24, Arvind Sankar 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.
>
> 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@xxxxxxxxxxxx/
>
> 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

All 4 commits look good.

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>