[lkml]   [2020]   [May]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH v2 0/4] x86/boot: Remove runtime relocations from compressed kernel
On Tue, May 26, 2020 at 12:59 AM 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, 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.


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?

- Sedat -


> 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/ | 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
> To view this discussion on the web visit

 \ /
  Last update: 2020-05-26 14:30    [W:0.174 / U:1.616 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site