[RFC PATCH 0/9] kexec x86 purgatory cleanup

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb at kernel.org>

The kexec purgatory is built like a kernel module, i.e., a partially
linked ELF object where each section is allocated and placed
individually, and all relocations need to be fixed up, even place
relative ones.

This makes sense for kernel modules, which share the address space with
the core kernel, and contain unresolved references that need to be wired
up to symbols in other modules or the kernel itself.

The purgatory, however, is a fully linked binary without any external
references, or any overlap with the kernel's virtual address space. So
it makes much more sense to create a fully linked ELF executable that
can just be loaded and run anywhere in memory.

The purgatory build on x86 has already switched over to position
independent codegen, which only leaves a handful of absolute references,
which can either be dropped (patch #3) or converted into a RIP-relative
one (patch #4). That leaves a purgatory executable that can run at any
offset in memory with applying any relocations whatsoever.

Some tweaks are needed to deal with the difference between partially
(ET_REL) and fully (ET_DYN/ET_EXEC) linked ELF objects, but with those
in place, a substantial amount of complicated ELF allocation, placement
and patching/relocation code can simply be dropped.

The last patch in the series removes this code from the generic kexec
implementation, but this can only be done once other architectures apply
the same changes proposed here for x86 (powerpc, s390 and riscv all
implement the purgatory using the shared logic)

Link: https://lore.kernel.org/all/CAKwvOd=3Jrzju++=Ve61=ZdeshxUM=K3-bGMNREnGOQgNw=aag@mail.gmail.com/
Link: https://lore.kernel.org/all/20240418201705.3673200-2-ardb+git@google.com/

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Eric Biederman <ebiederm at xmission.com>
Cc: kexec at lists.infradead.org
Cc: Nathan Chancellor <nathan at kernel.org>
Cc: Nick Desaulniers <ndesaulniers at google.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Bill Wendling <morbo at google.com>
Cc: Justin Stitt <justinstitt at google.com>
Cc: Masahiro Yamada <masahiroy at kernel.org>

Ard Biesheuvel (9):
  x86/purgatory: Drop function entry padding from purgatory
  x86/purgatory: Simplify stack handling
  x86/purgatory: Drop pointless GDT switch
  x86/purgatory: Avoid absolute reference to GDT
  x86/purgatory: Simplify GDT and drop data segment
  kexec: Add support for fully linked purgatory executables
  x86/purgatory: Use fully linked PIE ELF executable
  x86/purgatory: Simplify references to regs array
  kexec: Drop support for partially linked purgatory executables

 arch/x86/include/asm/kexec.h       |   8 -
 arch/x86/kernel/kexec-bzimage64.c  |   8 -
 arch/x86/kernel/machine_kexec_64.c | 127 ----------
 arch/x86/purgatory/Makefile        |  17 +-
 arch/x86/purgatory/entry64.S       |  96 ++++----
 arch/x86/purgatory/setup-x86_64.S  |  31 +--
 arch/x86/purgatory/stack.S         |  18 --
 include/asm-generic/purgatory.lds  |  34 +++
 kernel/kexec_file.c                | 255 +++-----------------
 9 files changed, 125 insertions(+), 469 deletions(-)
 delete mode 100644 arch/x86/purgatory/stack.S
 create mode 100644 include/asm-generic/purgatory.lds

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2.44.0.769.g3c40516874-goog