[RFC PATCH 0/9] arm64: Allocate .text and .init.text together

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

The arm64 module loader has to deal with a couple of corner cases that
may occur when .init.text is placed out of direct branch range of .text:

- ordinary direct branches from .init.text into .text may require the
  use of a PLT entry (i.e., a trampoline aka veneer), which means not
  only that additional PLT entries need to be allocated for
  cross-section calls, but also that .init.text needs its own PLT
  reservation, as the one in .text will be out of range as well;

- dynamic patching of the ftrace handler into .init.text code needs its
  own dedicated trampoline as the one in .text may be too far away.

- recent compilers may omit BTI veneers for static functions that never
  have their address taken, and so additional veneers will need to be
  added to .text in case cross-section direct branches from .init.text 
  require a PLT entry (and therefore a landing pad at the target end).

This is unfortunate, because it is actually somewhat unusual for .text
and .init.text to be so far away from each other: only when allocating
either of them (but not both) exhausts the 'near' (PLT-less) module
region, the other will be allocated from the spillover region, which is
not in direct branching range, and therefore requires PLT entries for
cross-section calls.

This series addresses this wart by allocating both of them as a single
chunk, and freeing the .init.text part along with the other init
sections at the appropriate time. This ensures that the two regions will
never require veneers for cross-section calls, allowing the arm64 module
loader to be simplified.

Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Petr Pavlu <petr.pavlu@xxxxxxxx>
Cc: Daniel Gomez <da.gomez@xxxxxxxxxx>
Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Cc: Aaron Tomlin <atomlin@xxxxxxxxxxx>
Cc: "Adrian Barnaś" <abarnas@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Kevin Brodsky <kevin.brodsky@xxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-modules@xxxxxxxxxxxxxxx

Ard Biesheuvel (9):
  mm: execmem: Add API to split an existing execmem cache allocation
  mm: execmem: Allow huge vmappings to be avoided for execmem caches
  module: Place MOD_TEXT before MOD_INIT_TEXT in enumeration
  module: Allocate MOD_INIT_TEXT from the MOD_TEXT ROX allocation
  arm64: mm: Permit permissions changes on huge vmappings
  arm64: Enable the execmem ROX cache for module text
  arm64: ftrace: Revert "fix unreachable PLT for ftrace_caller ..."
  arm64: module: Combine init and core PLT entries again
  arm64: ftrace: Simplify PLT handling

 arch/arm64/Kconfig                  |  1 +
 arch/arm64/include/asm/module.h     |  2 -
 arch/arm64/include/asm/module.lds.h |  3 -
 arch/arm64/kernel/ftrace.c          | 13 +---
 arch/arm64/kernel/module-plts.c     | 63 +++++---------------
 arch/arm64/kernel/module.c          | 20 +------
 arch/arm64/mm/init.c                | 19 +++++-
 arch/arm64/mm/pageattr.c            | 13 +++-
 include/linux/execmem.h             | 11 ++++
 include/linux/module.h              |  4 +-
 kernel/module/main.c                | 22 +++++--
 mm/execmem.c                        | 47 ++++++++++++++-
 12 files changed, 124 insertions(+), 94 deletions(-)

-- 
2.55.0.860.g4b6b3295ed-goog