[RFC][PATCH 00/22] x86: Remove anonymous out-of-line fixups

[Peter Zijlstra]

Hi,

Direct counterpart to the arm64 series from Mark:

  https://lkml.kernel.org/r/20211019160219.5202-1-mark.rutland@xxxxxxx

Since he already put it rather well:

"We recently realised that out-of-line extable fixups cause a number of problems
for backtracing (mattering both for developers and for RELIABLE_STACKTRACE and
LIVEPATCH). Dmitry spotted a confusing backtrace, which we identified was due
to problems with unwinding fixups, as summarized in:

  https://lore.kernel.org/linux-arm-kernel/20210927171812.GB9201@C02TD0UTHF1T.local/

The gist is that while backtracing through a fixup, the fixup gets symbolized
as an offset from the nearest prior symbol (which happens to be
`__entry_tramp_text_end`), and we the backtrace misses the function that was
being fixed up (because the fixup handling adjusts the PC, then the fixup does
a direct branch back to the original function). We can't reliably map from an
arbitrary PC in the fixup text back to the original function.

The way we create fixups is a bit unfortunate: most fixups are generated from
common templates, and only differ in register to be poked and the address to
branch back to, leading to redundant copies of the same logic that must pollute
Since the fixups are all written in assembly, and duplicated for each fixup
site, we can only perform very simple fixups, and can't handle any complex
triage that we might need for some exceptions (e.g. MTE faults)."



So far these patches have only been compile tested on x86_64
(defconfig,allyesconfig) and boot tested in kvm (defconfig) -- realy early
days.

Enjoy..

---
 arch/x86/entry/entry_32.S                  |  28 ++---
 arch/x86/entry/entry_64.S                  |  13 ++-
 arch/x86/entry/vdso/vdso-layout.lds.S      |   1 -
 arch/x86/include/asm/asm.h                 |  27 +++++
 arch/x86/include/asm/extable_fixup_types.h |  44 ++++++--
 arch/x86/include/asm/futex.h               |  28 ++---
 arch/x86/include/asm/msr.h                 |  26 ++---
 arch/x86/include/asm/segment.h             |   9 +-
 arch/x86/include/asm/sgx.h                 |  18 ++++
 arch/x86/include/asm/uaccess.h             |  35 +++---
 arch/x86/include/asm/word-at-a-time.h      |  29 ++---
 arch/x86/include/asm/xen/page.h            |  12 +--
 arch/x86/kernel/cpu/sgx/encls.h            |  36 +------
 arch/x86/kernel/fpu/legacy.h               |   6 +-
 arch/x86/kernel/fpu/xstate.h               |   6 +-
 arch/x86/kernel/ftrace.c                   |   9 +-
 arch/x86/kernel/vmlinux.lds.S              |   1 -
 arch/x86/kvm/emulate.c                     |  14 +--
 arch/x86/kvm/vmx/vmx_ops.h                 |  14 ++-
 arch/x86/lib/checksum_32.S                 |  19 +---
 arch/x86/lib/copy_mc_64.S                  |  12 +--
 arch/x86/lib/copy_user_64.S                |  32 ++----
 arch/x86/lib/mmx_32.c                      |  83 +++++----------
 arch/x86/lib/usercopy_32.c                 |  66 +++++-------
 arch/x86/lib/usercopy_64.c                 |   8 +-
 arch/x86/mm/extable.c                      | 166 ++++++++++++++++++++++++-----
 include/linux/bitfield.h                   |  19 +++-
 27 files changed, 385 insertions(+), 376 deletions(-)