The x86 signal frame is designed to be self-describing. The xstate_size
field in the software-reserved bytes indicates the actual size of the
xstate context and is used by the kernel to locate the FP_XSTATE_MAGIC2
marker during signal return.

This design is required to provide portability of signal frames across
different machines. For example, a process checkpointed on a system with
fewer xstate features and restored on a system with more features will
have a signal frame on its stack that is smaller than the destination
host's default. By relying on the frame's internal xstate_size, the
kernel can correctly validate and restore such frames.

This series addresses a regression introduced in commit dc8aa31a7ac2
("x86/fpu: Refine and simplify the magic number check during signal
return").

Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Chang S. Bae" <chang.seok.bae@intel.com>

Andrei Vagin (5):
  Revert "x86/fpu: Refine and simplify the magic number check during
    signal return"
  x86/fpu: Document signal frame portability
  selftests/x86: Add a test for signal frame portability
  x86/fpu: Add consistency check between xstate_size and xfeatures
  selftests/x86: Add a consistency test for signal frames

 Documentation/arch/x86/xstate.rst             |  14 +-
 arch/x86/include/uapi/asm/sigcontext.h        |  13 +-
 arch/x86/kernel/fpu/signal.c                  |  36 +++-
 arch/x86/kernel/fpu/xstate.c                  |   2 +-
 arch/x86/kernel/fpu/xstate.h                  |   2 +
 tools/testing/selftests/x86/Makefile          |   5 +-
 .../selftests/x86/sigframe_portability.c      | 189 ++++++++++++++++++
 tools/testing/selftests/x86/xstate.c          |   5 -
 tools/testing/selftests/x86/xstate.h          |  12 ++
 9 files changed, 260 insertions(+), 18 deletions(-)
 create mode 100644 tools/testing/selftests/x86/sigframe_portability.c

-- 
2.54.0.746.g67dd491aae-goog