[RFC 0/4] x86: sigcontext SS fixes, take 2

[Andy Lutomirski <luto@kernel.org>]

This is take 2 at fixing x86 64-bit signals wrt SS.  After a lot of
thought, this is not controlled by any flags -- I would much prefer
to avoid opt-in behavior.  Instead, it just tries hard to avoid
triggering the cases that break DOSEMU.

Stas, what do you think?  Could you test this?  It applies on top of
tip:x86/asm.  You can also find it at
https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/sigcontext&id=fd69bc4e6095d6a7cf2a0f03e69bace025505132

With this applied, all of the x86 selftests pass on x86_64.  That
wasn't the case before -- ldt_gdt_64 was broken.

This is a bit risky, and another option would be to do nothing at
all.  Then we'd disable the problematic self-tests (sigh), and
DOSEMU and similar tools will be stuck using gross hacks even on new
kernels.

Andy Lutomirski (4):
  x86/signal/64: Add a comment about sigcontext->fs and gs
  x86/signal/64: Fix SS if needed when delivering a 64-bit signal
  x86/signal/64: Re-add support for SS in the 64-bit signal context
  selftests/x86: Add tests for UC_SIGCONTEXT_SS and UC_STRICT_RESTORE_SS

 arch/x86/include/asm/desc_defs.h        |  23 +++
 arch/x86/include/asm/sigcontext.h       |   2 +-
 arch/x86/include/asm/sighandling.h      |   1 -
 arch/x86/include/uapi/asm/sigcontext.h  |  23 ++-
 arch/x86/include/uapi/asm/ucontext.h    |  41 +++++-
 arch/x86/kernel/signal.c                | 115 ++++++++++++---
 tools/testing/selftests/x86/Makefile    |   4 +-
 tools/testing/selftests/x86/sigreturn.c | 240 ++++++++++++++++++++++++++++----
 8 files changed, 389 insertions(+), 60 deletions(-)

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2.4.3