Improvements to X86FixupGadgets pass of clang(1)

Contributed by rueda on 2019-02-23 from the all your returns are belong to us dept.

Todd Mortimer (mortimer@) has committed improvements to (the anti-ROP) "X86FixupGadgets" pass of clang(1) for amd64 and i386:

 CVSROOT:	/cvs
Module name:	src
Changes by:	mortimer@cvs.openbsd.org	2019/02/22 08:28:43

Modified files:
	gnu/llvm/lib/Target/X86: X86FixupGadgets.cpp X86InstrCompiler.td 
	                         X86MCInstLower.cpp 
	gnu/llvm/tools/clang/include/clang/Driver: Options.td 
	gnu/llvm/tools/clang/lib/Driver/ToolChains: Clang.cpp 
	share/man/man1 : clang-local.1 

Log message:
Improve the X86FixupGadgets pass:
- Target all four kinds of return bytes (c2, c3, ca, cb)
- Fix up instructions using both ModR/M and SIB bytes
- Force alignment before instructions with return bytes in immediates
- Force alignment before instructions that have return bytes in their encoding
- Add a command line switch to toggle the functionality.

ok deraadt@

This extends the previous work to cover even more cases which (previously potentially) could be exploited as return instructions.

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