[PATCH] Add Forward-Edge Control-Flow Integrity support

[Tom Roeder]

================
Comment at: lib/CodeGen/ForwardControlFlowIntegrity.cpp:198
@@ +197,3 @@
+    // really get this information from the Target in some way.
+    int64_t MaskValue = ((TableSize << 3) - 1) & -8;
+    Constant *JumpTableMaskValue = ConstantInt::get(Int64Ty, MaskValue);
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JF Bastien wrote:
> Tom Roeder wrote:
> > JF Bastien wrote:
> > > Could you fix this?
> > I'd like to fix this, but I don't know where I can get this information in a principled manner. This requires the size of a jumptable entry, and that depends on the size of the jump instruction in bytes. In principle, this is available from MCInstrDesc::getSize(), but that returns 0 when I've tried it for JMP_4 on X86.
> > 
> > I can certainly encode it myself in the Targets, but that seems like dangerous duplication of functionality with the descriptions in the instruction tables.
> Oh I see.
> 
> It's not just a single instruction on each target though? If so then the content of jumptable entries does seem pretty target specific, so it kind of sounds like something that would belong in the target. Size is kind of icky though, it would be nice to factor out the target's instruction pattern from their actual encoding.
Well, the instruction itself is already generated in the Target; in the jumptable patch, I added a pair of virtual methods to include/llvm/Target/TargetInstrInfo.h and instantiated it for X86 and ARM in their Target directories. 

It does happen to be a single instruction on each architecture I've tried so far (X86, ARM, PPC), since it's just an unconditional branch to the target function. Maybe I should just add another method to TargetInstrInfo? But like I said, I don't know any clean way to get this from the encoding...I hope that's just my ignorance of the details of how Targets manage their instruction encodings: surely there must be a way to get the upper bound of the length of an unconditional jump instruction?

http://reviews.llvm.org/D4167