[llvm] [LFI][X86] Add control-flow rewrites (PR #210913)

[Zachary Yedidia via llvm-commits]

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@@ -81,43 +113,158 @@ void X86::X86MCLFIRewriter::rewriteSyscall(const MCInst &Inst, MCStreamer &Out,
   MCSymbol *Symbol = Out.getContext().createTempSymbol();
 
   // leaq .Ltmp(%rip), %r11
-  MCInst Lea;
-  Lea.setOpcode(X86::LEA64r);
-  Lea.addOperand(MCOperand::createReg(LFIScratchReg));
-  Lea.addOperand(MCOperand::createReg(X86::RIP));
-  Lea.addOperand(MCOperand::createImm(1));
-  Lea.addOperand(MCOperand::createReg(X86::NoRegister));
-  Lea.addOperand(
-      MCOperand::createExpr(MCSymbolRefExpr::create(Symbol, Out.getContext())));
-  Lea.addOperand(MCOperand::createReg(X86::NoRegister));
-  Out.emitInstruction(Lea, STI);
-
-  // jmpq *(%r14)
-  MCInst Jmp;
-  Jmp.setOpcode(X86::JMP64m);
-  Jmp.addOperand(MCOperand::createReg(LFIBaseReg));
-  Jmp.addOperand(MCOperand::createImm(1));
-  Jmp.addOperand(MCOperand::createReg(X86::NoRegister));
-  Jmp.addOperand(MCOperand::createImm(-8));
-  Jmp.addOperand(MCOperand::createReg(X86::NoRegister));
-  Out.emitInstruction(Jmp, STI);
+  Out.emitInstruction(
+      MCInstBuilder(X86::LEA64r)
+          .addReg(LFIScratchReg)
+          .addReg(X86::RIP)
+          .addImm(1)
+          .addReg(X86::NoRegister)
+          .addExpr(MCSymbolRefExpr::create(Symbol, Out.getContext()))
+          .addReg(X86::NoRegister),
+      STI);
+
+  // jmpq *-8(%r14)
+  Out.emitInstruction(MCInstBuilder(X86::JMP64m)
+                          .addReg(LFIBaseReg)
+                          .addImm(1)
+                          .addReg(X86::NoRegister)
+                          .addImm(-8)
+                          .addReg(X86::NoRegister),
+                      STI);
 
   Out.emitLabel(Symbol);
   Out.emitBundleUnlock(STI);
 }
 
+// andl $-LFIBundleSize, %eX
+// addq %r14, %rX
+void X86::X86MCLFIRewriter::emitSandboxBranchReg(MCRegister Reg,
+                                                 MCStreamer &Out,
+                                                 const MCSubtargetInfo &STI) {
+  MCRegister Reg32 = RegInfo->getSubReg(Reg, X86::sub_32bit);
+
+  Out.emitInstruction(MCInstBuilder(X86::AND32ri8)
+                          .addReg(Reg32)
+                          .addReg(Reg32)
+                          .addImm(-static_cast<int64_t>(LFIBundleSize)),
----------------
zyedidia wrote:

The cast is for the textual output rather than the encoding. `LFIBundleSize` is unsigned, so `-LFIBundleSize` is emitted as 4294967264 which I think is more confusing. We could switch `LFIBundleSize` to a signed type though if that is preferred.

https://github.com/llvm/llvm-project/pull/210913