Replace nop instructions with ud2 (#84)

facebookexperimental · Feb 28, 2023 · 9e1a9ed · 9e1a9ed
1 parent e3ff13f
commit 9e1a9ed
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Showing 3 changed files with 8 additions and 12 deletions.
diff --git a/src/OIDebugger.cpp b/src/OIDebugger.cpp
@@ -2113,7 +2113,6 @@ bool OIDebugger::writePrologue(
     const prequest &preq, const OICompiler::RelocResult::SymTable &jitSymbols) {
   size_t off = 0;
   uint8_t newInsts[prologueLength];
-  memset(newInsts, nopInst /* NOP */, sizeof(newInsts));
 
   /*
    * Global probes don't have multiple arguments, but calling `getReqForArg(X)`
@@ -2136,17 +2135,6 @@ bool OIDebugger::writePrologue(
     VLOG(1) << "Generating prologue for argument '" << req.arg
             << "', using probe at " << (void *)jitCodeStart->second;
 
-    /*
-     * With the move to an INT3 to regain control of the target thread I'm
-     * not convinced that we actually need to do any of this now. We may be
-     * able to simply tack an INT3 on to the end of the JIT'd code sequence
-     * (obviously we wouldn't ever execute the 'ret' there but that doesn't
-     * really matter).
-     */
-    /*
-     * movabs is really a synthetic for a REX prefixed mov instruction.
-     * The REX prefix opcode is 0x48 (REX.W == 1).
-     */
     newInsts[off++] = movabsrdi0Inst;
     newInsts[off++] = movabsrdi1Inst;
     remoteObjAddrs.emplace(std::move(jitCodeStart->first),
@@ -2177,6 +2165,11 @@ bool OIDebugger::writePrologue(
 
   newInsts[off++] = int3Inst;
 
+  while (off <= prologueLength - sizeofUd2) {
+    newInsts[off++] = ud2Inst0;
+    newInsts[off++] = ud2Inst1;
+  }
+
   assert(off <= prologueLength);
 
   return writeTargetMemory(&newInsts, (void *)segConfig.textSegBase,

diff --git a/src/OIDebugger.h b/src/OIDebugger.h
@@ -160,6 +160,7 @@ class OIDebugger {
   uint64_t count{};
   bool sigIntHandlerActive{false};
   const int sizeofInt3 = 1;
+  const int sizeofUd2 = 2;
   const int replayInstSize = 512;
   bool trapsRemoved{false};
   std::shared_ptr<SymbolService> symbols;

diff --git a/src/X86InstDefs.h b/src/X86InstDefs.h
@@ -24,3 +24,5 @@ static constexpr uint8_t movabsrax1Inst = 0xb8;
 static constexpr uint8_t callRaxInst0Inst = 0xff;
 static constexpr uint8_t callRaxInst1Inst = 0xd0;
 static constexpr long syscallInsts = 0x9090909090050fcc;
+static constexpr uint8_t ud2Inst0 = 0x0f;
+static constexpr uint8_t ud2Inst1 = 0x0b;