bpf: check illegal usage of XADD insn return value

Currently, BPF has XADD (locked add) insn support and the
asm looks like:
  lock *(u32 *)(r1 + 0) += r2
  lock *(u64 *)(r1 + 0) += r2
The instruction itself does not have a return value.

At the source code level, users often use
  __sync_fetch_and_add()
which eventually translates to XADD. The return value of
__sync_fetch_and_add() is supposed to be the old value
in the xadd memory location. Since BPF::XADD insn does not
support such a return value, this patch added a PreEmit
phase to check such a usage. If such an illegal usage
pattern is detected, a fatal error will be reported like
  line 4: Invalid usage of the XADD return value
if compiled with -g, or
  Invalid usage of the XADD return value
if compiled without -g.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 342692

4 files changed, 100 insertions, 0 deletions

diff --git a/llvm/lib/Target/BPF/BPF.h b/llvm/lib/Target/BPF/BPF.h
index 76d3e1ca5f6..9749e369c2c 100644
--- a/llvm/lib/Target/BPF/BPF.h
+++ b/llvm/lib/Target/BPF/BPF.h
@@ -19,9 +19,11 @@ class BPFTargetMachine;
 FunctionPass *createBPFISelDag(BPFTargetMachine &TM);
 FunctionPass *createBPFMIPeepholePass();
 FunctionPass *createBPFMIPreEmitPeepholePass();
+FunctionPass *createBPFMIPreEmitCheckingPass();
 
 void initializeBPFMIPeepholePass(PassRegistry&);
 void initializeBPFMIPreEmitPeepholePass(PassRegistry&);
+void initializeBPFMIPreEmitCheckingPass(PassRegistry&);
 }
 
 #endif
diff --git a/llvm/lib/Target/BPF/BPFMIChecking.cpp b/llvm/lib/Target/BPF/BPFMIChecking.cpp
new file mode 100644
index 00000000000..0a311378e77
--- /dev/null
+++ b/llvm/lib/Target/BPF/BPFMIChecking.cpp
@@ -0,0 +1,96 @@
+//===-------------- BPFMIChecking.cpp - MI Checking Legality -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs checking to signal errors for certain illegal usages at
+// MachineInstruction layer. Specially, the result of XADD{32,64} insn should
+// not be used. The pass is done at the PreEmit pass right before the
+// machine code is emitted at which point the register liveness information
+// is still available.
+//
+//===----------------------------------------------------------------------===//
+
+#include "BPF.h"
+#include "BPFInstrInfo.h"
+#include "BPFTargetMachine.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "bpf-mi-checking"
+
+namespace {
+
+struct BPFMIPreEmitChecking : public MachineFunctionPass {
+
+  static char ID;
+  MachineFunction *MF;
+  const TargetRegisterInfo *TRI;
+
+  BPFMIPreEmitChecking() : MachineFunctionPass(ID) {
+    initializeBPFMIPreEmitCheckingPass(*PassRegistry::getPassRegistry());
+  }
+
+private:
+  // Initialize class variables.
+  void initialize(MachineFunction &MFParm);
+
+  void checkingIllegalXADD(void);
+
+public:
+
+  // Main entry point for this pass.
+  bool runOnMachineFunction(MachineFunction &MF) override {
+    if (!skipFunction(MF.getFunction())) {
+      initialize(MF);
+      checkingIllegalXADD();
+    }
+    return false;
+  }
+};
+
+// Initialize class variables.
+void BPFMIPreEmitChecking::initialize(MachineFunction &MFParm) {
+  MF = &MFParm;
+  TRI = MF->getSubtarget<BPFSubtarget>().getRegisterInfo();
+  LLVM_DEBUG(dbgs() << "*** BPF PreEmit checking pass ***\n\n");
+}
+
+void BPFMIPreEmitChecking::checkingIllegalXADD(void) {
+  for (MachineBasicBlock &MBB : *MF) {
+    for (MachineInstr &MI : MBB) {
+      if (MI.getOpcode() != BPF::XADD32 && MI.getOpcode() != BPF::XADD64)
+        continue;
+
+      LLVM_DEBUG(MI.dump());
+      if (!MI.allDefsAreDead()) {
+        DebugLoc Empty;
+        const DebugLoc &DL = MI.getDebugLoc();
+        if (DL != Empty)
+          report_fatal_error("line " + std::to_string(DL.getLine()) +
+                             ": Invalid usage of the XADD return value", false);
+        else
+          report_fatal_error("Invalid usage of the XADD return value", false);
+      }
+    }
+  }
+
+  return;
+}
+
+} // end default namespace
+
+INITIALIZE_PASS(BPFMIPreEmitChecking, "bpf-mi-pemit-checking",
+                "BPF PreEmit Checking", false, false)
+
+char BPFMIPreEmitChecking::ID = 0;
+FunctionPass* llvm::createBPFMIPreEmitCheckingPass()
+{
+  return new BPFMIPreEmitChecking();
+}
diff --git a/llvm/lib/Target/BPF/BPFTargetMachine.cpp b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
index 84d89bff74f..ffcb1302533 100644
--- a/llvm/lib/Target/BPF/BPFTargetMachine.cpp
+++ b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
@@ -115,6 +115,7 @@ void BPFPassConfig::addMachineSSAOptimization() {
 void BPFPassConfig::addPreEmitPass() {
   const BPFSubtarget *Subtarget = getBPFTargetMachine().getSubtargetImpl();
 
+  addPass(createBPFMIPreEmitCheckingPass());
   if (getOptLevel() != CodeGenOpt::None)
     if (Subtarget->getHasAlu32() && !DisableMIPeephole)
       addPass(createBPFMIPreEmitPeepholePass());
diff --git a/llvm/lib/Target/BPF/CMakeLists.txt b/llvm/lib/Target/BPF/CMakeLists.txt
index ee01b4b7b80..5e2ae533ea6 100644
--- a/llvm/lib/Target/BPF/CMakeLists.txt
+++ b/llvm/lib/Target/BPF/CMakeLists.txt
@@ -24,6 +24,7 @@ add_llvm_target(BPFCodeGen
   BPFSubtarget.cpp
   BPFTargetMachine.cpp
   BPFMIPeephole.cpp
+  BPFMIChecking.cpp
   )
 
 add_subdirectory(AsmParser)