Commit 22467e25 authored by Fangrui Song's avatar Fangrui Song
Browse files

Add function attribute "patchable-function-prefix" to support... 

Add function attribute "patchable-function-prefix" to support -fpatchable-function-entry=N,M where M>0

Similar to the function attribute `prefix` (prefix data),
"patchable-function-prefix" inserts data (M NOPs) before the function
entry label.

-fpatchable-function-entry=2,1 (1 NOP before entry, 1 NOP after entry)
will look like:

```
  .type	foo,@function
.Ltmp0:               # @foo
  nop
foo:
.Lfunc_begin0:
  # optional `bti c` (AArch64 Branch Target Identification) or
  # `endbr64` (Intel Indirect Branch Tracking)
  nop

  .section  __patchable_function_entries,"awo",@progbits,get,unique,0
  .p2align  3
  .quad .Ltmp0
```

-fpatchable-function-entry=N,0 + -mbranch-protection=bti/-fcf-protection=branch has two reasonable
placements (https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01185.html):

```
(a)         (b)

func:       func:
.Ltmp0:     bti c
  bti c     .Ltmp0:
  nop       nop
```

(a) needs no additional code. If the consensus is to go for (b), we will
need more code in AArch64BranchTargets.cpp / X86IndirectBranchTracking.cpp .

Differential Revision: https://reviews.llvm.org/D73070
parent f394d22f

llvm/include/llvm/CodeGen/AsmPrinter.h

+6 −0
Original line number Diff line number Diff line
@@ -113,6 +113,9 @@ public: 


  ProfileSummaryInfo *PSI;



  /// The symbol for the entry in __patchable_function_entires.

  MCSymbol *CurrentPatchableFunctionEntrySym = nullptr;



  /// The symbol for the current function. This is recalculated at the beginning

  /// of each call to runOnMachineFunction().

  MCSymbol *CurrentFnSym = nullptr;
@@ -449,6 +452,9 @@ public: 
  /// instructions in verbose mode.

  virtual void emitImplicitDef(const MachineInstr *MI) const;



  /// Emit N NOP instructions.

  void emitNops(unsigned N);



  //===------------------------------------------------------------------===//

  // Symbol Lowering Routines.

  //===------------------------------------------------------------------===//

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

+29 −4
Original line number Diff line number Diff line
@@ -706,6 +706,21 @@ void AsmPrinter::EmitFunctionHeader() { 
    }

  }



  // Emit M NOPs for -fpatchable-function-entry=N,M where M>0. We arbitrarily

  // place prefix data before NOPs.

  unsigned PatchableFunctionPrefix = 0;

  (void)F.getFnAttribute("patchable-function-prefix")

      .getValueAsString()

      .getAsInteger(10, PatchableFunctionPrefix);

  if (PatchableFunctionPrefix) {

    CurrentPatchableFunctionEntrySym =

        OutContext.createLinkerPrivateTempSymbol();

    OutStreamer->EmitLabel(CurrentPatchableFunctionEntrySym);

    emitNops(PatchableFunctionPrefix);

  } else {

    CurrentPatchableFunctionEntrySym = CurrentFnBegin;

  }



  // Emit the function descriptor. This is a virtual function to allow targets

  // to emit their specific function descriptor.

  if (MAI->needsFunctionDescriptors())
@@ -1157,7 +1172,7 @@ void AsmPrinter::EmitFunctionBody() { 
    // unspecified.

    if (Noop.getOpcode()) {

      OutStreamer->AddComment("avoids zero-length function");

      OutStreamer->EmitInstruction(Noop, getSubtargetInfo());

      emitNops(1);

    }

  }
@@ -2787,6 +2802,13 @@ void AsmPrinter::printOffset(int64_t Offset, raw_ostream &OS) const { 
    OS << Offset;

}



void AsmPrinter::emitNops(unsigned N) {

  MCInst Nop;

  MF->getSubtarget().getInstrInfo()->getNoop(Nop);

  for (; N; --N)

    EmitToStreamer(*OutStreamer, Nop);

}



//===----------------------------------------------------------------------===//

// Symbol Lowering Routines.

//===----------------------------------------------------------------------===//
@@ -3189,11 +3211,14 @@ void AsmPrinter::recordSled(MCSymbol *Sled, const MachineInstr &MI, 


void AsmPrinter::emitPatchableFunctionEntries() {

  const Function &F = MF->getFunction();

  unsigned PatchableFunctionEntry = 0;

  unsigned PatchableFunctionPrefix = 0, PatchableFunctionEntry = 0;

  (void)F.getFnAttribute("patchable-function-prefix")

      .getValueAsString()

      .getAsInteger(10, PatchableFunctionPrefix);

  (void)F.getFnAttribute("patchable-function-entry")

      .getValueAsString()

      .getAsInteger(10, PatchableFunctionEntry);

  if (!PatchableFunctionEntry)

  if (!PatchableFunctionPrefix && !PatchableFunctionEntry)

    return;

  const unsigned PointerSize = getPointerSize();

  if (TM.getTargetTriple().isOSBinFormatELF()) {
@@ -3222,7 +3247,7 @@ void AsmPrinter::emitPatchableFunctionEntries() { 
          "__patchable_function_entries", ELF::SHT_PROGBITS, Flags));

    }

    EmitAlignment(Align(PointerSize));

    OutStreamer->EmitSymbolValue(CurrentFnBegin, PointerSize);

    OutStreamer->EmitSymbolValue(CurrentPatchableFunctionEntrySym, PointerSize);

  }

}

llvm/lib/IR/Verifier.cpp

+15 −6
Original line number Diff line number Diff line
@@ -1852,16 +1852,25 @@ void Verifier::verifyFunctionAttrs(FunctionType *FT, AttributeList Attrs, 
      CheckFailed("invalid value for 'frame-pointer' attribute: " + FP, V);

  }



  if (Attrs.hasFnAttribute("patchable-function-prefix")) {

    StringRef S = Attrs

                      .getAttribute(AttributeList::FunctionIndex,

                                    "patchable-function-prefix")

                      .getValueAsString();

    unsigned N;

    if (S.getAsInteger(10, N))

      CheckFailed(

          "\"patchable-function-prefix\" takes an unsigned integer: " + S, V);

  }

  if (Attrs.hasFnAttribute("patchable-function-entry")) {

    StringRef S0 = Attrs

    StringRef S = Attrs

                      .getAttribute(AttributeList::FunctionIndex,

                                    "patchable-function-entry")

                      .getValueAsString();

    StringRef S = S0;

    unsigned N;

    if (S.getAsInteger(10, N))

      CheckFailed(

          "\"patchable-function-entry\" takes an unsigned integer: " + S0, V);

          "\"patchable-function-entry\" takes an unsigned integer: " + S, V);

  }

}

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

+1 −2
Original line number Diff line number Diff line
@@ -250,8 +250,7 @@ void AArch64AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI) 
            .getValueAsString()

            .getAsInteger(10, Num))

      return;

    for (; Num; --Num)

      EmitToStreamer(*OutStreamer, MCInstBuilder(AArch64::HINT).addImm(0));

    emitNops(Num);

    return;

  }

llvm/lib/Target/ARM/ARMMCInstLower.cpp

+1 −4
Original line number Diff line number Diff line
@@ -207,10 +207,7 @@ void ARMAsmPrinter::EmitSled(const MachineInstr &MI, SledKind Kind) 
  EmitToStreamer(*OutStreamer, MCInstBuilder(ARM::Bcc).addImm(20)

    .addImm(ARMCC::AL).addReg(0));



  MCInst Noop;

  Subtarget->getInstrInfo()->getNoop(Noop);

  for (int8_t I = 0; I < NoopsInSledCount; I++)

    OutStreamer->EmitInstruction(Noop, getSubtargetInfo());

  emitNops(NoopsInSledCount);



  OutStreamer->EmitLabel(Target);

  recordSled(CurSled, MI, Kind);