dwarf/frame: handle unknown opcodes more gracefully (#4037)

Instead of panic'ing on unknown opcodes (there are a few around which
gdb handles) report an error and continue as if the FDE did not exist.

Fixes #4035

pkg/dwarf/frame/entries.go

@@ -55,8 +55,12 @@ func (fde *FrameDescriptionEntry) Translate(delta uint64) {
 }
 
 // EstablishFrame set up frame for the given PC.
-func (fde *FrameDescriptionEntry) EstablishFrame(pc uint64) *FrameContext {
-	return executeDwarfProgramUntilPC(fde, pc)
+func (fde *FrameDescriptionEntry) EstablishFrame(pc uint64) (*FrameContext, error) {
+	fctxt := executeDwarfProgramUntilPC(fde, pc)
+	if fctxt.err != nil {
+		return nil, fctxt.err
+	}
+	return fctxt, nil
 }
 
 type FrameDescriptionEntries []*FrameDescriptionEntry

pkg/dwarf/frame/table.go

@@ -30,6 +30,7 @@ type FrameContext struct {
 	codeAlignment   uint64
 	dataAlignment   int64
 	rememberedState *stateStack
+	err             error
 }
 
 type rowState struct {
@@ -179,7 +180,7 @@ func (frame *FrameContext) executeDwarfProgram() {
 }
 
 // ExecuteUntilPC execute dwarf instructions.
-func (frame *FrameContext) ExecuteUntilPC(instructions []byte) {
+func (frame *FrameContext) ExecuteUntilPC(instructions []byte) error {
 	frame.buf.Truncate(0)
 	frame.buf.Write(instructions)
 
@@ -189,9 +190,13 @@ func (frame *FrameContext) ExecuteUntilPC(instructions []byte) {
 	for frame.address >= frame.loc && frame.buf.Len() > 0 {
 		executeDwarfInstruction(frame)
 	}
+	return frame.err
 }
 
 func executeDwarfInstruction(frame *FrameContext) {
+	if frame.err != nil {
+		return
+	}
 	instruction, err := frame.buf.ReadByte()
 	if err != nil {
 		panic("Could not read from instruction buffer")
@@ -203,6 +208,11 @@ func executeDwarfInstruction(frame *FrameContext) {
 
 	fn := lookupFunc(instruction, frame.buf)
 
+	if fn == nil {
+		frame.err = fmt.Errorf("encountered an unexpected DWARF CFA opcode: %#v", instruction)
+		return
+	}
+
 	fn(frame)
 }
 
@@ -233,12 +243,7 @@ func lookupFunc(instruction byte, buf *bytes.Buffer) instruction {
 		}
 	}
 
-	fn, ok := fnlookup[instruction]
-	if !ok {
-		panic(fmt.Sprintf("Encountered an unexpected DWARF CFA opcode: %#v", instruction))
-	}
-
-	return fn
+	return fnlookup[instruction]
 }
 
 func advanceloc(frame *FrameContext) {

pkg/proc/stack.go

@@ -508,16 +508,20 @@ func (it *stackIterator) appendInlineCalls(callback func(Stackframe) bool, frame
 // it.regs.CFA; the caller has to eventually switch it.regs when the iterator
 // advances to the next frame.
 func (it *stackIterator) advanceRegs() (callFrameRegs op.DwarfRegisters, ret uint64, retaddr uint64) {
+	logger := logflags.StackLogger()
+
 	fde, err := it.bi.frameEntries.FDEForPC(it.pc)
 	var framectx *frame.FrameContext
 	if _, nofde := err.(*frame.ErrNoFDEForPC); nofde {
 		framectx = it.bi.Arch.fixFrameUnwindContext(nil, it.pc, it.bi)
 	} else {
-		framectx = it.bi.Arch.fixFrameUnwindContext(fde.EstablishFrame(it.pc), it.pc, it.bi)
+		fctxt, err := fde.EstablishFrame(it.pc)
+		if err != nil {
+			logger.Errorf("Error executing Frame Debug Entry for PC %x: %v", it.pc, err)
+		}
+		framectx = it.bi.Arch.fixFrameUnwindContext(fctxt, it.pc, it.bi)
 	}
 
-	logger := logflags.StackLogger()
-
 	logger.Debugf("advanceRegs at %#x", it.pc)
 
 	cfareg, err := it.executeFrameRegRule(0, framectx.CFA, 0)