proc/*: allow stepping into functions without debug_info symbols

If proc.Step encounters a CALL instruction that points to an address
that isn't associated with any function it should still follow the
CALL.

The circumstances creating this problem do not normally occur, it was
encountered in the process of fixing a bug created by Go1.12.

pkg/proc/breakpoints.go

@@ -252,8 +252,13 @@ func (bpmap *BreakpointMap) Set(addr uint64, kind BreakpointKind, cond ast.Expr,
 		return nil, err
 	}
 
+	fnName := ""
+	if fn != nil {
+		fnName = fn.Name
+	}
+
 	newBreakpoint := &Breakpoint{
-		FunctionName: fn.Name,
+		FunctionName: fnName,
 		File:         f,
 		Line:         l,
 		Addr:         addr,

pkg/proc/disasm_amd64.go

@@ -118,7 +118,7 @@ func resolveCallArg(inst *archInst, currentGoroutine bool, regs Registers, mem M
 
 	file, line, fn := bininfo.PCToLine(pc)
 	if fn == nil {
-		return nil
+		return &Location{PC: pc}
 	}
 	return &Location{PC: pc, File: file, Line: line, Fn: fn}
 }

pkg/proc/gdbserial/gdbserver.go

@@ -1028,9 +1028,6 @@ func (p *Process) FindBreakpoint(pc uint64) (*proc.Breakpoint, bool) {
 
 func (p *Process) writeBreakpoint(addr uint64) (string, int, *proc.Function, []byte, error) {
 	f, l, fn := p.bi.PCToLine(uint64(addr))
-	if fn == nil {
-		return "", 0, nil, nil, proc.InvalidAddressError{Address: addr}
-	}
 
 	if err := p.conn.setBreakpoint(addr); err != nil {
 		return "", 0, nil, nil, err

pkg/proc/native/proc.go

@@ -211,9 +211,6 @@ func (dbp *Process) CheckAndClearManualStopRequest() bool {
 
 func (dbp *Process) writeBreakpoint(addr uint64) (string, int, *proc.Function, []byte, error) {
 	f, l, fn := dbp.bi.PCToLine(uint64(addr))
-	if fn == nil {
-		return "", 0, nil, nil, proc.InvalidAddressError{Address: addr}
-	}
 
 	originalData := make([]byte, dbp.bi.Arch.BreakpointSize())
 	_, err := dbp.currentThread.ReadMemory(originalData, uintptr(addr))

pkg/proc/threads.go

@@ -379,21 +379,16 @@ func setStepIntoBreakpoint(dbp Process, text []AsmInstruction, cond ast.Expr) er
 
 	instr := text[0]
 
-	if instr.DestLoc == nil || instr.DestLoc.Fn == nil {
+	if instr.DestLoc == nil {
+		// Call destination couldn't be resolved because this was not the
+		// current instruction, therefore the step-into breakpoint can not be set.
 		return nil
 	}
 
 	fn := instr.DestLoc.Fn
 
-	// Ensure PC and Entry match, otherwise StepInto is likely to set
-	// its breakpoint before DestLoc.PC and hence run too far ahead.
-	// Calls to runtime.duffzero and duffcopy have this problem.
-	if fn.Entry != instr.DestLoc.PC {
-		return nil
-	}
-
 	// Skip unexported runtime functions
-	if strings.HasPrefix(fn.Name, "runtime.") && !isExportedRuntime(fn.Name) {
+	if fn != nil && strings.HasPrefix(fn.Name, "runtime.") && !isExportedRuntime(fn.Name) {
 		return nil
 	}
 
@@ -401,8 +396,19 @@ func setStepIntoBreakpoint(dbp Process, text []AsmInstruction, cond ast.Expr) er
 	// or entire packages from being stepped into with 'step'
 	// those extra checks should be done here.
 
+	pc := instr.DestLoc.PC
+
+	// We want to skip the function prologue but we should only do it if the
+	// destination address of the CALL instruction is the entry point of the
+	// function.
+	// Calls to runtime.duffzero and duffcopy inserted by the compiler can
+	// sometimes point inside the body of those functions, well after the
+	// prologue.
+	if fn != nil && fn.Entry == instr.DestLoc.PC {
+		pc, _ = FirstPCAfterPrologue(dbp, fn, false)
+	}
+
 	// Set a breakpoint after the function's prologue
-	pc, _ := FirstPCAfterPrologue(dbp, fn, false)
 	if _, err := dbp.SetBreakpoint(pc, NextBreakpoint, cond); err != nil {
 		if _, ok := err.(BreakpointExistsError); !ok {
 			return err