go/usr/austin/ogle/frame.go

// Copyright 2009 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ogle

import (
	"fmt";
	"ptrace";
	"sym";
)

// A Frame represents a single frame on a remote call stack.
type Frame struct {
	// pc is the PC of the next instruction that will execute in
	// this frame.  For lower frames, this is the instruction
	// following the CALL instruction.
	pc, sp, fp ptrace.Word;
	// The runtime.Stktop of the active stack segment
	stk remoteStruct;
	// The function this stack frame is in
	fn *sym.TextSym;
	// The path and line of the CALL or current instruction.  Note
	// that this differs slightly from the meaning of Frame.pc.
	path string;
	line int;
	// The inner and outer frames of this frame.  outer is filled
	// in lazily.
	inner, outer *Frame;
}

// NewFrame returns the top-most Frame of the given g's thread.
// This function can abort.
func NewFrame(g remoteStruct) *Frame {
	p := g.r.p;
	var pc, sp ptrace.Word;

	// Is this G alive?
	switch g.Field(p.f.G.Status).(remoteInt).Get() {
	case p.runtime.Gidle, p.runtime.Gmoribund, p.runtime.Gdead:
		return nil;
	}

	// Find the OS thread for this G

	// TODO(austin) Ideally, we could look at the G's state and
	// figure out if it's on an OS thread or not.  However, this
	// is difficult because the state isn't updated atomically
	// with scheduling changes.
	for _, t := range p.Threads() {
		regs, err := t.Regs();
		if err != nil {
			// TODO(austin) What to do?
			continue;
		}
		thisg := p.G(regs);
		if thisg == g.addr().base {
			// Found this G's OS thread
			pc = regs.PC();
			sp = regs.SP();

			// If this thread crashed, try to recover it
			if pc == 0 {
				pc = p.peekUintptr(pc);
				sp += 8;
			}

			break;
		}
	}

	if pc == 0 && sp == 0 {
		// G is not mapped to an OS thread.  Use the
		// scheduler's stored PC and SP.
		sched := g.Field(p.f.G.Sched).(remoteStruct);
		pc = ptrace.Word(sched.Field(p.f.Gobuf.Pc).(remoteUint).Get());
		sp = ptrace.Word(sched.Field(p.f.Gobuf.Sp).(remoteUint).Get());
	}

	// Get Stktop
	stk := g.Field(p.f.G.Stackbase).(remotePtr).Get().(remoteStruct);

	return prepareFrame(pc, sp, stk, nil);
}

// prepareFrame creates a Frame from the PC and SP within that frame,
// as well as the active stack segment.  This function takes care of
// traversing stack breaks and unwinding closures.  This function can
// abort.
func prepareFrame(pc, sp ptrace.Word, stk remoteStruct, inner *Frame) *Frame {
	// Based on src/pkg/runtime/amd64/traceback.c:traceback
	p := stk.r.p;
	top := inner == nil;

	// Get function
	var path string;
	var line int;
	var fn *sym.TextSym;

	for i := 0; i < 100; i++ {
		// Traverse segmented stack breaks
		if p.sys.lessstack != nil && pc == ptrace.Word(p.sys.lessstack.Value) {
			// Get stk->gobuf.pc
			pc = ptrace.Word(stk.Field(p.f.Stktop.Gobuf).(remoteStruct).Field(p.f.Gobuf.Pc).(remoteUint).Get());
			// Get stk->gobuf.sp
			sp = ptrace.Word(stk.Field(p.f.Stktop.Gobuf).(remoteStruct).Field(p.f.Gobuf.Sp).(remoteUint).Get());
			// Get stk->stackbase
			stk = stk.Field(p.f.Stktop.Stackbase).(remotePtr).Get().(remoteStruct);
			continue;
		}

		// Get the PC of the call instruction
		callpc := pc;
		if !top && (p.sys.goexit == nil || pc != ptrace.Word(p.sys.goexit.Value)) {
			callpc--;
		}

		// Look up function
		path, line, fn = p.syms.LineFromPC(uint64(callpc));
		if fn != nil {
			break;
		}

		// Closure?
		var buf = make([]byte, p.ClosureSize());
		if _, err := p.Peek(pc, buf); err != nil {
			break;
		}
		spdelta, ok := p.ParseClosure(buf);
		if ok {
			sp += ptrace.Word(spdelta);
			pc = p.peekUintptr(sp - ptrace.Word(p.PtrSize()));
		}
	}
	if fn == nil {
		return nil;
	}

	// Compute frame pointer
	var fp ptrace.Word;
	if fn.FrameSize < p.PtrSize() {
		fp = sp + ptrace.Word(p.PtrSize());
	} else {
		fp = sp + ptrace.Word(fn.FrameSize);
	}
	// TODO(austin) To really figure out if we're in the prologue,
	// we need to disassemble the function and look for the call
	// to morestack.  For now, just special case the entry point.
	//
	// TODO(austin) What if we're in the call to morestack in the
	// prologue?  Then top == false.
	if top && pc == ptrace.Word(fn.Entry()) {
		// We're in the function prologue, before SP
		// has been adjusted for the frame.
		fp -= ptrace.Word(fn.FrameSize - p.PtrSize());
	}

	return &Frame{pc, sp, fp, stk, fn, path, line, inner, nil};
}

// Outer returns the Frame that called this Frame, or nil if this is
// the outermost frame.  This function can abort.
func (f *Frame) Outer() *Frame {
	// Is there a cached outer frame
	if f.outer != nil {
		return f.outer;
	}

	p := f.stk.r.p;

	sp := f.fp;
	if f.fn == p.sys.newproc && f.fn == p.sys.deferproc {
		// TODO(rsc) The compiler inserts two push/pop's
		// around calls to go and defer.  Russ says this
		// should get fixed in the compiler, but we account
		// for it for now.
		sp += ptrace.Word(2 * p.PtrSize());
	}

	pc := p.peekUintptr(f.fp - ptrace.Word(p.PtrSize()));
	if pc < 0x1000 {
		return nil;
	}

	f.outer = prepareFrame(pc, sp, f.stk, f);
	return f.outer;
}

// Inner returns the Frame called by this Frame, or nil if this is the
// innermost frame.
func (f *Frame) Inner() *Frame {
	return f.inner;
}

func (f *Frame) String() string {
	res := f.fn.Name;
	if f.pc > ptrace.Word(f.fn.Value) {
		res += fmt.Sprintf("+%#x", f.pc - ptrace.Word(f.fn.Entry()));
	}
	return res + fmt.Sprintf(" %s:%d", f.path, f.line);
}