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// Copyright 2014 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 server
import (
"errors"
"fmt"
"golang.org/x/debug/dwarf"
)
func (s *Server) functionStartAddress(name string) (uint64, error) {
entry, err := s.dwarfData.LookupFunction(name)
if err != nil {
return 0, err
}
addrAttr := entry.Val(dwarf.AttrLowpc)
if addrAttr == nil {
return 0, fmt.Errorf("symbol %q has no LowPC attribute", name)
}
addr, ok := addrAttr.(uint64)
if !ok {
return 0, fmt.Errorf("symbol %q has non-uint64 LowPC attribute", name)
}
return addr, nil
}
// evalLocation parses a DWARF location description encoded in v. It works for
// cases where the variable is stored at an offset from the Canonical Frame
// Address. The return value is this offset.
// TODO: a more general location-description-parsing function.
func evalLocation(v []uint8) (int64, error) {
// Some DWARF constants.
const (
opConsts = 0x11
opPlus = 0x22
opCallFrameCFA = 0x9C
)
if len(v) == 0 {
return 0, errors.New("empty location specifier")
}
if v[0] != opCallFrameCFA {
return 0, errors.New("unsupported location specifier")
}
if len(v) == 1 {
// The location description was just DW_OP_call_frame_cfa, so the location is exactly the CFA.
return 0, nil
}
if v[1] != opConsts {
return 0, errors.New("unsupported location specifier")
}
offset, v, err := sleb128(v[2:])
if err != nil {
return 0, err
}
if len(v) == 1 && v[0] == opPlus {
// The location description was DW_OP_call_frame_cfa, DW_OP_consts <offset>, DW_OP_plus.
// So return the offset.
return offset, nil
}
return 0, errors.New("unsupported location specifier")
}
func uleb128(v []uint8) (u uint64) {
var shift uint
for _, x := range v {
u |= (uint64(x) & 0x7F) << shift
shift += 7
if x&0x80 == 0 {
break
}
}
return u
}
// sleb128 parses a signed integer encoded with sleb128 at the start of v, and
// returns the integer and the remainder of v.
func sleb128(v []uint8) (s int64, rest []uint8, err error) {
var shift uint
var sign int64 = -1
var i int
var x uint8
for i, x = range v {
s |= (int64(x) & 0x7F) << shift
shift += 7
sign <<= 7
if x&0x80 == 0 {
if x&0x40 != 0 {
s |= sign
}
break
}
}
if i == len(v) {
return 0, nil, errors.New("truncated sleb128")
}
return s, v[i+1:], nil
}
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