1// Copyright 2019 Google LLC 2// 3// Licensed under the Apache License, Version 2.0 (the "License"); 4// you may not use this file except in compliance with the License. 5// You may obtain a copy of the License at 6// 7// http://www.apache.org/licenses/LICENSE-2.0 8// 9// Unless required by applicable law or agreed to in writing, software 10// distributed under the License is distributed on an "AS IS" BASIS, 11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12// See the License for the specific language governing permissions and 13// limitations under the License. 14 15// +build linux 16 17package server 18 19import ( 20 "errors" 21 "fmt" 22 23 "cloud.google.com/go/cmd/go-cloud-debug-agent/internal/debug/dwarf" 24) 25 26func (s *Server) functionStartAddress(name string) (uint64, error) { 27 entry, err := s.dwarfData.LookupFunction(name) 28 if err != nil { 29 return 0, err 30 } 31 addrAttr := entry.Val(dwarf.AttrLowpc) 32 if addrAttr == nil { 33 return 0, fmt.Errorf("symbol %q has no LowPC attribute", name) 34 } 35 addr, ok := addrAttr.(uint64) 36 if !ok { 37 return 0, fmt.Errorf("symbol %q has non-uint64 LowPC attribute", name) 38 } 39 return addr, nil 40} 41 42// evalLocation parses a DWARF location description encoded in v. It works for 43// cases where the variable is stored at an offset from the Canonical Frame 44// Address. The return value is this offset. 45// TODO: a more general location-description-parsing function. 46func evalLocation(v []uint8) (int64, error) { 47 // Some DWARF constants. 48 const ( 49 opConsts = 0x11 50 opPlus = 0x22 51 opCallFrameCFA = 0x9C 52 ) 53 if len(v) == 0 { 54 return 0, errors.New("empty location specifier") 55 } 56 if v[0] != opCallFrameCFA { 57 return 0, errors.New("unsupported location specifier") 58 } 59 if len(v) == 1 { 60 // The location description was just DW_OP_call_frame_cfa, so the location is exactly the CFA. 61 return 0, nil 62 } 63 if v[1] != opConsts { 64 return 0, errors.New("unsupported location specifier") 65 } 66 offset, v, err := sleb128(v[2:]) 67 if err != nil { 68 return 0, err 69 } 70 if len(v) == 1 && v[0] == opPlus { 71 // The location description was DW_OP_call_frame_cfa, DW_OP_consts <offset>, DW_OP_plus. 72 // So return the offset. 73 return offset, nil 74 } 75 return 0, errors.New("unsupported location specifier") 76} 77 78func uleb128(v []uint8) (u uint64) { 79 var shift uint 80 for _, x := range v { 81 u |= (uint64(x) & 0x7F) << shift 82 shift += 7 83 if x&0x80 == 0 { 84 break 85 } 86 } 87 return u 88} 89 90// sleb128 parses a signed integer encoded with sleb128 at the start of v, and 91// returns the integer and the remainder of v. 92func sleb128(v []uint8) (s int64, rest []uint8, err error) { 93 var shift uint 94 var sign int64 = -1 95 var i int 96 var x uint8 97 for i, x = range v { 98 s |= (int64(x) & 0x7F) << shift 99 shift += 7 100 sign <<= 7 101 if x&0x80 == 0 { 102 if x&0x40 != 0 { 103 s |= sign 104 } 105 break 106 } 107 } 108 if i == len(v) { 109 return 0, nil, errors.New("truncated sleb128") 110 } 111 return s, v[i+1:], nil 112} 113