1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //
8 //  Parses ELF .eh_frame_hdr sections.
9 //
10 //===----------------------------------------------------------------------===//
11 
12 #ifndef __EHHEADERPARSER_HPP__
13 #define __EHHEADERPARSER_HPP__
14 
15 #include "libunwind.h"
16 
17 #include "DwarfParser.hpp"
18 
19 namespace libunwind {
20 
21 /// \brief EHHeaderParser does basic parsing of an ELF .eh_frame_hdr section.
22 ///
23 /// See DWARF spec for details:
24 ///    http://refspecs.linuxbase.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html
25 ///
26 template <typename A> class EHHeaderParser {
27 public:
28   typedef typename A::pint_t pint_t;
29 
30   /// Information encoded in the EH frame header.
31   struct EHHeaderInfo {
32     pint_t eh_frame_ptr;
33     size_t fde_count;
34     pint_t table;
35     uint8_t table_enc;
36   };
37 
38   static bool decodeEHHdr(A &addressSpace, pint_t ehHdrStart, pint_t ehHdrEnd,
39                           EHHeaderInfo &ehHdrInfo);
40   static bool findFDE(A &addressSpace, pint_t pc, pint_t ehHdrStart,
41                       uint32_t sectionLength,
42                       typename CFI_Parser<A>::FDE_Info *fdeInfo,
43                       typename CFI_Parser<A>::CIE_Info *cieInfo);
44 
45 private:
46   static bool decodeTableEntry(A &addressSpace, pint_t &tableEntry,
47                                pint_t ehHdrStart, pint_t ehHdrEnd,
48                                uint8_t tableEnc,
49                                typename CFI_Parser<A>::FDE_Info *fdeInfo,
50                                typename CFI_Parser<A>::CIE_Info *cieInfo);
51   static size_t getTableEntrySize(uint8_t tableEnc);
52 };
53 
54 template <typename A>
55 bool EHHeaderParser<A>::decodeEHHdr(A &addressSpace, pint_t ehHdrStart,
56                                     pint_t ehHdrEnd, EHHeaderInfo &ehHdrInfo) {
57   pint_t p = ehHdrStart;
58   uint8_t version = addressSpace.get8(p++);
59   if (version != 1) {
60     _LIBUNWIND_LOG("unsupported .eh_frame_hdr version: %" PRIu8 " at %" PRIx64,
61                    version, static_cast<uint64_t>(ehHdrStart));
62     return false;
63   }
64 
65   uint8_t eh_frame_ptr_enc = addressSpace.get8(p++);
66   uint8_t fde_count_enc = addressSpace.get8(p++);
67   ehHdrInfo.table_enc = addressSpace.get8(p++);
68 
69   ehHdrInfo.eh_frame_ptr =
70       addressSpace.getEncodedP(p, ehHdrEnd, eh_frame_ptr_enc, ehHdrStart);
71   ehHdrInfo.fde_count =
72       fde_count_enc == DW_EH_PE_omit
73           ? 0
74           : addressSpace.getEncodedP(p, ehHdrEnd, fde_count_enc, ehHdrStart);
75   ehHdrInfo.table = p;
76 
77   return true;
78 }
79 
80 template <typename A>
81 bool EHHeaderParser<A>::decodeTableEntry(
82     A &addressSpace, pint_t &tableEntry, pint_t ehHdrStart, pint_t ehHdrEnd,
83     uint8_t tableEnc, typename CFI_Parser<A>::FDE_Info *fdeInfo,
84     typename CFI_Parser<A>::CIE_Info *cieInfo) {
85   // Have to decode the whole FDE for the PC range anyway, so just throw away
86   // the PC start.
87   addressSpace.getEncodedP(tableEntry, ehHdrEnd, tableEnc, ehHdrStart);
88   pint_t fde =
89       addressSpace.getEncodedP(tableEntry, ehHdrEnd, tableEnc, ehHdrStart);
90   const char *message =
91       CFI_Parser<A>::decodeFDE(addressSpace, fde, fdeInfo, cieInfo);
92   if (message != NULL) {
93     _LIBUNWIND_DEBUG_LOG("EHHeaderParser::decodeTableEntry: bad fde: %s",
94                          message);
95     return false;
96   }
97 
98   return true;
99 }
100 
101 template <typename A>
102 bool EHHeaderParser<A>::findFDE(A &addressSpace, pint_t pc, pint_t ehHdrStart,
103                                 uint32_t sectionLength,
104                                 typename CFI_Parser<A>::FDE_Info *fdeInfo,
105                                 typename CFI_Parser<A>::CIE_Info *cieInfo) {
106   pint_t ehHdrEnd = ehHdrStart + sectionLength;
107 
108   EHHeaderParser<A>::EHHeaderInfo hdrInfo;
109   if (!EHHeaderParser<A>::decodeEHHdr(addressSpace, ehHdrStart, ehHdrEnd,
110                                       hdrInfo))
111     return false;
112 
113   if (hdrInfo.fde_count == 0) return false;
114 
115   size_t tableEntrySize = getTableEntrySize(hdrInfo.table_enc);
116   pint_t tableEntry;
117 
118   size_t low = 0;
119   for (size_t len = hdrInfo.fde_count; len > 1;) {
120     size_t mid = low + (len / 2);
121     tableEntry = hdrInfo.table + mid * tableEntrySize;
122     pint_t start = addressSpace.getEncodedP(tableEntry, ehHdrEnd,
123                                             hdrInfo.table_enc, ehHdrStart);
124 
125     if (start == pc) {
126       low = mid;
127       break;
128     } else if (start < pc) {
129       low = mid;
130       len -= (len / 2);
131     } else {
132       len /= 2;
133     }
134   }
135 
136   tableEntry = hdrInfo.table + low * tableEntrySize;
137   if (decodeTableEntry(addressSpace, tableEntry, ehHdrStart, ehHdrEnd,
138                        hdrInfo.table_enc, fdeInfo, cieInfo)) {
139     if (pc >= fdeInfo->pcStart && pc < fdeInfo->pcEnd)
140       return true;
141   }
142 
143   return false;
144 }
145 
146 template <typename A>
147 size_t EHHeaderParser<A>::getTableEntrySize(uint8_t tableEnc) {
148   switch (tableEnc & 0x0f) {
149   case DW_EH_PE_sdata2:
150   case DW_EH_PE_udata2:
151     return 4;
152   case DW_EH_PE_sdata4:
153   case DW_EH_PE_udata4:
154     return 8;
155   case DW_EH_PE_sdata8:
156   case DW_EH_PE_udata8:
157     return 16;
158   case DW_EH_PE_sleb128:
159   case DW_EH_PE_uleb128:
160     _LIBUNWIND_ABORT("Can't binary search on variable length encoded data.");
161   case DW_EH_PE_omit:
162     return 0;
163   default:
164     _LIBUNWIND_ABORT("Unknown DWARF encoding for search table.");
165   }
166 }
167 
168 }
169 
170 #endif
171