1 //===-- ubsan_type_hash.cc ------------------------------------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // Implementation of a hash table for fast checking of inheritance 11 // relationships. This file is only linked into C++ compilations, and is 12 // permitted to use language features which require a C++ ABI library. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "ubsan_type_hash.h" 17 18 #include "sanitizer_common/sanitizer_common.h" 19 20 // The following are intended to be binary compatible with the definitions 21 // given in the Itanium ABI. We make no attempt to be ODR-compatible with 22 // those definitions, since existing ABI implementations aren't. 23 24 namespace std { 25 class type_info { 26 public: 27 virtual ~type_info(); 28 29 const char *__type_name; 30 }; 31 } 32 33 namespace __cxxabiv1 { 34 35 /// Type info for classes with no bases, and base class for type info for 36 /// classes with bases. 37 class __class_type_info : public std::type_info { 38 virtual ~__class_type_info(); 39 }; 40 41 /// Type info for classes with simple single public inheritance. 42 class __si_class_type_info : public __class_type_info { 43 public: 44 virtual ~__si_class_type_info(); 45 46 const __class_type_info *__base_type; 47 }; 48 49 class __base_class_type_info { 50 public: 51 const __class_type_info *__base_type; 52 long __offset_flags; 53 54 enum __offset_flags_masks { 55 __virtual_mask = 0x1, 56 __public_mask = 0x2, 57 __offset_shift = 8 58 }; 59 }; 60 61 /// Type info for classes with multiple, virtual, or non-public inheritance. 62 class __vmi_class_type_info : public __class_type_info { 63 public: 64 virtual ~__vmi_class_type_info(); 65 66 unsigned int flags; 67 unsigned int base_count; 68 __base_class_type_info base_info[1]; 69 }; 70 71 } 72 73 namespace abi = __cxxabiv1; 74 75 // We implement a simple two-level cache for type-checking results. For each 76 // (vptr,type) pair, a hash is computed. This hash is assumed to be globally 77 // unique; if it collides, we will get false negatives, but: 78 // * such a collision would have to occur on the *first* bad access, 79 // * the probability of such a collision is low (and for a 64-bit target, is 80 // negligible), and 81 // * the vptr, and thus the hash, can be affected by ASLR, so multiple runs 82 // give better coverage. 83 // 84 // The first caching layer is a small hash table with no chaining; buckets are 85 // reused as needed. The second caching layer is a large hash table with open 86 // chaining. We can freely evict from either layer since this is just a cache. 87 // 88 // FIXME: Make these hash table accesses thread-safe. The races here are benign 89 // (worst-case, we could miss a bug or see a slowdown) but we should 90 // avoid upsetting race detectors. 91 92 /// Find a bucket to store the given hash value in. 93 static __ubsan::HashValue *getTypeCacheHashTableBucket(__ubsan::HashValue V) { 94 static const unsigned HashTableSize = 65537; 95 static __ubsan::HashValue __ubsan_vptr_hash_set[HashTableSize] = { 1 }; 96 97 unsigned Probe = V & 65535; 98 for (int Tries = 5; Tries; --Tries) { 99 if (!__ubsan_vptr_hash_set[Probe] || __ubsan_vptr_hash_set[Probe] == V) 100 return &__ubsan_vptr_hash_set[Probe]; 101 Probe += ((V >> 16) & 65535) + 1; 102 if (Probe >= HashTableSize) 103 Probe -= HashTableSize; 104 } 105 // FIXME: Pick a random entry from the probe sequence to evict rather than 106 // just taking the first. 107 return &__ubsan_vptr_hash_set[V]; 108 } 109 110 /// A cache of recently-checked hashes. Mini hash table with "random" evictions. 111 __ubsan::HashValue 112 __ubsan::__ubsan_vptr_type_cache[__ubsan::VptrTypeCacheSize] = { 1 }; 113 114 /// \brief Determine whether \p Derived has a \p Base base class subobject at 115 /// offset \p Offset. 116 static bool isDerivedFromAtOffset(const abi::__class_type_info *Derived, 117 const abi::__class_type_info *Base, 118 sptr Offset) { 119 if (Derived->__type_name == Base->__type_name) 120 return Offset == 0; 121 122 if (const abi::__si_class_type_info *SI = 123 dynamic_cast<const abi::__si_class_type_info*>(Derived)) 124 return isDerivedFromAtOffset(SI->__base_type, Base, Offset); 125 126 const abi::__vmi_class_type_info *VTI = 127 dynamic_cast<const abi::__vmi_class_type_info*>(Derived); 128 if (!VTI) 129 // No base class subobjects. 130 return false; 131 132 // Look for a base class which is derived from \p Base at the right offset. 133 for (unsigned int base = 0; base != VTI->base_count; ++base) { 134 // FIXME: Curtail the recursion if this base can't possibly contain the 135 // given offset. 136 sptr OffsetHere = VTI->base_info[base].__offset_flags >> 137 abi::__base_class_type_info::__offset_shift; 138 if (VTI->base_info[base].__offset_flags & 139 abi::__base_class_type_info::__virtual_mask) 140 // For now, just punt on virtual bases and say 'yes'. 141 // FIXME: OffsetHere is the offset in the vtable of the virtual base 142 // offset. Read the vbase offset out of the vtable and use it. 143 return true; 144 if (isDerivedFromAtOffset(VTI->base_info[base].__base_type, 145 Base, Offset - OffsetHere)) 146 return true; 147 } 148 149 return false; 150 } 151 152 /// \brief Find the derived-most dynamic base class of \p Derived at offset 153 /// \p Offset. 154 static const abi::__class_type_info *findBaseAtOffset( 155 const abi::__class_type_info *Derived, sptr Offset) { 156 if (!Offset) 157 return Derived; 158 159 if (const abi::__si_class_type_info *SI = 160 dynamic_cast<const abi::__si_class_type_info*>(Derived)) 161 return findBaseAtOffset(SI->__base_type, Offset); 162 163 const abi::__vmi_class_type_info *VTI = 164 dynamic_cast<const abi::__vmi_class_type_info*>(Derived); 165 if (!VTI) 166 // No base class subobjects. 167 return 0; 168 169 for (unsigned int base = 0; base != VTI->base_count; ++base) { 170 sptr OffsetHere = VTI->base_info[base].__offset_flags >> 171 abi::__base_class_type_info::__offset_shift; 172 if (VTI->base_info[base].__offset_flags & 173 abi::__base_class_type_info::__virtual_mask) 174 // FIXME: Can't handle virtual bases yet. 175 continue; 176 if (const abi::__class_type_info *Base = 177 findBaseAtOffset(VTI->base_info[base].__base_type, 178 Offset - OffsetHere)) 179 return Base; 180 } 181 182 return 0; 183 } 184 185 namespace { 186 187 struct VtablePrefix { 188 /// The offset from the vptr to the start of the most-derived object. 189 /// This should never be greater than zero, and will usually be exactly 190 /// zero. 191 sptr Offset; 192 /// The type_info object describing the most-derived class type. 193 std::type_info *TypeInfo; 194 }; 195 VtablePrefix *getVtablePrefix(void *Object) { 196 VtablePrefix **VptrPtr = reinterpret_cast<VtablePrefix**>(Object); 197 if (!*VptrPtr) 198 return 0; 199 VtablePrefix *Prefix = *VptrPtr - 1; 200 if (Prefix->Offset > 0 || !Prefix->TypeInfo) 201 // This can't possibly be a valid vtable. 202 return 0; 203 return Prefix; 204 } 205 206 } 207 208 bool __ubsan::checkDynamicType(void *Object, void *Type, HashValue Hash) { 209 // A crash anywhere within this function probably means the vptr is corrupted. 210 // FIXME: Perform these checks more cautiously. 211 212 // Check whether this is something we've evicted from the cache. 213 HashValue *Bucket = getTypeCacheHashTableBucket(Hash); 214 if (*Bucket == Hash) { 215 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 216 return true; 217 } 218 219 VtablePrefix *Vtable = getVtablePrefix(Object); 220 if (!Vtable) 221 return false; 222 223 // Check that this is actually a type_info object for a class type. 224 abi::__class_type_info *Derived = 225 dynamic_cast<abi::__class_type_info*>(Vtable->TypeInfo); 226 if (!Derived) 227 return false; 228 229 abi::__class_type_info *Base = (abi::__class_type_info*)Type; 230 if (!isDerivedFromAtOffset(Derived, Base, -Vtable->Offset)) 231 return false; 232 233 // Success. Cache this result. 234 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 235 *Bucket = Hash; 236 return true; 237 } 238 239 __ubsan::DynamicTypeInfo __ubsan::getDynamicTypeInfo(void *Object) { 240 VtablePrefix *Vtable = getVtablePrefix(Object); 241 if (!Vtable) 242 return DynamicTypeInfo(0, 0, 0); 243 const abi::__class_type_info *ObjectType = findBaseAtOffset( 244 static_cast<const abi::__class_type_info*>(Vtable->TypeInfo), 245 -Vtable->Offset); 246 return DynamicTypeInfo(Vtable->TypeInfo->__type_name, -Vtable->Offset, 247 ObjectType ? ObjectType->__type_name : "<unknown>"); 248 } 249