1 //===-- ubsan_type_hash_itanium.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 type hashing/lookup for Itanium C++ ABI. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "sanitizer_common/sanitizer_platform.h" 15 #include "ubsan_platform.h" 16 #if CAN_SANITIZE_UB && !SANITIZER_WINDOWS 17 #include "ubsan_type_hash.h" 18 19 #include "sanitizer_common/sanitizer_common.h" 20 21 // The following are intended to be binary compatible with the definitions 22 // given in the Itanium ABI. We make no attempt to be ODR-compatible with 23 // those definitions, since existing ABI implementations aren't. 24 25 namespace std { 26 class type_info { 27 public: 28 virtual ~type_info(); 29 30 const char *__type_name; 31 }; 32 } 33 34 namespace __cxxabiv1 { 35 36 /// Type info for classes with no bases, and base class for type info for 37 /// classes with bases. 38 class __class_type_info : public std::type_info { 39 ~__class_type_info() override; 40 }; 41 42 /// Type info for classes with simple single public inheritance. 43 class __si_class_type_info : public __class_type_info { 44 public: 45 ~__si_class_type_info() override; 46 47 const __class_type_info *__base_type; 48 }; 49 50 class __base_class_type_info { 51 public: 52 const __class_type_info *__base_type; 53 long __offset_flags; 54 55 enum __offset_flags_masks { 56 __virtual_mask = 0x1, 57 __public_mask = 0x2, 58 __offset_shift = 8 59 }; 60 }; 61 62 /// Type info for classes with multiple, virtual, or non-public inheritance. 63 class __vmi_class_type_info : public __class_type_info { 64 public: 65 ~__vmi_class_type_info() override; 66 67 unsigned int flags; 68 unsigned int base_count; 69 __base_class_type_info base_info[1]; 70 }; 71 72 } 73 74 namespace abi = __cxxabiv1; 75 76 // We implement a simple two-level cache for type-checking results. For each 77 // (vptr,type) pair, a hash is computed. This hash is assumed to be globally 78 // unique; if it collides, we will get false negatives, but: 79 // * such a collision would have to occur on the *first* bad access, 80 // * the probability of such a collision is low (and for a 64-bit target, is 81 // negligible), and 82 // * the vptr, and thus the hash, can be affected by ASLR, so multiple runs 83 // give better coverage. 84 // 85 // The first caching layer is a small hash table with no chaining; buckets are 86 // reused as needed. The second caching layer is a large hash table with open 87 // chaining. We can freely evict from either layer since this is just a cache. 88 // 89 // FIXME: Make these hash table accesses thread-safe. The races here are benign: 90 // assuming the unsequenced loads and stores don't misbehave too badly, 91 // the worst case is false negatives or poor cache behavior, not false 92 // positives or crashes. 93 94 /// Find a bucket to store the given hash value in. 95 static __ubsan::HashValue *getTypeCacheHashTableBucket(__ubsan::HashValue V) { 96 static const unsigned HashTableSize = 65537; 97 static __ubsan::HashValue __ubsan_vptr_hash_set[HashTableSize]; 98 99 unsigned First = (V & 65535) ^ 1; 100 unsigned Probe = First; 101 for (int Tries = 5; Tries; --Tries) { 102 if (!__ubsan_vptr_hash_set[Probe] || __ubsan_vptr_hash_set[Probe] == V) 103 return &__ubsan_vptr_hash_set[Probe]; 104 Probe += ((V >> 16) & 65535) + 1; 105 if (Probe >= HashTableSize) 106 Probe -= HashTableSize; 107 } 108 // FIXME: Pick a random entry from the probe sequence to evict rather than 109 // just taking the first. 110 return &__ubsan_vptr_hash_set[First]; 111 } 112 113 /// \brief Determine whether \p Derived has a \p Base base class subobject at 114 /// offset \p Offset. 115 static bool isDerivedFromAtOffset(const abi::__class_type_info *Derived, 116 const abi::__class_type_info *Base, 117 sptr Offset) { 118 if (Derived->__type_name == Base->__type_name || 119 (SANITIZER_NON_UNIQUE_TYPEINFO && 120 !internal_strcmp(Derived->__type_name, Base->__type_name))) 121 return Offset == 0; 122 123 if (const abi::__si_class_type_info *SI = 124 dynamic_cast<const abi::__si_class_type_info*>(Derived)) 125 return isDerivedFromAtOffset(SI->__base_type, Base, Offset); 126 127 const abi::__vmi_class_type_info *VTI = 128 dynamic_cast<const abi::__vmi_class_type_info*>(Derived); 129 if (!VTI) 130 // No base class subobjects. 131 return false; 132 133 // Look for a base class which is derived from \p Base at the right offset. 134 for (unsigned int base = 0; base != VTI->base_count; ++base) { 135 // FIXME: Curtail the recursion if this base can't possibly contain the 136 // given offset. 137 sptr OffsetHere = VTI->base_info[base].__offset_flags >> 138 abi::__base_class_type_info::__offset_shift; 139 if (VTI->base_info[base].__offset_flags & 140 abi::__base_class_type_info::__virtual_mask) 141 // For now, just punt on virtual bases and say 'yes'. 142 // FIXME: OffsetHere is the offset in the vtable of the virtual base 143 // offset. Read the vbase offset out of the vtable and use it. 144 return true; 145 if (isDerivedFromAtOffset(VTI->base_info[base].__base_type, 146 Base, Offset - OffsetHere)) 147 return true; 148 } 149 150 return false; 151 } 152 153 /// \brief Find the derived-most dynamic base class of \p Derived at offset 154 /// \p Offset. 155 static const abi::__class_type_info *findBaseAtOffset( 156 const abi::__class_type_info *Derived, sptr Offset) { 157 if (!Offset) 158 return Derived; 159 160 if (const abi::__si_class_type_info *SI = 161 dynamic_cast<const abi::__si_class_type_info*>(Derived)) 162 return findBaseAtOffset(SI->__base_type, Offset); 163 164 const abi::__vmi_class_type_info *VTI = 165 dynamic_cast<const abi::__vmi_class_type_info*>(Derived); 166 if (!VTI) 167 // No base class subobjects. 168 return 0; 169 170 for (unsigned int base = 0; base != VTI->base_count; ++base) { 171 sptr OffsetHere = VTI->base_info[base].__offset_flags >> 172 abi::__base_class_type_info::__offset_shift; 173 if (VTI->base_info[base].__offset_flags & 174 abi::__base_class_type_info::__virtual_mask) 175 // FIXME: Can't handle virtual bases yet. 176 continue; 177 if (const abi::__class_type_info *Base = 178 findBaseAtOffset(VTI->base_info[base].__base_type, 179 Offset - OffsetHere)) 180 return Base; 181 } 182 183 return 0; 184 } 185 186 namespace { 187 188 struct VtablePrefix { 189 /// The offset from the vptr to the start of the most-derived object. 190 /// This will only be greater than zero in some virtual base class vtables 191 /// used during object con-/destruction, and will usually be exactly zero. 192 sptr Offset; 193 /// The type_info object describing the most-derived class type. 194 std::type_info *TypeInfo; 195 }; 196 VtablePrefix *getVtablePrefix(void *Vtable) { 197 VtablePrefix *Vptr = reinterpret_cast<VtablePrefix*>(Vtable); 198 if (!Vptr) 199 return 0; 200 VtablePrefix *Prefix = Vptr - 1; 201 if (!Prefix->TypeInfo) 202 // This can't possibly be a valid vtable. 203 return 0; 204 return Prefix; 205 } 206 207 } 208 209 bool __ubsan::checkDynamicType(void *Object, void *Type, HashValue Hash) { 210 // A crash anywhere within this function probably means the vptr is corrupted. 211 // FIXME: Perform these checks more cautiously. 212 213 // Check whether this is something we've evicted from the cache. 214 HashValue *Bucket = getTypeCacheHashTableBucket(Hash); 215 if (*Bucket == Hash) { 216 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 217 return true; 218 } 219 220 void *VtablePtr = *reinterpret_cast<void **>(Object); 221 VtablePrefix *Vtable = getVtablePrefix(VtablePtr); 222 if (!Vtable) 223 return false; 224 if (Vtable->Offset < -VptrMaxOffsetToTop || Vtable->Offset > VptrMaxOffsetToTop) { 225 // Too large or too small offset are signs of Vtable corruption. 226 return false; 227 } 228 229 // Check that this is actually a type_info object for a class type. 230 abi::__class_type_info *Derived = 231 dynamic_cast<abi::__class_type_info*>(Vtable->TypeInfo); 232 if (!Derived) 233 return false; 234 235 abi::__class_type_info *Base = (abi::__class_type_info*)Type; 236 if (!isDerivedFromAtOffset(Derived, Base, -Vtable->Offset)) 237 return false; 238 239 // Success. Cache this result. 240 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 241 *Bucket = Hash; 242 return true; 243 } 244 245 __ubsan::DynamicTypeInfo 246 __ubsan::getDynamicTypeInfoFromVtable(void *VtablePtr) { 247 VtablePrefix *Vtable = getVtablePrefix(VtablePtr); 248 if (!Vtable) 249 return DynamicTypeInfo(0, 0, 0); 250 if (Vtable->Offset < -VptrMaxOffsetToTop || Vtable->Offset > VptrMaxOffsetToTop) 251 return DynamicTypeInfo(0, Vtable->Offset, 0); 252 const abi::__class_type_info *ObjectType = findBaseAtOffset( 253 static_cast<const abi::__class_type_info*>(Vtable->TypeInfo), 254 -Vtable->Offset); 255 return DynamicTypeInfo(Vtable->TypeInfo->__type_name, -Vtable->Offset, 256 ObjectType ? ObjectType->__type_name : "<unknown>"); 257 } 258 259 #endif // CAN_SANITIZE_UB && !SANITIZER_WINDOWS 260
