ubsan_type_hash.cc revision 26a725f0b2fd30b98adae6353b432d0c955e10bd
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 24namespace std { 25 class type_info { 26 public: 27 virtual ~type_info(); 28 private: 29 const char *__type_name; 30 }; 31} 32 33namespace __cxxabiv1 { 34 35/// Type info for classes with no bases, and base class for type info for 36/// classes with bases. 37class __class_type_info : public std::type_info { 38 virtual ~__class_type_info(); 39}; 40 41/// Type info for classes with simple single public inheritance. 42class __si_class_type_info : public __class_type_info { 43public: 44 virtual ~__si_class_type_info(); 45 46 const __class_type_info *__base_type; 47}; 48 49class __base_class_type_info { 50public: 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. 62class __vmi_class_type_info : public __class_type_info { 63public: 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 73namespace 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. 93static __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 __ubsan_vptr_type_cache[__ubsan::VptrTypeCacheSize] = { 1 }; 112 113/// \brief Determine whether \p Derived has a \p Base base class subobject at 114/// offset \p Offset. 115static bool isDerivedFromAtOffset(const abi::__class_type_info *Derived, 116 const abi::__class_type_info *Base, 117 sptr Offset) { 118 if (Derived == Base) 119 return Offset == 0; 120 121 if (const abi::__si_class_type_info *SI = 122 dynamic_cast<const abi::__si_class_type_info*>(Derived)) 123 return isDerivedFromAtOffset(SI->__base_type, Base, Offset); 124 125 const abi::__vmi_class_type_info *VTI = 126 dynamic_cast<const abi::__vmi_class_type_info*>(Derived); 127 if (!VTI) 128 // No base class subobjects. 129 return false; 130 131 // Look for a zero-offset base class which is derived from \p Base. 132 for (unsigned int base = 0; base != VTI->base_count; ++base) { 133 // FIXME: Curtail the recursion if this base can't possibly contain the 134 // given offset. 135 sptr OffsetHere = VTI->base_info[base].__offset_flags >> 136 abi::__base_class_type_info::__offset_shift; 137 if (VTI->base_info[base].__offset_flags & 138 abi::__base_class_type_info::__virtual_mask) 139 // For now, just punt on virtual bases and say 'yes'. 140 // FIXME: OffsetHere is the offset in the vtable of the virtual base 141 // offset. Read the vbase offset out of the vtable and use it. 142 return true; 143 if (isDerivedFromAtOffset(VTI->base_info[base].__base_type, 144 Base, Offset - OffsetHere)) 145 return true; 146 } 147 148 return false; 149} 150 151namespace { 152 153struct VtablePrefix { 154 /// The offset from the vptr to the start of the most-derived object. 155 /// This should never be greater than zero, and will usually be exactly 156 /// zero. 157 sptr Offset; 158 /// The type_info object describing the most-derived class type. 159 std::type_info *TypeInfo; 160}; 161VtablePrefix *getVtablePrefix(void *Object) { 162 VtablePrefix **Ptr = reinterpret_cast<VtablePrefix**>(Object); 163 return *Ptr - 1; 164} 165 166} 167 168bool __ubsan::checkDynamicType(void *Object, void *Type, HashValue Hash) { 169 // A crash anywhere within this function probably means the vptr is corrupted. 170 // FIXME: Perform these checks more cautiously. 171 172 // Check whether this is something we've evicted from the cache. 173 HashValue *Bucket = getTypeCacheHashTableBucket(Hash); 174 if (*Bucket == Hash) { 175 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 176 return true; 177 } 178 179 VtablePrefix *Vtable = getVtablePrefix(Object); 180 if (Vtable + 1 == 0 || Vtable->Offset > 0) 181 // This can't possibly be a valid vtable. 182 return false; 183 184 // Check that this is actually a type_info object for a class type. 185 abi::__class_type_info *Derived = 186 dynamic_cast<abi::__class_type_info*>(Vtable->TypeInfo); 187 if (!Derived) 188 return false; 189 190 abi::__class_type_info *Base = (abi::__class_type_info*)Type; 191 if (!isDerivedFromAtOffset(Derived, Base, -Vtable->Offset)) 192 return false; 193 194 // Success. Cache this result. 195 __ubsan_vptr_type_cache[Hash % VptrTypeCacheSize] = Hash; 196 *Bucket = Hash; 197 return true; 198} 199