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1 | //===-- ubsan_type_hash.cc ------------------------------------------------===// |
2 | // | |
3 | // This file is distributed under the University of Illinois Open Source | |
4 | // License. See LICENSE.TXT for details. | |
5 | // | |
6 | //===----------------------------------------------------------------------===// | |
7 | // | |
8 | // Implementation of a hash table for fast checking of inheritance | |
9 | // relationships. This file is only linked into C++ compilations, and is | |
10 | // permitted to use language features which require a C++ ABI library. | |
11 | // | |
12 | //===----------------------------------------------------------------------===// | |
13 | ||
14 | #include "ubsan_type_hash.h" | |
15 | ||
16 | #include "sanitizer_common/sanitizer_common.h" | |
17 | ||
18 | // The following are intended to be binary compatible with the definitions | |
19 | // given in the Itanium ABI. We make no attempt to be ODR-compatible with | |
20 | // those definitions, since existing ABI implementations aren't. | |
21 | ||
22 | namespace std { | |
23 | class type_info { | |
24 | public: | |
25 | virtual ~type_info(); | |
26 | ||
27 | const char *__type_name; | |
28 | }; | |
29 | } | |
30 | ||
31 | namespace __cxxabiv1 { | |
32 | ||
33 | /// Type info for classes with no bases, and base class for type info for | |
34 | /// classes with bases. | |
35 | class __class_type_info : public std::type_info { | |
36 | virtual ~__class_type_info(); | |
37 | }; | |
38 | ||
39 | /// Type info for classes with simple single public inheritance. | |
40 | class __si_class_type_info : public __class_type_info { | |
41 | public: | |
42 | virtual ~__si_class_type_info(); | |
43 | ||
44 | const __class_type_info *__base_type; | |
45 | }; | |
46 | ||
47 | class __base_class_type_info { | |
48 | public: | |
49 | const __class_type_info *__base_type; | |
50 | long __offset_flags; | |
51 | ||
52 | enum __offset_flags_masks { | |
53 | __virtual_mask = 0x1, | |
54 | __public_mask = 0x2, | |
55 | __offset_shift = 8 | |
56 | }; | |
57 | }; | |
58 | ||
59 | /// Type info for classes with multiple, virtual, or non-public inheritance. | |
60 | class __vmi_class_type_info : public __class_type_info { | |
61 | public: | |
62 | virtual ~__vmi_class_type_info(); | |
63 | ||
64 | unsigned int flags; | |
65 | unsigned int base_count; | |
66 | __base_class_type_info base_info[1]; | |
67 | }; | |
68 | ||
69 | } | |
70 | ||
71 | namespace abi = __cxxabiv1; | |
72 | ||
73 | // We implement a simple two-level cache for type-checking results. For each | |
74 | // (vptr,type) pair, a hash is computed. This hash is assumed to be globally | |
75 | // unique; if it collides, we will get false negatives, but: | |
76 | // * such a collision would have to occur on the *first* bad access, | |
77 | // * the probability of such a collision is low (and for a 64-bit target, is | |
78 | // negligible), and | |
79 | // * the vptr, and thus the hash, can be affected by ASLR, so multiple runs | |
80 | // give better coverage. | |
81 | // | |
82 | // The first caching layer is a small hash table with no chaining; buckets are | |
83 | // reused as needed. The second caching layer is a large hash table with open | |
84 | // chaining. We can freely evict from either layer since this is just a cache. | |
85 | // | |
df77f0e4 KS |
86 | // FIXME: Make these hash table accesses thread-safe. The races here are benign: |
87 | // assuming the unsequenced loads and stores don't misbehave too badly, | |
88 | // the worst case is false negatives or poor cache behavior, not false | |
89 | // positives or crashes. | |
de5a5fa1 MP |
90 | |
91 | /// Find a bucket to store the given hash value in. | |
92 | static __ubsan::HashValue *getTypeCacheHashTableBucket(__ubsan::HashValue V) { | |
93 | static const unsigned HashTableSize = 65537; | |
df77f0e4 | 94 | static __ubsan::HashValue __ubsan_vptr_hash_set[HashTableSize]; |
de5a5fa1 | 95 | |
df77f0e4 KS |
96 | unsigned First = (V & 65535) ^ 1; |
97 | unsigned Probe = First; | |
de5a5fa1 MP |
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. | |
df77f0e4 | 107 | return &__ubsan_vptr_hash_set[First]; |
de5a5fa1 MP |
108 | } |
109 | ||
110 | /// A cache of recently-checked hashes. Mini hash table with "random" evictions. | |
111 | __ubsan::HashValue | |
df77f0e4 | 112 | __ubsan::__ubsan_vptr_type_cache[__ubsan::VptrTypeCacheSize]; |
de5a5fa1 MP |
113 | |
114 | /// \brief Determine whether \p Derived has a \p Base base class subobject at | |
115 | /// offset \p Offset. | |
3ea36418 | 116 | static bool isDerivedFromAtOffset(const abi::__class_type_info *Derived, |
de5a5fa1 MP |
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)) | |
3ea36418 | 124 | return isDerivedFromAtOffset(SI->__base_type, Base, Offset); |
de5a5fa1 MP |
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 & | |
3ea36418 JJ |
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)) | |
de5a5fa1 MP |
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. | |
3ea36418 JJ |
154 | static const abi::__class_type_info *findBaseAtOffset( |
155 | const abi::__class_type_info *Derived, sptr Offset) { | |
de5a5fa1 MP |
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)) | |
3ea36418 | 161 | return findBaseAtOffset(SI->__base_type, Offset); |
de5a5fa1 MP |
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 & | |
3ea36418 JJ |
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)) | |
de5a5fa1 MP |
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; | |
3ea36418 | 230 | if (!isDerivedFromAtOffset(Derived, Base, -Vtable->Offset)) |
de5a5fa1 MP |
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( | |
3ea36418 JJ |
244 | static_cast<const abi::__class_type_info*>(Vtable->TypeInfo), |
245 | -Vtable->Offset); | |
de5a5fa1 MP |
246 | return DynamicTypeInfo(Vtable->TypeInfo->__type_name, -Vtable->Offset, |
247 | ObjectType ? ObjectType->__type_name : "<unknown>"); | |
248 | } |