1 <html> 2 <head> 3 <title>Dalvik Bytecode Verifier Notes</title> 4 </head> 5 6 <body> 7 <h1>Dalvik Bytecode Verifier Notes</h1> 8 9 <p> 10 The bytecode verifier in the Dalvik VM attempts to provide the same sorts 11 of checks and guarantees that other popular virtual machines do. We 12 perform generally the same set of checks as are described in _The Java 13 Virtual Machine Specification, Second Edition_, including the updates 14 planned for the Third Edition. 15 16 <p> 17 Verification can be enabled for all classes, disabled for all, or enabled 18 only for "remote" (non-bootstrap) classes. It should be performed for any 19 class that will be processed with the DEX optimizer, and in fact the 20 default VM behavior is to only optimize verified classes. 21 22 23 <h2>Why Verify?</h2> 24 25 <p> 26 The verification process adds additional time to the build and to 27 the installation of new applications. It's fairly quick for app-sized 28 DEX files, but rather slow for the big "core" and "framework" files. 29 Why do it all, when our system relies on UNIX processes for security? 30 <p> 31 <ol> 32 <li>Optimizations. The interpreter can ignore a lot of potential 33 error cases because the verifier guarantees that they are impossible. 34 Also, we can optimize the DEX file more aggressively if we start 35 with a stronger set of assumptions about the bytecode. 36 <li>"Precise" GC. The work peformed during verification has significant 37 overlap with the work required to compute register use maps for 38 type-precise GC. 39 <li>Intra-application security. If an app wants to download bits 40 of interpreted code over the network and execute them, it can safely 41 do so using well-established security mechanisms. 42 <li>3rd party app failure analysis. We have no way to control the 43 tools and post-processing utilities that external developers employ, 44 so when we get bug reports with a weird exception or native crash 45 it's very helpful to start with the assumption that the bytecode 46 is valid. 47 </ol> 48 <p> 49 It's also a convenient framework to deal with certain situations, notably 50 replacement of instructions that access volatile 64-bit fields with 51 more rigorous versions that guarantee atomicity. 52 53 54 <h2>Verifier Differences</h2> 55 56 <p> 57 There are a few checks that the Dalvik bytecode verifier does not perform, 58 because they're not relevant. For example: 59 <ul> 60 <li>Type restrictions on constant pool references are not enforced, 61 because Dalvik does not have a pool of typed constants. (Dalvik 62 uses a simple index into type-specific pools.) 63 <li>Verification of the operand stack size is not performed, because 64 Dalvik does not have an operand stack. 65 <li>Limitations on <code>jsr</code> and <code>ret</code> do not apply, 66 because Dalvik doesn't support subroutines. 67 </ul> 68 69 In some cases they are implemented differently, e.g.: 70 <ul> 71 <li>In a conventional VM, backward branches and exceptions are 72 forbidden when a local variable holds an uninitialized reference. The 73 restriction was changed to mark registers as invalid when they hold 74 references to the uninitialized result of a previous invocation of the 75 same <code>new-instance</code> instruction. 76 This solves the same problem -- trickery potentially allowing 77 uninitialized objects to slip past the verifier -- without unduly 78 limiting branches. 79 </ul> 80 81 There are also some new ones, such as: 82 <ul> 83 <li>The <code>move-exception</code> instruction can only appear as 84 the first instruction in an exception handler. 85 <li>The <code>move-result*</code> instructions can only appear 86 immediately after an appropriate <code>invoke-*</code> 87 or <code>filled-new-array</code> instruction. 88 </ul> 89 90 <p> 91 The VM is permitted but not required to enforce "structured locking" 92 constraints, which are designed to ensure that, when a method returns, all 93 monitors locked by the method have been unlocked an equal number of times. 94 This is not currently implemented. 95 96 <p> 97 The Dalvik verifier is more restrictive than other VMs in one area: 98 type safety on sub-32-bit integer widths. These additional restrictions 99 should make it impossible to, say, pass a value outside the range 100 [-128, 127] to a function that takes a <code>byte</code> as an argument. 101 102 103 <h2>Verification Failures</h2> 104 105 <p> 106 The verifier may reject a class immediately, or it may defer throwing 107 an exception until the code is actually used. For example, if a class 108 attempts to perform an illegal access on a field, the VM should throw 109 an IllegalAccessError the first time the instruction is encountered. 110 On the other hand, if a class contains an invalid bytecode, it should be 111 rejected immediately with a VerifyError. 112 113 <p> 114 Immediate VerifyErrors are accompanied by detailed, if somewhat cryptic, 115 information in the log file. From this it's possible to determine the 116 exact instruction that failed, and the reason for the failure. 117 118 <p> 119 It's a bit tricky to implement deferred verification errors in Dalvik. 120 A few approaches were considered: 121 122 <ol> 123 <li>We could replace the invalid field access instruction with a special 124 instruction that generates an illegal access error, and allow class 125 verification to complete successfully. This type of verification must 126 be deferred to first class load, rather than be performed ahead of time 127 during DEX optimization, because some failures will depend on the current 128 execution environment (e.g. not all classes are available at dexopt time). 129 At that point the bytecode instructions are mapped read-only during 130 verification, so rewriting them isn't possible. 131 </li> 132 133 <li>We can perform the access checks when the field/method/class is 134 resolved. In a typical VM implementation we would do the check when the 135 entry is resolved in the context of the current classfile, but our DEX 136 files combine multiple classfiles together, merging the field/method/class 137 resolution results into a single large table. Once one class successfully 138 resolves the field, every other class in the same DEX file would be able 139 to access the field. This is incorrect. 140 </li> 141 142 <li>Perform the access checks on every field/method/class access. 143 This adds significant overhead. This is mitigated somewhat by the DEX 144 optimizer, which will convert many field/method/class accesses into a 145 simpler form after performing the access check. However, not all accesses 146 can be optimized (e.g. accesses to classes unknown at dexopt time), 147 and we don't currently have an optimized form of certain instructions 148 (notably static field operations). 149 </li> 150 </ol> 151 152 <p> 153 In early versions of Dalvik (as found in Android 1.6 and earlier), the verifier 154 simply regarded all problems as immediately fatal. This generally worked, 155 but in some cases the VM was rejecting classes because of bits of code 156 that were never used. The VerifyError itself was sometimes difficult to 157 decipher, because it was thrown during verification rather than at the 158 point where the problem was first noticed during execution. 159 <p> 160 The current version uses a variation of approach #1. The dexopt 161 command works the way it did before, leaving the code untouched and 162 flagging fully-correct classes as "pre-verified". When the VM loads a 163 class that didn't pass pre-verification, the verifier is invoked. If a 164 "deferrable" problem is detected, a modifiable copy of the instructions 165 in the problematic method is made. In that copy, the troubled instruction 166 is replaced with an "always throw" opcode, and verification continues. 167 168 <p> 169 In the example used earlier, an attempt to read from an inaccessible 170 field would result in the "field get" instruction being replaced by 171 "always throw IllegalAccessError on field X". Creating copies of method 172 bodies requires additional heap space, but since this affects very few 173 methods overall the memory impact should be minor. 174 175 <p> 176 <address>Copyright © 2008 The Android Open Source Project</address> 177 178 </body> 179 </html> 180
