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The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	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.
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	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.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	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
				49
				50	<h2>Verifier Differences</h2>
				51
				52	<p>
				53	There are a few checks that the Dalvik bytecode verifier does not perform,
				54	because they're not relevant. For example:
				55	<ul>
				56	<li>Type restrictions on constant pool references are not enforced,
				57	because Dalvik does not have a pool of typed constants. (Dalvik
				58	uses a simple index into type-specific pools.)
				59	<li>Verification of the operand stack size is not performed, because
				60	Dalvik does not have an operand stack.
				61	<li>Limitations on <code>jsr</code> and <code>ret</code> do not apply,
				62	because Dalvik doesn't support subroutines.
				63	</ul>
				64
				65	In some cases they are implemented differently, e.g.:
				66	<ul>
				67	<li>In a conventional VM, backward branches and exceptions are
				68	forbidden when a local variable holds an uninitialized reference. The
				69	restriction was changed to mark registers as invalid when they hold
				70	references to the uninitialized result of a previous invocation of the
				71	same <code>new-instance</code> instruction.
				72	This solves the same problem -- trickery potentially allowing
				73	uninitialized objects to slip past the verifier -- without unduly
				74	limiting branches.
				75	</ul>
				76
				77	There are also some new ones, such as:
				78	<ul>
				79	<li>The <code>move-exception</code> instruction can only appear as
				80	the first instruction in an exception handler.
				81	<li>The <code>move-result*</code> instructions can only appear
				82	immediately after an appropriate <code>invoke-*</code>
				83	or <code>filled-new-array</code> instruction.
				84	</ul>
				85
				86	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	87	The VM is permitted but not required to enforce "structured locking"
				88	constraints, which are designed to ensure that, when a method returns, all
				89	monitors locked by the method have been unlocked an equal number of times.
				90	This is not currently implemented.
				91
				92	<p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	93	The Dalvik verifier is more restrictive than other VMs in one area:
				94	type safety on sub-32-bit integer widths. These additional restrictions
				95	should make it impossible to, say, pass a value outside the range
				96	[-128, 127] to a function that takes a <code>byte</code> as an argument.
				97
				98
				99	<h2>Verification Failures</h2>
				100
				101	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	102	The verifier may reject a class immediately, or it may defer throwing
				103	an exception until the code is actually used. For example, if a class
				104	attempts to perform an illegal access on a field, the VM should throw
				105	an IllegalAccessError the first time the instruction is encountered.
				106	On the other hand, if a class contains an invalid bytecode, it should be
				107	rejected immediately with a VerifyError.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	108
				109	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	110	Immediate VerifyErrors are accompanied by detailed, if somewhat cryptic,
				111	information in the log file. From this it's possible to determine the
				112	exact instruction that failed, and the reason for the failure.
				113
				114	<p>
				115	It's a bit tricky to implement deferred verification errors in Dalvik.
				116	A few approaches were considered:
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	117
				118	<ol>
				119	<li>We could replace the invalid field access instruction with a special
				120	instruction that generates an illegal access error, and allow class
				121	verification to complete successfully. This type of verification must
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	122	be deferred to first class load, rather than be performed ahead of time
				123	during DEX optimization, because some failures will depend on the current
				124	execution environment (e.g. not all classes are available at dexopt time).
				125	At that point the bytecode instructions are mapped read-only during
				126	verification, so rewriting them isn't possible.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	127	</li>
				128
				129	<li>We can perform the access checks when the field/method/class is
				130	resolved. In a typical VM implementation we would do the check when the
				131	entry is resolved in the context of the current classfile, but our DEX
				132	files combine multiple classfiles together, merging the field/method/class
				133	resolution results into a single large table. Once one class successfully
				134	resolves the field, every other class in the same DEX file would be able
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	135	to access the field. This is incorrect.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	136	</li>
				137
				138	<li>Perform the access checks on every field/method/class access.
				139	This adds significant overhead. This is mitigated somewhat by the DEX
				140	optimizer, which will convert many field/method/class accesses into a
				141	simpler form after performing the access check. However, not all accesses
				142	can be optimized (e.g. accesses to classes unknown at dexopt time),
				143	and we don't currently have an optimized form of certain instructions
				144	(notably static field operations).
				145	</li>
				146	</ol>
				147
				148	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	149	In early versions of Dalvik (as shipped with Android 1.0/1.5), the verifier
				150	simply regarded all problems as immediately fatal. This generally worked,
				151	but in some cases the VM was rejecting classes because of bits of code
				152	that were never used. The VerifyError itself was sometimes difficult to
				153	decipher, because it was thrown during verification rather than at the
				154	point where the problem was first noticed during execution.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	155	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	156	The current version uses a variation of approach #1. The dexopt
				157	command works the way it did before, leaving the code untouched and
				158	flagging fully-correct classes as "pre-verified". When the VM loads a
				159	class that didn't pass pre-verification, the verifier is invoked. If a
				160	"deferrable" problem is detected, a modifiable copy of the instructions
				161	in the problematic method is made. In that copy, the troubled instruction
				162	is replaced with an "always throw" opcode, and verification continues.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	163
				164	<p>
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	165	In the example used earlier, an attempt to read from an inaccessible
				166	field would result in the "field get" instruction being replaced by
				167	"always throw IllegalAccessError on field X". Creating copies of method
				168	bodies requires additional heap space, but since this affects very few
				169	methods overall the memory impact should be minor.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	170
Andy McFadden	0df4413	2009-07-29 16:07:01 -0700	[diff] [blame^]	171	<p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	172	<address>Copyright © 2008 The Android Open Source Project</address>
				173
				174	</body>
				175	</html>