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<title>Dalvik Bytecode Verifier Notes</title>
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<h1>Dalvik Bytecode Verifier Notes</h1>

<p>
The bytecode verifier in the Dalvik VM attempts to provide the same sorts
of checks and guarantees that other popular virtual machines do.  We
perform generally the same set of checks as are described in _The Java
Virtual Machine Specification, Second Edition_, including the updates
planned for the Third Edition.

<p>
Verification can be enabled for all classes, disabled for all, or enabled
only for "remote" (non-bootstrap) classes.  It should be performed for any
class that will be processed with the DEX optimizer, and in fact the
default VM behavior is to only optimize verified classes.


<h2>Why Verify?</h2>

<p>
The verification process adds additional time to the build and to
the installation of new applications.  It's fairly quick for app-sized
DEX files, but rather slow for the big "core" and "framework" files.
Why do it all, when our system relies on UNIX processes for security?
<p>
<ol>
    <li>Optimizations.  The interpreter can ignore a lot of potential
    error cases because the verifier guarantees that they are impossible.
    Also, we can optimize the DEX file more aggressively if we start
    with a stronger set of assumptions about the bytecode.
    <li>"Precise" GC.  The work peformed during verification has significant
    overlap with the work required to compute register use maps for
    type-precise GC.
    <li>Intra-application security.  If an app wants to download bits
    of interpreted code over the network and execute them, it can safely
    do so using well-established security mechanisms.
    <li>3rd party app failure analysis.  We have no way to control the
    tools and post-processing utilities that external developers employ,
    so when we get bug reports with a weird exception or native crash
    it's very helpful to start with the assumption that the bytecode
    is valid.
</ol>
<p>
It's also a convenient framework to deal with certain situations, notably
replacement of instructions that access volatile 64-bit fields with
more rigorous versions that guarantee atomicity.


<h2>Verifier Differences</h2>

<p>
There are a few checks that the Dalvik bytecode verifier does not perform,
because they're not relevant.  For example:
<ul>
    <li>Type restrictions on constant pool references are not enforced,
    because Dalvik does not have a pool of typed constants.  (Dalvik
    uses a simple index into type-specific pools.)
    <li>Verification of the operand stack size is not performed, because
    Dalvik does not have an operand stack.
    <li>Limitations on <code>jsr</code> and <code>ret</code> do not apply,
    because Dalvik doesn't support subroutines.
</ul>

In some cases they are implemented differently, e.g.:
<ul>
    <li>In a conventional VM, backward branches and exceptions are
    forbidden when a local variable holds an uninitialized reference.  The
    restriction was changed to mark registers as invalid when they hold
    references to the uninitialized result of a previous invocation of the
    same <code>new-instance</code> instruction.
    This solves the same problem -- trickery potentially allowing
    uninitialized objects to slip past the verifier -- without unduly
    limiting branches.
</ul>

There are also some new ones, such as:
<ul>
    <li>The <code>move-exception</code> instruction can only appear as
    the first instruction in an exception handler.
    <li>The <code>move-result*</code> instructions can only appear
    immediately after an appropriate <code>invoke-*</code>
    or <code>filled-new-array</code> instruction.
</ul>

<p>
The VM is permitted but not required to enforce "structured locking"
constraints, which are designed to ensure that, when a method returns, all
monitors locked by the method have been unlocked an equal number of times.
This is not currently implemented.

<p>
The Dalvik verifier is more restrictive than other VMs in one area:
type safety on sub-32-bit integer widths.  These additional restrictions
should make it impossible to, say, pass a value outside the range
[-128, 127] to a function that takes a <code>byte</code> as an argument.


<h2>Monitor Verification</h2>

<p>
If a method locks an object with a <code>synchronized</code> statement, the
object must be unlocked before the method returns.  At the bytecode level,
this means the method must execute a matching <code>monitor-exit</code>
for every <code>monitor-enter</code> instruction, whether the function
completes normally or abnormally.  The bytecode verifier optionally
enforces this.

<p>
The verifier uses a fairly simple-minded model.  If you enter a monitor
held in register N, you can exit the monitor using register N or any
subsequently-made copies of register N.  The verifier does not attempt
to identify previously-made copies, track loads and stores through
fields, or recognize identical constant values (for example, the result
values from two <code>const-class</code> instructions on the same class
will be the same reference, but the verifier doesn't recognize this).

<p>
Further, you may only exit the monitor most recently entered.  "Hand
over hand" locking techniques, e.g. "lock A; lock B; unlock A; unlock B",
are not allowed.

<p>
This means that there are a number of situations in which the verifier
will throw an exception on code that would execute correctly at run time.
This is not expected to be an issue for compiler-generated bytecode.

<p>
For implementation convenience, the maximum nesting depth of
<code>synchronized</code> statements has been set to 32.  This is not
a limitation on the recursion count.  The only way to trip this would be
to have a single method with more than 32 nested <code>synchronized</code>
statements, something that is unlikely to occur.


<h2>Verification Failures</h2>

<p>
The verifier may reject a class immediately, or it may defer throwing
an exception until the code is actually used.  For example, if a class
attempts to perform an illegal access on a field, the VM should throw
an IllegalAccessError the first time the instruction is encountered.
On the other hand, if a class contains an invalid bytecode, it should be
rejected immediately with a VerifyError.

<p>
Immediate VerifyErrors are accompanied by detailed, if somewhat cryptic,
information in the log file.  From this it's possible to determine the
exact instruction that failed, and the reason for the failure.

<p>
It's a bit tricky to implement deferred verification errors in Dalvik.
A few approaches were considered:

<ol>
<li>We could replace the invalid field access instruction with a special
instruction that generates an illegal access error, and allow class
verification to complete successfully.  This type of verification must
be deferred to first class load, rather than be performed ahead of time
during DEX optimization, because some failures will depend on the current
execution environment (e.g. not all classes are available at dexopt time).
At that point the bytecode instructions are mapped read-only during
verification, so rewriting them isn't possible.
</li>

<li>We can perform the access checks when the field/method/class is
resolved.  In a typical VM implementation we would do the check when the
entry is resolved in the context of the current classfile, but our DEX
files combine multiple classfiles together, merging the field/method/class
resolution results into a single large table.  Once one class successfully
resolves the field, every other class in the same DEX file would be able
to access the field.  This is incorrect.
</li>

<li>Perform the access checks on every field/method/class access.
This adds significant overhead.  This is mitigated somewhat by the DEX
optimizer, which will convert many field/method/class accesses into a
simpler form after performing the access check.  However, not all accesses
can be optimized (e.g. accesses to classes unknown at dexopt time),
and we don't currently have an optimized form of certain instructions
(notably static field operations).
</li>
</ol>

<p>
In early versions of Dalvik (as found in Android 1.6 and earlier), the verifier
simply regarded all problems as immediately fatal.  This generally worked,
but in some cases the VM was rejecting classes because of bits of code
that were never used.  The VerifyError itself was sometimes difficult to
decipher, because it was thrown during verification rather than at the
point where the problem was first noticed during execution.
<p>
The current version uses a variation of approach #1.  The dexopt
command works the way it did before, leaving the code untouched and
flagging fully-correct classes as "pre-verified".  When the VM loads a
class that didn't pass pre-verification, the verifier is invoked.  If a
"deferrable" problem is detected, a modifiable copy of the instructions
in the problematic method is made.  In that copy, the troubled instruction
is replaced with an "always throw" opcode, and verification continues.

<p>
In the example used earlier, an attempt to read from an inaccessible
field would result in the "field get" instruction being replaced by
"always throw IllegalAccessError on field X".  Creating copies of method
bodies requires additional heap space, but since this affects very few
methods overall the memory impact should be minor.

<p>
<address>Copyright &copy; 2008 The Android Open Source Project</address>

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