Daneel: Type inference for Dalvik bytecode
In the last blog post about Daneel I mentioned one particular caveat of Dalvik bytecode, namely the existence of untyped instructions, which has a huge impact on how we transform bytecode. I want to take a similar approach as last time and look at one specific example to illustrate those implications. So let us take a look at the following Java method.
public float untyped(float[] array, boolean flag) {
if (flag) {
float delta = 0.5f;
return array[7] + delta;
} else {
return 0.2f;
}
}
The above is a straightforward snippet and most of you probably know how the generated Java bytecode will look like. So let’s jump right to the Dalvik bytecode and discuss that in detail.
UntypedSample.untyped:([FZ)F: [regs=5, ins=3, outs=0] 0000: if-eqz v4, 0009 0002: const/high16 v0, #0x3f000000 0004: const/4 v1, #0x7 0005: aget v1, v3, v1 0007: add-float/2addr v0, v1 0008: return v0 0009: const v0, #0x3e4ccccd 000c: goto 0008
Keep in mind that Daneel doesn’t like to remember things, so he wants to look through the code just once from top to bottom and emit Java bytecode while doing so. He gets really puzzled at certain points in the code.
- Label 2: What is the type of register
v0? - Label 4: What is the type of register
v1? - Label 9: Register
v0again? What’s the type at this point?
You, as a reader, do have the answer because you know and understand the semantic of the underlying Java code, but Daneel doesn’t, so he tries to infer the types. Let’s look through the code in the same way Daneel does.
At method entry he knows about the types of method parameters. Dalvik passes parameters in the last registers (in this case in v3 and v4). Also we have a register (in this case v2) holding a this reference. So we start out with the following register types at method entry.
UntypedSample.untyped:([FZ)F: [regs=5, ins=3, outs=0] uninit uninit object [float bool
The array to the right represents the inferred register types at each point in the instruction stream as determined by the abstract interpreter. Note that we also have to keep track of the dimension count and the element type for array references. Now let’s look at the first block of instructions.
0002: const/high16 v0, #0x3f000000 u32 uninit object [float bool 0004: const/4 v1, #0x7 u32 u32 object [float bool 0005: aget v1, v3, v1 u32 float object [float bool 0007: add-float/2addr v0, v1 float float object [float bool
Each line shows the register type after the instruction has been processed. At each line Daneel learns something new about the register types.
- Label 2: I don’t know the type of
v0, only that it holds an untyped 32-bit value. - Label 4: Same applies for
v1here, it’s an untyped 32-bit value as well. - Label 5: Now I know
v1is used as an array index, it must have been an integer value. Also the array reference in registerv3is accessed, so I know the result is a float value. The result is stored inv1, overwriting it’s previous content. - Label 7: Now I know
v0is used in a floating-point addition, it must have been a float value.
Keep in mind that at each line, Daneel emits appropriate Java bytecode. So whenever he learns the concrete type of a register, he might need to retroactively patch previously emitted instructions, because some of his assumptions about the type were broken.
Finally we look at the second block of instructions reached through the conditional branch as part of the if-statement.
0009: const v0, #0x3e4ccccd u32 uninit object [float bool 000c: goto 0008 float uninit object [float bool
When reaching this block we basically have the same information as at method entry. Again Daneel learns in the process.
- Label 9: I don’t know the type of
v0, only that it holds an untyped 32-bit value. - Label 12: Now I know that
v0has to be a float value because the unconditional branch targets the join-point at label 8. And I already looked at that code and know that we expect a float value in that register at that point.
This illustrates why our abstract interpreter also has to remember and merge register type information at each join-point. It’s important to keep in mind that Daneel follows the instruction stream from top to bottom, as opposed to the control-flow of the code.
Now imagine scrambling up the code so that instruction stream and control-flow are vastly different from each other, together with a few exception handlers and an optimal register re-usage as produced by some SSA representation. That’s where Daneel still keeps choking at the moment. But we can handle most of the code produced by the dx tool already and will hunt down all those nasty bugs triggered by obfuscated code as well.
Disclaimer: The abstract interpreter and the method rewriter were mostly written by Rémi Forax, with this post I take no credit for it’s implementation whatsoever, I just want to explain how it works.
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