Refactor memory manipulation in evm #442

s1na · 2019-02-20T17:10:01Z

This PR:

Introduces the Memory class used for evm memory manipulation
Removes memStore and memLoad and moves the logic to opcode handlers. The goal is to increase verbosity and reduce code hidden in utility functions
In some opcodes, it reduces copy gas before attempting memory write
The test cases ExtCodeCopyTargetRangeLongerThanCodeTests and codeCopyZero which were skipped previously have been fixed and are no longer skipped
This is part of the same effort as in Refactor evm and prepare for ewasm integration #424 and Basic support for ewasm precompiles #431.

Similar to before, offset and length are cast from BN to normal JS numbers, which means if they're bigger than 53 bits BN throws an exception. Gas limit should probably cap how bug offset and length can become, but it might be better to address this issue somehow in future. The only solution that came to my mind is to use a Map instead of a normal array for memory, and to store indices as a Buffer which can be arbitrary long. But I imagine this would be less performant.

Guard mem read against zero length Fix saving call return data to memory Minor improvements Convert offset and length to number before mem write

holgerd77 · 2019-02-21T09:00:14Z

Will directly take a look here once after we settled out the Blockchain stuff (probably we more or less have).

holgerd77 · 2019-02-21T10:36:14Z

We have memory exposed in the the step event. This should probably be changed to runState.memory._store (or some getter) in runStepHook (this is equivalent to the old runState.memory, do I get this right?) to not introduce some breaking change here.

holgerd77 · 2019-02-21T10:46:52Z

Since this is now so beautifully isolated, can you add some few simple tests for extend, write and read? Think this would be worth it, also to have some more explicit usage examples where devs can more easily see how memory works.

Should be ok to add these to the API suite though strictly speaking no (public) API.

s1na · 2019-02-21T11:52:39Z

Both good points, updated.

coveralls · 2019-02-21T11:58:10Z

Coverage increased (+0.1%) to 89.272% when pulling 14a3505 on refactor/vm-mem into b03957c on master.

holgerd77

I could only grab after the review how much of an improvement this actually is, thanks Sina for this, great! 😄 👍

Some smaller questions.

holgerd77 · 2019-02-21T11:26:51Z

lib/vm/memory.js

+    }
+    return Buffer.from(loaded)
+  }
+}


Ok, equivalent to the old memLoad function, subMemUsage and zero check extracted to opcodes.

Second note: signature changed from Buffer to Number for offset and size, but better to have this explicit here than having this inner conversion, good.

holgerd77 · 2019-02-21T11:29:30Z

lib/vm/opFns.js

+    let data = Buffer.alloc(0)
+    if (!length.isZero()) {
+      data = runState.memory.read(offset.toNumber(), length.toNumber())
+    }


holgerd77 · 2019-02-21T11:38:01Z

lib/vm/opFns.js

@@ -252,17 +256,27 @@ module.exports = {
    }
  },
  CALLDATACOPY: function (memOffset, dataOffset, dataLength, runState) {
-    memStore(runState, memOffset, runState.callData, dataOffset, dataLength)
-    // sub the COPY fee
+    subMemUsage(runState, memOffset, dataLength)


Had a really hard time to understand why this is now included until I understood that the following code from the old memStore is actually evaluating to false by skipSubMem being undefined when not passed to the function:

function memStore (runState, offset, val, valOffset, length, skipSubMem) { if (skipSubMem !== false) { subMemUsage(runState, offset, length) } //... }

So so glad that we get this out here. Phew. 😄

holgerd77 · 2019-02-21T11:44:08Z

lib/vm/opFns.js

-function memStore (runState, offset, val, valOffset, length, skipSubMem) {
-  if (skipSubMem !== false) {
-    subMemUsage(runState, offset, length)
+function getDataSlice (offset, length, data) {


My intuition here would be to have the parameter sorting the other way around, so function getDataSlice (data, offset, length).

Yeah, that makes more sense, updated.

holgerd77 · 2019-02-21T11:55:14Z

lib/vm/opFns.js

+    memOffset = memOffset.toNumber()
+    dataLength = dataLength.toNumber()
+    runState.memory.extend(memOffset, dataLength)
+    runState.memory.write(memOffset, dataLength, data)


Just some exemplary analysis for my own understanding: the old memStore functionality is separated into getDataSlice (taking the valOffset from the memStore signature) and then the extend and write functionality.

This makes things much more explicit by breaking up the old memStore functionality which is actually currently to a greater extend unreadable and pretty hard to decipher.

👍

holgerd77 · 2019-02-21T12:01:36Z

lib/vm/opFns.js

@@ -576,7 +617,8 @@ module.exports = {
      trap(ERROR.STATIC_STATE_CHANGE)
    }

-    var data = memLoad(runState, inOffset, inLength)
+    subMemUsage(runState, inOffset, inLength)
+    const data = runState.memory.read(inOffset.toNumber(), inLength.toNumber())


Why are these zero checks not necessary here?

Good question, because no test failed for them 😅

Fixed all reads that had no length check, thanks for catching!

holgerd77 · 2019-02-21T12:01:50Z

lib/vm/opFns.js

+    let data = Buffer.alloc(0)
+    if (!inLength.isZero()) {
+      data = runState.memory.read(inOffset.toNumber(), inLength.toNumber())
+    }


holgerd77 · 2019-02-21T12:02:14Z

lib/vm/opFns.js

@@ -728,7 +775,8 @@ module.exports = {
    var value = new BN(0)
    toAddress = addressToBuffer(toAddress)

-    var data = memLoad(runState, inOffset, inLength)
+    subMemUsage(runState, inOffset, inLength)
+    const data = runState.memory.read(inOffset.toNumber(), inLength.toNumber())


Same questions here.

holgerd77 · 2019-02-21T12:02:30Z

lib/vm/opFns.js

+    runState.returnValue = Buffer.alloc(0)
+    if (!length.isZero()) {
+      runState.returnValue = runState.memory.read(offset.toNumber(), length.toNumber())
+    }


holgerd77 · 2019-02-21T12:03:20Z

lib/vm/opFns.js

+        const dataLength = localOpts.outLength.toNumber()
+        runState.memory.extend(memOffset, dataLength)
+        runState.memory.write(memOffset, dataLength, data)
+      }


holgerd77

So this generally looks really good, have gone through all the code. Since this is also highly covered by the consensus test suite it should be ok to merge once last things are addressed.

holgerd77 · 2019-02-21T12:07:08Z

tests/api/vm/memory.js

+    st.throws(() => m.write(0, 5, Buffer.from([8, 8, 8])), /size/)
+    st.end()
+  })
+})


Cool, thanks! 😄

holgerd77

Looks good.

axic · 2019-02-26T12:00:18Z

lib/vm/opFns.js

@@ -587,7 +587,10 @@ module.exports = {
    }

    subMemUsage(runState, offset, length)
-    const data = runState.memory.read(offset.toNumber(), length.toNumber())
+    let data = Buffer.alloc(0)
+    if (!length.isZero()) {


Why would it be a problem allowing reading 0 length?

Checking for 0 length was being done in memLoad. I tried omitting it, but faced a problem: tests with an offset wider than 53 bits would fail (BN.toNumber() would throw). In these test cases the length is 0 (as even length 1 would need a lot of gas I think). So I found this check is basically a way to make the tests pass! I'm thinking about how we can improve on this.

s1na added 2 commits February 20, 2019 18:01

Add memory class for evm, use in opFns

2e2c817

Guard mem read against zero length Fix saving call return data to memory Minor improvements Convert offset and length to number before mem write

Re-add two skipped tests

362edb0

s1na added 2 commits February 21, 2019 12:51

runCode: return memory._store on step hook

8ae5df0

test: add basic unit tests for vm/memory

c45b02e

holgerd77 requested changes Feb 21, 2019

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holgerd77 reviewed Feb 21, 2019

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tests/api/vm/memory.js

st.throws(() => m.write(0, 5, Buffer.from([8, 8, 8])), /size/)

st.end()

})

})

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Cool, thanks! 😄

s1na added 2 commits February 21, 2019 13:57

opFns: guard mem reads against zero length

c02adf1

opFns: change getDataSlice parameter order

14a3505

holgerd77 approved these changes Feb 21, 2019

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holgerd77 merged commit d0ad8d1 into master Feb 21, 2019

holgerd77 deleted the refactor/vm-mem branch February 21, 2019 13:28

axic reviewed Feb 26, 2019

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holgerd77 mentioned this pull request Feb 28, 2019

Major Version Release Planning / Refactor Strategy / v3 and v4 Releases #455

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s1na mentioned this pull request Mar 3, 2019

MetaVM: initial base class for an extensible EVM #441

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evertonfraga added the package: vm label Feb 27, 2020

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Refactor memory manipulation in evm #442

Refactor memory manipulation in evm #442

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Refactor memory manipulation in evm #442

Refactor memory manipulation in evm #442

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s1na commented Feb 20, 2019 •

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