EVM: bn256ScalarMul precompile (0x07) does not behave correctly when scalar=0

[raulk]

Original report

https://filecoinproject.slack.com/archives/CRK2LKYHW/p1675441024104129

Investigation

Our implementation of the ecMul precompile attempts to place the result of the multiplication (zero) on the alt_bn128 curve, which fails with an EcErr(InvalidEncoding) error. Instead, we should successfully return zero. Other implementations return zero.

Note that according to EIP-196, where this precompile was introduced, zero is a valid scalar argument.

[INFO] Call Precompile:
	address: 0000000000000000000000000000000000000007
	context: PrecompileContext { call_type: StaticCall, gas_limit: 9484863179, value: 0 }
	input: 035cf447ec2f8f21e6ea3d49d80a4a823834b1a776ab1733731587613f5065f821c972c4e0c8eb2430171599b1f4900601fdb8f4b2d248d22ebefe3d5368a80000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
[WARN] Precompile failed: error EcErr(InvalidEncoding)

Separately, we should investigate why the input sent to this precompile is longer than it needs to be (appears to include zero padding of some kind)?

Repro

filecoin-project/builtin-actors#1164

[raulk]

@Stebalien -- I am not well-versed in EC multiplication, so would appreciate your input. I'm not sure if it makes sense to shortcut and return 0 if the supplied point is valid and the scalar is zero.

[raulk]

Can use fvm-bench to reproduce with the attached contract.

fvm-bench --debug -b /path/to/builtin-actors-devnet.car /path/to/contract/Verifier.bin 8d1b7790 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

Verifier.bin.zip