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@0xkarmacoma
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recovering skeptic // research engineer @a16zcrypto
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Last Seen Profiles
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Here are the main points from my Solidity Summit talk:
Symbolic execution is a generic technique that can be used to explore paths in your program
Symbolic execution treats your program like a dungeon in an RPG: it explores everything until it finds and defeats the boss.
The…
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Let's build a smol EVM from scratch:
- stack
- memory
- code buffer
- a few instructions
- an instruction decoder
- a basic fetch-decode-eval loop
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A bit of exciting personal news 😁
I accepted an offer as a research engineer at ✨
@a16zcrypto
✨
I'm super excited to work with
@eddylazzarin
's incredible research, engineering and security teams
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Let's build a smol EVM from scratch, part 2:
- branching instructions
- looping and conditionals
- jump destination validation
- stack manipulation instructions
This part is brought to you by
@MilkRoadDaily
✌️
🟩🟩🟩🟩🟩🟨⬜⬜⬜⬜ 77/142 instructions
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Quick cheat sheet on calldata vs memory function args
- calldata args are immutable and more gas efficient
- default to calldata args unless you _need_ to mutate the args
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⛽ fun way to run out of gas
It attempts to store a 1 bit "very far" in memory (not(0) is 0xffff...ffff), and the memory expansion function has exponential cost, so this consumes trillions of gas units
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Introducing SS2ERC721, a new base contract that allows:
- minting up to 1200 NFTs to distinct recipients per tx
- with >90% gas savings compared to a regular ERC721 (10-12x cheaper) 🤌
How? By using SSTORE2 for the primary recipients of the drop!
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Corruptions by
@dhof
have a certain number of iterations that "deep-fry" the rendering.
4139 has 0 iterations (cleanest output)
808 has 1023 iterations (the max)
Sheet with the iterations for all token ids:
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I had an amazing time working on distributed systems at
@NetflixEng
and I will be forever grateful to the great people there, but it is time for me to start a new adventure.
Today is my first day as a smart contract engineer at
@tryShowtime
and I couldn't be more excited 🙌💖
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Let's build a smol EVM from scratch, part 3:
- calldata
- comparison instructions
- the Solidity function dispatcher
- a kickass script that uses hooks into our interpreter
This part is brought to you by
@MilkRoadDaily
✌️
🟩🟩🟩🟩🟩🟩⬜⬜⬜⬜ 85/142 instructions
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I can't stand on the sidelines anymore while billions are lost to reentrancy attacks. I had to do my part.
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we had fun didn't we
1⃣ Unchecked loop increments:
This version release adds an overflow check optimization that auto-generates unchecked arithmetic increment of 'for' loop counters.
This new optimization removes the need for unchecked increment patterns in for loop bodies.
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I hope this gist about the different ways to send Ether in Solidity finds you well
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A look at the tricks used by
@samczsun
and the contract that beat him:
- tiny handcoded EVM-level contract, no Solidity
- no function selector (a single function implements both owner() and solve() based on calldatasize 🤯)
- no storage
- optimal memory size (3 words)
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I might mess around and take a snapshot of my followers with a .eth address 👀
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POV: you use formal methods to show the teacher that the homework has NO SOLUTION
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I'm a simple guy, all I want is an EVM memory profiler that looks like defrag.exe. Who is working on this?
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I have given my 2 week notice at my web2 job, and I'm going web3 full time 🌈
Let's play a game! This contract on Goerli lets you guess what web3 company I'll be joining 👀
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🤏 smol deep dive into EVM disassembly, in which we learn to navigate metadata and why bytecode '6142' breaks most disassemblers
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I feel bad solving this CTF with Halmos, I am number 2 on the leaderboard yet I had no idea that this was a sudoku.
It feels like the meta will change and CTFs will adjust to avoid being trivially solvable by automated tooling. In the mean time, let me show you how this works
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I come across a lot of devs who haven't setup git commit signing. It takes 2 min, you should do it.
If you don't, anybody can impersonate you on github.
If you don't believe me, reply with your github account below and I'll make a silly commit as you 👇
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So here I am, playing
@fvictorio_nan
's EVM puzzle and I reach a level that includes an XOR to find the right jump value. What am I supposed to do, XOR in my head? No way, mhh mhh
Let's use
@daejunpark
's Halmos to do the work for us instead.
Time to go ✨symbolic✨
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*slaps roof of Seaport* this bad boy can fit so much contract code in it
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Solidity people are doing crazy gas optimizations
Meanwhile some frontends still suggest transactions with a max priority fee set to max fee
Example:
- this user could have paid 68.2 base fee + 1.5 priority fee = 69.7 gwei
- instead they paid 162.1 gwei, 2.3x too high
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I'm starting to think that the market is not demanding formal verification this cycle
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I got tired of stringing together cast commands so I came up with this as a poor man's Solidity REPL 🙃 Feelin' dirty lol
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Why you need to pay attention to
@darkforest_eth
, the most fascinating blockchain game by far
🧵
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The Unreasonable Effectiveness of Assembly Code
Check out this algorithm for escaping json strings by
@optimizoor
:
- O(n)
- no spurious memory copy
- only 1 (one) memory allocation
- actually respects the json spec by escaping U+0000 to U+001F
Just look at it 🤌
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Ever wondered what values like msg.sender and tx.gasprice are when you're in a view function?
TLDR they can be anything when they're evaluated off-chain, and indeed they vary wildly by provider. Which opens up the possibility to fingerprint when we're evaluated off-chain (🧵)
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they don't want you to know this but the key to successful smart contract development is actually to make your contracts as dumb as possible
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gas golfing is a self-indulgent pleasure that doesn’t matter nearly as much as people think
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this will create an unresolved tension that will stay with you all day
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Formal verification to the people
There is a huge opportunity for applying formal methods to smart contracts:
- smart contracts are orders of magnitude less complex than traditional software (no concurrency, exceptions, small ISA, small memory footprint, no network...)
- bugs…
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Halmos v0.1.0 is out, and it’s a big release focused on performance and developer experience
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@RaoulGMI
I love that a leader in this space such as yourself is able to:
1. Say I don’t know
2. Stay humble
3. Stay curious
Definitely GMI 👏
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✨ halmos v0.1.10 release ✨
- shared solver instance during path exploration: this is a big one that solves the memory explosion problem many people reported
- --match-test and --match-contract options by
@agfviggiano
🫡
- friendship ended with python 3.8
- not just PASS/FAIL…
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Love the focus on testing at this year's Solidity Summit
- dos and don'ts of testing by
@lovethewired
- BTT structured tests by
@PaulRBerg
- differential testing by
@annascarroll
- mutation testing by
@ChandrakanaNaN
- symbolic testing by
@daejunpark
and me
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oh you're into solidity? name the unspecified evaluation order rules for event parameters
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a study of return bombing
- how to write an effective bomb
- how to safely process the revert data
@ricocreditsys
@0xkarmacoma
If you do a raw call and was to use the result, all the returndata is copied to memory. This makes you vulnerable to returnbombing.
AFAIK, try catch limits the returndata size if the return type is « bonded ».
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Is this a thing that exists?
- input: EVM instructions
- output: annotated EVM instructions with what the stack layout would look like
Right now I'm annotating by hand like an animal, but I could probably build this into smol-evm
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If I see SafeMath along with solc 0.8, I assume one of these:
- you copied old code and just bumped solc
- you don't know about checked math in recent solc
- you know about checked math but think it's good to be extra safe
They're all bad reasons
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I had no idea that a deep tech topic such as writing a smol EVM from scratch would resonate with so many people 💖
How are you all liking Notion as the publishing platform?
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men would rather printf-debug on mainnet than go to therapy
2.4 million dollars invested into a pre-sale using a contract where the developer:
1. Didn't even bother removing commented lines of code
2. Is literally console.logging multiple times in most functions on mainnet.
We're so back.
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any idea why in Solidity we can define errors either at the top level or inside a contract, but events only inside contracts?
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mad science 👨🔬
minting 1M NFTs to distinct recipients from an open edition
814 transactions
6.35k gas/mint
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I'll be talking about symbolic execution at the Solidity Summit in Istanbul 🥳
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Food for thought:
- mining addresses with 4 leading 0 bytes is cheap
- can be stored as bytes16
- can pack 2 in the same storage slot
Any contract doing this in the wild?
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EnterpriseCounters.sol
- on-chain Inversion-of-Control (IoC) containers
- default increment strategy with built-in retries
- swappable storage backends
- XML-based configuration
- access control
- logging
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halmos v0.1.11
Adds support for
- the ecrecover precompile
- the vm.addr(key) and vm.sign(key, digest) cheats
- the makeAddr functions from forge-std
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What's next? I will support the engineers and founders at
@a16zcrypto
portfolio companies and turn research into usable open source tools for the working smart contract engineer.
Can't wait to get started!
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TIL 0xEF is considered an invalid code prefix, and trying to create code that starts with 0xEF will revert (EIP-3541)
AFAIK all SSTORE2 implementations (OG/solmate/solady) prefix the data with 0x00 so should be safe
beware if you roll your own code-as-data solution though 🐉
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Here is a fun little test that shows a fundamental difference between fuzzing and symbolic testing.
The test accepts two inputs x and y, and checks that if x and y are different then their keccak256 hashes don’t collide.
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@saxenism
Can we put this idea to rest that ++i is faster than i++?
With via_ir = true, they literally compile to the same thing
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Can be solved entirely with the foundry stack:
- cast call to retrieve values on chain
- chisel to twiddle bits
- cast create2 to find interesting addresses
- forge script to deploy
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writing tests that can be run by both foundry and halmos takes a little bit of practice but is very satisfying:
- for the postconditions, assert() > require()
- for the preconditions, vm.assume() > bound()
- use test_ as a prefix
- avoid other reverts other than the assertions
💡Easy Stateless Fuzz & Symbolic Test💡
Foundry + Halmos => use the same test contract for both stateless fuzz & symbolic tests.
Here is an example from my current private audit where I'm using Halmos to verify a hashing algo produces unique results for the possible input set:
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self-own but true: all the best devs I have ever worked with have like 137 followers on here
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✅ Shipped my first contract in a product
- non-upgradable
- on-chain sell orders
- non-escrow (never takes ownership of your NFTs or ERC20s)
- hardcoded 0% commission
- supports EIP-2981 royalties
- supports payments in multiple ERC20 currencies
Gm everybody! Today, we’re announcing our NFT marketplace at .
✦ Gasless: free minting + listing, no gas fees
✦ Instant royalties via an on-chain royalty standard
✦ Up to 10,000 editions
✦ Eco-friendly NFTs on the
@0xPolygon
network
Live now. More:
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335
1K
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reverse optimizors: is this the highest gas per byte of code ratio we can possibly get?
465f0351 would consume 2.5e+148 gas on mainnet with just 4 bytes
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Sometimes it feels silly adding tests like “only the owner should be able to call X()”, but is it?
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boss: you need to tweet more relatable memes
me: you're not gonna believe this
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Today is my last day at
@Showtime_xyz
and I couldn't be more proud of the incredibly user-friendly web3 infra we built over the last 18 months. Thank you
@AlexMasmej
for betting on me as a smart contract eng and being the most wholesome and driven founder in web3!
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I'll meet you at the intersection of research and engineering (っ◔◡◔)っ ♥
Introducing our 2nd round of speakers for the Solidity Summit!
Shoutout to
@lovethewired
,
@annascarroll
,
@PaulRBerg
&
@0xkarmacoma
for contributing valuable content. This batch focuses on testing, tooling & coding patterns!
Learn more 👇🧵
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halmos v0.1.2 just released 🎁
- added support for DELEGATECALL, CALLCODE (lol) and CREATE2
- support for struct types as test parameters
- library linking is now supported
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- OSS
- no false positives
- integrates with Foundry
- we focus on performance and a good devex
- write your assertions in Solidity, not a spec language
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A guide to Corruption(s) Insight 🔮
Each token has an insight score. At first, it increases linearly with the time passed since the last transfer, roughly 1 point every 2 days.
After a month, things get interesting...
More Corruptions: they all have insight 1 for now
For NFTs not getting transferred, their insight will start increasing in about 2d (10k blocks)
Their insight multiplier will start increasing in 34d
They will reach max insight multiplier in 833d
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reject modernity (forge test --debug)
embrace tradition (console2.log("here"))
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🪂 SS2ERC721 starter kit
10x cheaper NFT airdrops, made easy
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