4 ms·

It’s not too surprising that SF can be exploited. The interface is designed to be operated from a (non-adversarial) GUI, and efficiency is paramount, so it unde

It’s not too surprising that SF can be exploited. The interface is designed to be operated from a (non-adversarial) GUI, and efficiency is paramount, so it understandably has little sanitization. Also, this would be more interesting on previous versions of SF before the NN eval, as the handcrafted eval has been battle hardened for years. That said, this is still a cool analysis to identify a concrete exploit!

rmgl 6y ago
You can easily crash Stockfish from the xboard GUI by editing the position and playing with two kings on each side. Illegal position, sure, but validating it first would not affect the fast core logic. The lack of validation is surprising because the code looks extremely clean and well structured otherwise.
- nemo44x 6y ago
  The British rules of Chess or some British federation) used to allow promotion to a second king. Oddly you could also promote to your opponents color if you wanted too. These rules were abandon in the 19th century though.
  - saagarjha 6y ago
    I assume those rules also let you capture the king?
    - nemo44x 6y ago
      I forget the nuance but yes, so long as there were 2 kings of a color they were treated like other pieces until 1 remained I think. The promotion of an opponents piece could end in a game where you promote your pawn to their knight which becomes a checkmate as the pieces were organized in a way that blocked the king.
    - jandrese 6y ago
      It would take a careful reading of the rules, but I think it is more likely that the wording would make it such that putting either king in checkmate would win the game. So you could promote to king, but that would be a liability so it made no sense. A better exploit would be to promote to your opponent’s king in a way that immediately puts it in checkmate.
  - jsmith45 6y ago
    Yeah that would be Law XIII of the Code of Laws of the British Chess Association. Law XIII said, "When a pawn has reached the eighth square, the player has the option of selecting a piece, whether such piece has previously been lost or not, whose names and powers it shall then assume, or of deciding that it shall remain a pawn." No restriction on piece type or color. So under those laws, promoting to a king or a piece of the other color would have been legal. From what I've read though, this sounds like it was sloppy wording, and the normal promotion rules we are familiar with today were actually intended, except for the option of remaining a pawn, which was obviously intentional since it is explicitly called out, and was actually the controversial part of that rule.
    - Enginerrrd 6y ago
      Interesting. I suppose that could let one draw an otherwise dominated game where an opponent is up by more than a queen. Under such circumstances, it seems like it would be quite unlikely to come up in practice. The promoting to an opponents color is interesting, though I don't know that I'd be inherently opposed to it. Who gets to move that piece? It would be your opponent wouldn't it?
      - jsmith45 6y ago
        >Who gets to move that piece? It would be your opponent wouldn't it? More an amusing curiosity than anything truly interesting. It would rarely be practical to give your opponent another piece. Puzzle creators have apparently come up with a few ways to make a scenario where that would be a move required to win, but my understanding is that none of them are terribly realistic.
      - LorenPechtel 6y ago
        Yup. I can't imagine it happening in reality but I have seen a puzzle that's mate in one by promoting to an enemy knight, causing a discovered check and the knight can't be captured because it is the opponent's piece.
  - fn1 6y ago
    That is like the requirement which was added in FIDE-rules 1972 stating that king and rook need to be on the same rank while castling. Before that you could promote a pawn to a rook and then castle vertically across the board.
    - SomeoneFromCA 6y ago
      That would have been an interesting novelty, for sure.
0xdada 6y ago
I think it's a misconception that safe handling of input will always cost performance. For Stockfish, I imagine the performance critical part is analysis, where input should already have been parsed.
- deleted 6y ago
  [deleted]

rmgl 6y ago

You can easily crash Stockfish from the xboard GUI by editing the position and playing with two kings on each side. Illegal position, sure, but validating it first would not affect the fast core logic. The lack of validation is surprising because the code looks extremely clean and well structured otherwise.

nemo44x 6y ago
The British rules of Chess or some British federation) used to allow promotion to a second king. Oddly you could also promote to your opponents color if you wanted too. These rules were abandon in the 19th century though.
- saagarjha 6y ago
  I assume those rules also let you capture the king?
  - nemo44x 6y ago
    I forget the nuance but yes, so long as there were 2 kings of a color they were treated like other pieces until 1 remained I think. The promotion of an opponents piece could end in a game where you promote your pawn to their knight which becomes a checkmate as the pieces were organized in a way that blocked the king.
  - jandrese 6y ago
    It would take a careful reading of the rules, but I think it is more likely that the wording would make it such that putting either king in checkmate would win the game. So you could promote to king, but that would be a liability so it made no sense. A better exploit would be to promote to your opponent’s king in a way that immediately puts it in checkmate.
- jsmith45 6y ago
  Yeah that would be Law XIII of the Code of Laws of the British Chess Association. Law XIII said, "When a pawn has reached the eighth square, the player has the option of selecting a piece, whether such piece has previously been lost or not, whose names and powers it shall then assume, or of deciding that it shall remain a pawn." No restriction on piece type or color. So under those laws, promoting to a king or a piece of the other color would have been legal. From what I've read though, this sounds like it was sloppy wording, and the normal promotion rules we are familiar with today were actually intended, except for the option of remaining a pawn, which was obviously intentional since it is explicitly called out, and was actually the controversial part of that rule.
  - Enginerrrd 6y ago
    Interesting. I suppose that could let one draw an otherwise dominated game where an opponent is up by more than a queen. Under such circumstances, it seems like it would be quite unlikely to come up in practice. The promoting to an opponents color is interesting, though I don't know that I'd be inherently opposed to it. Who gets to move that piece? It would be your opponent wouldn't it?
    - jsmith45 6y ago
      >Who gets to move that piece? It would be your opponent wouldn't it? More an amusing curiosity than anything truly interesting. It would rarely be practical to give your opponent another piece. Puzzle creators have apparently come up with a few ways to make a scenario where that would be a move required to win, but my understanding is that none of them are terribly realistic.
    - LorenPechtel 6y ago
      Yup. I can't imagine it happening in reality but I have seen a puzzle that's mate in one by promoting to an enemy knight, causing a discovered check and the knight can't be captured because it is the opponent's piece.
- fn1 6y ago
  That is like the requirement which was added in FIDE-rules 1972 stating that king and rook need to be on the same rank while castling. Before that you could promote a pawn to a rook and then castle vertically across the board.
  - SomeoneFromCA 6y ago
    That would have been an interesting novelty, for sure.

0xdada 6y ago

I think it's a misconception that safe handling of input will always cost performance. For Stockfish, I imagine the performance critical part is analysis, where input should already have been parsed.

deleted 6y ago
[deleted]