Malcolm: Prevent Trojan Source attacks with GCC 12

David Malcolm describes some GCC improvements to defend against bidirectional-text attacks in source code.

My colleague Marek Polacek and I implemented a new warning for GCC 12, -Wbidi-chars, for detecting Trojan Source attacks involving Unicode control characters. Marek implemented the guts of the warning, but when I tried it out on the examples provided by the Trojan Source researchers, I found I had trouble understanding the initial results—precisely because of the obfuscation itself.

So for GCC 12, I've added a new flag to GCC diagnostics, indicating that the diagnostic itself relates to source code encoding. When any such diagnostic is printed, GCC will now escape non-ASCII characters in the source code.

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