fuzz: Use FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION for pow checks #29305

1. 
   dergoegge commented at 2:18 pm on January 24, 2024: member

   Alternative to the mocking of CheckProofOfWork in #28043 for avoiding fuzzers to be blocked on proof-of-work checks.

   More on FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION: https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode

2. fuzz: Use FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION for pow checks

   Unless we are fuzzing `CheckProofOfWork` itself, it makes little sense
   to have it be a blocker when fuzzing. We therefore use
   `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` (only defined for fuzz builds
   by e.g. clang with -fsanitize=fuzzer, afl-clang-lto, etc.) to replace
   the actual proof-of-work checks with checking if the first bit on
   `nNonce` is set. This allows the fuzzer to create PoW (in)valid blocks
   by controling that one bit instead of having to grind for actual PoW.
   
   Checking for a bit in `nNonce` is preferable over checking for a bit in
   the block hash as `nNonce` is chosen by the fuzzer and when whole chains
   of blocks are created during fuzzing a change in an old block won't
   invalidate the fake PoW on newer blocks.

   37e5ebe60b
3. 
   DrahtBot commented at 2:18 pm on January 24, 2024: contributor

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4. DrahtBot added the label Tests on Jan 24, 2024
5. dergoegge marked this as a draft on Jan 24, 2024
6. 
   dergoegge commented at 3:07 pm on January 24, 2024: member

   Using clang with -fsanitize=fuzzer does actually not define FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION by default.
7. dergoegge closed this on Jan 24, 2024

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