fuzz: refactor memcpy to std::ranges::copy to work around ubsan warn #33743

1. 
   maflcko commented at 10:10 am on October 30, 2025: member

   Using std::ranges::copy from the C++ standard library has a few benefits here:

   • It has the additional benefit of being a bit more type safe and document the byte cast explicitly.
   • The compiler will likely optimize it to the same asm, but performance doesn’t really matter here anyway.
   • It has defined semantics for empty source ranges.

   Fixes https://github.com/bitcoin/bitcoin/issues/33643

2. fuzz: refactor memcpy to std::ranges::copy to work around ubsan warn

   Using std::ranges::copy from the C++ standard library has a few benefits
   here:
   
   * It has the additional benefit of being a bit more type safe and
     document the byte cast explicitly.
   * The compiler will likely optimize it to the same asm, but performance
     doesn't really matter here anyway.
   * It works around an UB-Sanitizer bug, when the source range is empty.
   
   Fixes https://github.com/bitcoin/bitcoin/issues/33643

   fa4b52bd16
3. DrahtBot added the label Tests on Oct 30, 2025
4. 
   DrahtBot commented at 10:10 am on October 30, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33743.

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5. maflcko added this to the milestone 31.0 on Oct 30, 2025
6. 
   fanquake commented at 10:11 am on October 30, 2025: member

   cc @dergoegge @marcofleon
7. maflcko force-pushed on Oct 30, 2025
8. 
   purpleKarrot commented at 1:30 pm on October 30, 2025: contributor

   • It works around an UB-Sanitizer bug, when the source range is empty.

   I guess what you want to express is:

   • It has defined semantics for empty source ranges.

   Passing a zero length to memcpy is UB per the language standard. This is not an “UB-Sanitizer bug”.

9. fanquake added the label Fuzzing on Oct 30, 2025
10. 
    maflcko commented at 2:18 pm on October 30, 2025: member

    Yeah, I guess it is too early to call this a bug, but the language standard has been changed, see #33644 (comment).

    I’ve used your wording for now.

11. 
    marcofleon commented at 3:01 pm on October 30, 2025: contributor

    tACK fa4b52bd16189d40761c5976b8427e30779aba23

    Tested this branch and didn’t see the null pointer error. The change to std::byte is for modernization and the change to std::ranges addresses the issue.

12. dergoegge approved
13. 
    dergoegge commented at 9:54 am on October 31, 2025: member

    utACK fa4b52bd16189d40761c5976b8427e30779aba23
14. fanquake merged this on Oct 31, 2025
15. fanquake closed this on Oct 31, 2025

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16. maflcko deleted the branch on Oct 31, 2025

Contributors
maflcko DrahtBot fanquake purpleKarrot marcofleon dergoegge

Labels
Tests Fuzzing

Milestone
31.0