util: use explicit cast in MultiIntBitSet::Fill() #31431

1. 
   vasild commented at 4:01 pm on December 5, 2024: contributor

   The current code does not have a bug, but is implicitly casting -1 to 65535 and the sanitizer has no way to know whether we intend that or not.

   0FUZZ=bitset src/test/fuzz/fuzz /tmp/fuz
   1
   2error: implicit conversion from type 'int' of value -1 (32-bit, signed)
   3to type 'value_type' (aka 'unsigned short') changed the value to 65535
   4(16-bit, unsigned)
   5
   6Base64: Qv7bX/8=
   

   https://api.cirrus-ci.com/v1/task/5685829642747904/logs/ci.log

2. util: use explicit cast in MultiIntBitSet::Fill()

   The current code does not have a bug, but is implicitly casting -1 to
   65535 and the sanitizer has no way to know whether we intend that or
   not.
   
   ```
   FUZZ=bitset src/test/fuzz/fuzz /tmp/fuz
   
   error: implicit conversion from type 'int' of value -1 (32-bit, signed)
   to type 'value_type' (aka 'unsigned short') changed the value to 65535
   (16-bit, unsigned)
   
   Base64: Qv7bX/8=
   ```

   edb41e4814
3. 
   DrahtBot commented at 4:01 pm on December 5, 2024: 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/31431.

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4. DrahtBot added the label Utils/log/libs on Dec 5, 2024
5. 
   vasild commented at 4:03 pm on December 5, 2024: contributor

   An alternative:

   16:53 < sipa> vasild: i would also be ok with a suppression, because this isn’t UB

   The typecast is used also elsewhere and is more robust wrt moving the code around.

6. 
   sipa commented at 4:03 pm on December 5, 2024: member

   ACK edb41e4814ccc2c06a5694b2d2632dbbd22bc0cf
7. 
   maflcko commented at 4:24 pm on December 5, 2024: member

   lgtm ACK edb41e4814ccc2c06a5694b2d2632dbbd22bc0cf
8. 
   Empact commented at 8:04 pm on December 5, 2024: contributor

   ACK edb41e4814ccc2c06a5694b2d2632dbbd22bc0cf
9. tdb3 approved
10. 
    tdb3 commented at 2:40 am on December 6, 2024: contributor

    code review ACK edb41e4814ccc2c06a5694b2d2632dbbd22bc0cf
11. fanquake merged this on Dec 6, 2024
12. fanquake closed this on Dec 6, 2024

    ---

13. vasild deleted the branch on Dec 6, 2024
14. achow101 added the label Needs backport (28.x) on Dec 11, 2024
15. achow101 referenced this in commit b8112cf422 on Dec 11, 2024
16. fanquake removed the label Needs backport (28.x) on Dec 17, 2024
17. 
    fanquake commented at 11:15 am on December 17, 2024: member

    Backported in #31469.

Contributors
vasild DrahtBot sipa maflcko Empact tdb3 fanquake

Labels
Utils/log/libs