kernel: Avoid an out-of-range shift in CopyBits

[fingolfin]

CopyBits and its helpers shift by amounts derived from <frombit>, <tobit> and the alignment difference between the two blocks. Several of those can reach BIPEB, where 1 << BIPEB and x >> BIPEB are undefined:

• After a partial first destination word is filled, <frombit> can be left equal to BIPEB rather than wrapping to the next source word. MaskForCopyBits then evaluates (UInt)1 << BIPEB.
• In the main loop <frombit> equal to 0 makes the second half of the word assembly shift by BIPEB - 0.
• CopyInWord shifts by <shift>. The callers do keep it in range, but only indirectly: in the tobit + BIPEB - frombit case it takes the branch condition frombit + tailbits > BIPEB to show frombit > tobit.

Normalise <frombit> to the start of the next word, make MaskForCopyBits total so an out-of-range bound yields an empty mask, special-case the aligned main loop, and bound the shift in CopyInWord directly.

An exhaustive sweep over frombit, tobit in [0,BIPEB) and nbits in [1,5*BIPEB] hits the undefined shift in 1953 of 1310720 cases, and gives identical results before and after: in every one of those cases the mask being computed is dead, because no whole words remain to copy. So this changes no observable behaviour on current compilers.

The guards also do a second job. They are what lets a static analyzer establish the bounds: without them clang's core.BitwiseShift reports "Left shift overflows the capacity of 'UInt'" for paths reaching here from blister.c and vecgf2.c. Please keep them even where a local reading suggests the branch is dead.

Co-authored-by: Claude Opus 5 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.97%. Comparing base (989cc35) to head (77a2746).

Files with missing lines	Patch %	Lines
src/bits_intern.h	66.66%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6534      +/-   ##
==========================================
- Coverage   78.98%   78.97%   -0.01%     
==========================================
  Files         684      684              
  Lines      294205   294226      +21     
  Branches     8647     8678      +31     
==========================================
+ Hits       232370   232377       +7     
- Misses      60028    60034       +6     
- Partials     1807     1815       +8     

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[lgoettgens]

I don't understand all the nitty bitty details, but looks sensible overall