[rajath201]

msgpack_vrefbuffer_migrate decides whether to grow to->array by comparing to->tail + nused against vbuf->end, the end of the source's iovec array, which is a separate allocation. Whether to grows therefore depends on heap layout: when its array sits above the source's, the realloc is skipped and the memcpy writes nused iovecs past to->end. Two default-initialized buffers holding 20 refs each are enough for ASan to report a 320-byte heap-buffer-overflow write at vrefbuffer.c:227, the overflow described in #1197.

Compare nused against the free slots in to, the same fix ee24845 made to the C++ vrefbuffer::migrate(), written as a count so the check never forms a pointer past to->end (the < to->end form suggested in the issue would still only grow when there is already room). The realloc now happens exactly when to is short, so the common path does no extra work. The new vrefbuffer_c_migrate_grows_destination test migrates into whichever array sits higher in memory so the old check fails deterministically: on the current tree it overflows under ASan and fails tail <= end without it, including gcc -m32, and with the fix ctest is 6/6 under clang and gcc with and without ASan/UBSan.

Fixes #1197.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.53%. Comparing base (eabfe29) to head (1c47ce0).
⚠️ Report is 6 commits behind head on c_master.
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Additional details and impacted files

@@             Coverage Diff              @@
##           c_master    #1200      +/-   ##
============================================
+ Coverage     57.47%   61.53%   +4.06%     
============================================
  Files            10       10              
  Lines          1124     1144      +20     
  Branches        129      140      +11     
============================================
+ Hits            646      704      +58     
+ Misses          478      440      -38     

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[redboltz]

@rajath201 , thank you for sending the PR. It looks good to me.