mruby-string-ext: fix String#prepend with self-referencing arguments#6780
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String#prepend(s, s) read RSTRING_LEN(argv[i]) in the copy loop after
mrb_str_resize had already updated the receiver's length, causing the
memcpy to write past the allocated buffer.
Detect self-references with mrb_obj_eq() and read from the memmoved
original data at p + total_prepend_len using the captured self_len.
This also handles mixed cases like s.prepend("X", s) where earlier
writes would otherwise corrupt the source of later reads.
Co-authored-by: Claude <noreply@anthropic.com>
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This pull request fixes a bug in String#prepend where self-referencing arguments (e.g., s.prepend(s)) could lead to incorrect data being copied because the original string's content was being overwritten during the prepend operation. The fix ensures that if an argument is the string itself, the source pointer is adjusted to the new location of the original data. New test cases have been added to verify this behavior. I have no feedback to provide.
matz
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Summary
String#prepend(s, s)readRSTRING_LEN(argv[i])in the copy loop aftermrb_str_resizehad already updated the receiver's length, causingmemcpyto write past the allocated buffer. Introduced in2cd8eb386, only on master/stable (not yet released).Detect self-references with
mrb_obj_eq()and read from the memmoved original data atp + total_prepend_lenusing the capturedself_len. This also handles mixed cases likes.prepend("X", s)where earlier writes would otherwise corrupt the source of later reads.Test plan
rake CONFIG=host-debug all test:run:serial— 1809 lib + 100 bintest OKs.prepend(s, s),s.prepend("XYZ", s), ands.prepend(s, "X", s)