mruby-sprintf: protect format string from mutation during callbacks#6781
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mrb_str_format captured raw C pointers (p, end) into the format string's buffer before the main loop. The %s and %p specifiers call to_s and inspect, which can invoke Ruby code that mutates the format string via String#replace, freeing or reallocating its buffer. The loop then continued iterating with dangling pointers, reading freed memory and potentially leaking adjacent heap contents into the result. Duplicate the format string with mrb_str_dup() before the loop. This is O(1) because mrb_str_dup shares the underlying buffer; if the original is later mutated via String#replace, str_replace decrements the shared refcount, leaving our duplicate's buffer intact. Co-authored-by: Claude <noreply@anthropic.com>
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This pull request addresses a potential memory safety issue in mrb_str_format where callbacks could mutate the format string and invalidate internal pointers. It introduces a string duplication step to ensure the format string remains stable during processing and adds a corresponding test case to verify the fix. I have no feedback to provide.
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`mrb_str_dup()` always duplicates string objects in an unfrozen state, and the class is also set. Therefore, it can be observed and modified from the Ruby side using the `ObjectSpace.each_object` method. By using `mrb_str_dup_frozen()`, unnecessary duplication can be avoided, and modifications to the string can also be prevented.
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`mrb_str_dup()` always duplicates string objects in an unfrozen state, and the class is also set. Therefore, it can be observed and modified from the Ruby side using the `ObjectSpace.each_object` method. By using `mrb_str_dup_frozen()`, unnecessary duplication can be avoided, and modifications to the string can also be prevented.
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Summary
mrb_str_formatcaptured raw C pointers (p,end) into the format string's buffer before the main loop. The%sand%pspecifiers callto_sandinspect, which can invoke Ruby code that mutates the format string viaString#replace, freeing or reallocating its buffer. The loop then continued iterating with dangling pointers, reading freed memory and potentially leaking adjacent heap contents into the result string.Fix
Duplicate the format string with
mrb_str_dup()before the loop. This is O(1) becausemrb_str_dupshares the underlying buffer; if the original is later mutated viaString#replace,str_replacedecrements the shared refcount, leaving our duplicate's buffer intact.Test plan
rake CONFIG=host-debug all test:run:serial— 1810 lib + 100 bintest OKto_scallback calling$fmt.replace("Z")no longer corrupts sprintf output