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mruby-sprintf: protect format string from mutation during callbacks #6781

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Merged
matz merged 1 commit into from
Apr 11, 2026
Merged

mruby-sprintf: protect format string from mutation during callbacks #6781

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7 changes: 7 additions & 0 deletions mrbgems/mruby-sprintf/src/sprintf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -380,6 +380,13 @@ mrb_str_format(mrb_state *mrb, mrb_int argc, const mrb_value *argv, mrb_value fm
argc++;
argv--;
mrb_ensure_string_type(mrb, fmt);
/* Duplicate the format string so that to_s/inspect callbacks invoked
during the loop cannot invalidate p/end by mutating the original
via String#replace or similar. mrb_str_dup shares the underlying
buffer, so this is O(1); String#replace on the original goes
through str_replace which decrements the shared refcount, leaving
our copy's buffer intact. */
fmt = mrb_str_dup(mrb, fmt);
p = RSTRING_PTR(fmt);
end = p + RSTRING_LEN(fmt);
blen = 0;
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16 changes: 16 additions & 0 deletions mrbgems/mruby-sprintf/test/sprintf.rb
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Expand Up @@ -90,3 +90,19 @@
"%?" % ""
end
end

assert("sprintf with to_s mutating format string") do
# The to_s callback must not be able to invalidate sprintf's internal
# iteration pointers by mutating the format string.
fmt = "%s" + "B" * 200
mutator = Object.new
$sprintf_test_fmt = fmt
def mutator.to_s
$sprintf_test_fmt.replace("Z")
"ok"
end
result = sprintf(fmt, mutator)
assert_equal 202, result.length
assert_equal "ok", result[0, 2]
assert_equal "B" * 200, result[2..]
end
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