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mruby-string-ext: fix String#prepend with self-referencing arguments #6780

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

mruby-string-ext: fix String#prepend with self-referencing arguments #6780

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18 changes: 15 additions & 3 deletions mrbgems/mruby-string-ext/src/string.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2201,15 +2201,27 @@ str_prepend(mrb_state *mrb, mrb_value self)

char *p = RSTRING_PTR(self);

/* Move original content to the end */
/* Move original content to the end. The original self data now lives
at p + total_prepend_len, which we use as the source for any
self-referencing arguments (e.g., s.prepend(s, s)) to avoid reading
data that has already been overwritten by earlier copies. */
memmove(p + total_prepend_len, p, self_len);

/* Copy prepended strings in order */
mrb_int offset = 0;
for (mrb_int i = 0; i < argc; i++) {
mrb_int arg_len = RSTRING_LEN(argv[i]);
const char *src;
mrb_int arg_len;
if (mrb_obj_eq(mrb, self, argv[i])) {
src = p + total_prepend_len;
arg_len = self_len;
}
else {
src = RSTRING_PTR(argv[i]);
arg_len = RSTRING_LEN(argv[i]);
}
if (arg_len > 0) {
memcpy(p + offset, RSTRING_PTR(argv[i]), arg_len);
memcpy(p + offset, src, arg_len);
offset += arg_len;
}
}
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15 changes: 15 additions & 0 deletions mrbgems/mruby-string-ext/test/string.rb
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Expand Up @@ -492,6 +492,21 @@ def assert_upto(exp, receiver, *args)
g = "world"
assert_equal "hello world", g.prepend("", "hello ", "")
assert_equal "hello world", g

# Self-referencing arguments (GHSA-3hgj-g76g-878c)
h = "A" * 100
h.prepend(h, h)
assert_equal 300, h.length
assert_equal "A" * 300, h

# Mixed self-reference and literal
i = "AB"
i.prepend("XYZ", i)
assert_equal "XYZABAB", i

j = "AB"
j.prepend(j, "X", j)
assert_equal "ABXABAB", j
end

assert('String#ljust') do
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