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Avoid recursion in parsing escapes #4317

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Merged
kddnewton merged 1 commit into from
Oct 29, 2026
Merged

Avoid recursion in parsing escapes #4317

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24 changes: 15 additions & 9 deletions src/prism.c
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Original file line number Diff line number Diff line change
Expand Up @@ -9135,12 +9135,18 @@ escape_read_warn(pm_parser_t *parser, uint8_t flags, uint8_t flag, const char *t
}

/**
* Read the value of an escape into the buffer.
* Read the value of an escape into the buffer. The control and meta forms can
* wrap another escape (as in \C-\M-x). Those loop back to the top with the
* accumulated flags rather than recursing, so that arbitrarily long chains of
* prefixes use constant stack space regardless of whether the compiler
* eliminates tail calls.
*/
static void
escape_read(pm_parser_t *parser, pm_buffer_t *buffer, pm_buffer_t *regular_expression_buffer, uint8_t flags) {
uint8_t peeked = peek(parser);
switch (peeked) {
uint8_t peeked;

escape_read_next:
switch (peeked = peek(parser)) {
case '\\': {
parser->current.end++;
escape_write_byte(parser, buffer, regular_expression_buffer, flags, escape_byte('\\', flags));
Expand Down Expand Up @@ -9397,8 +9403,8 @@ escape_read(pm_parser_t *parser, pm_buffer_t *buffer, pm_buffer_t *regular_expre
return;
}

escape_read(parser, buffer, regular_expression_buffer, flags | PM_ESCAPE_FLAG_CONTROL);
return;
flags |= PM_ESCAPE_FLAG_CONTROL;
goto escape_read_next;
case ' ':
parser->current.end++;
escape_read_warn(parser, flags, PM_ESCAPE_FLAG_CONTROL, "\\s");
Expand Down Expand Up @@ -9455,8 +9461,8 @@ escape_read(pm_parser_t *parser, pm_buffer_t *buffer, pm_buffer_t *regular_expre
return;
}

escape_read(parser, buffer, regular_expression_buffer, flags | PM_ESCAPE_FLAG_CONTROL);
return;
flags |= PM_ESCAPE_FLAG_CONTROL;
goto escape_read_next;
case ' ':
parser->current.end++;
escape_read_warn(parser, flags, PM_ESCAPE_FLAG_CONTROL, "\\s");
Expand Down Expand Up @@ -9509,8 +9515,8 @@ escape_read(pm_parser_t *parser, pm_buffer_t *buffer, pm_buffer_t *regular_expre
return;
}

escape_read(parser, buffer, regular_expression_buffer, flags | PM_ESCAPE_FLAG_META);
return;
flags |= PM_ESCAPE_FLAG_META;
goto escape_read_next;
case ' ':
parser->current.end++;
escape_read_warn(parser, flags, PM_ESCAPE_FLAG_META, "\\s");
Expand Down
13 changes: 13 additions & 0 deletions test/prism/api/parse_test.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -210,8 +210,21 @@ def test_scopes
assert Prism.parse_success?("foo(...)", scopes: [Prism.scope(forwarding: [:"..."])])
end

def test_repeated_escape_prefixes
count = 100_000

assert_repeated_escape_prefixes("\"#{"\\M-" * count}a\"", "meta cannot be repeated", count - 1)
assert_repeated_escape_prefixes("?#{"\\C-" * count}a", "control cannot be repeated", count - 1)
assert_repeated_escape_prefixes("/#{"\\c" * count}a/", "control cannot be repeated", count - 1)
end

private

def assert_repeated_escape_prefixes(source, message, expected)
messages = Prism.parse(source).errors.map(&:message)
assert_equal expected, messages.count { |m| m.end_with?(message) }
end

def find_source_file_node(program)
queue = [program]
while (node = queue.shift)
Expand Down
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