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gh-127971: fix off-by-one read beyond the end of a string during search #132574

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Merged
merged 7 commits into from
Jul 13, 2025
Merged

gh-127971: fix off-by-one read beyond the end of a string during search #132574

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f0d20b2
gh-127971: do not read past the end of a string
duaneg Apr 15, 2025
462dddf
Add test case
duaneg Apr 16, 2025
1d0210a
Add blurb
duaneg Apr 16, 2025
212452d
Add test case for adaptive_find and comments better explaining the re…
duaneg Jun 21, 2025
c07c23e
Tweak conditional phrasing to match loop terminating criteria, add co…
duaneg Jun 21, 2025
338f32d
Address more reviewer feedback: add conditional to adaptive find, tweak
duaneg Jul 12, 2025
9884ec1
Always run test, not just under ASAN
duaneg Jul 13, 2025
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11 changes: 11 additions & 0 deletions Lib/test/string_tests.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@

import unittest, string, sys, struct
from test import support
from test.support import check_sanitizer
from test.support import import_helper
from collections import UserList
import random
Expand Down Expand Up @@ -767,6 +768,16 @@ def test_replace(self):
self.checkraises(TypeError, 'hello', 'replace', 42, 'h')
self.checkraises(TypeError, 'hello', 'replace', 'h', 42)

@unittest.skipUnless(check_sanitizer(address=True), "AddressSanitizer required")
def test_replacement_on_buffer_boundary(self):
# gh-127971: Check we don't read past the end of the buffer when a
# potential match misses on the last character.
any_3_nonblank_codepoints = '!!!'
seven_codepoints = any_3_nonblank_codepoints + ' ' + any_3_nonblank_codepoints
a = (' ' * 243) + seven_codepoints + (' ' * 7)
b = ' ' * 6 + chr(256)
a.replace(seven_codepoints, b)

def test_replace_uses_two_way_maxcount(self):
# Test that maxcount works in _two_way_count in fastsearch.h
A, B = "A"*1000, "B"*1000
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-04-16-12-01-13.gh-issue-127971.pMDOQ0.rst
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@@ -0,0 +1 @@
Fix off-by-one read beyond the end of a string in string search.
8 changes: 4 additions & 4 deletions Objects/stringlib/fastsearch.h
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Original file line number Diff line number Diff line change
Expand Up @@ -595,7 +595,7 @@ STRINGLIB(default_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
continue;
}
/* miss: check if next character is part of pattern */
if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
i = i + m;
}
else {
Expand All @@ -604,7 +604,7 @@ STRINGLIB(default_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
}
else {
/* skip: check if next character is part of pattern */
if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
i = i + m;
}
}
Expand Down Expand Up @@ -668,7 +668,7 @@ STRINGLIB(adaptive_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
}
}
/* miss: check if next character is part of pattern */
if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
i = i + m;
}
else {
Expand All @@ -677,7 +677,7 @@ STRINGLIB(adaptive_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
}
else {
/* skip: check if next character is part of pattern */
if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
i = i + m;
}
}
Expand Down
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