[3.13] pythongh-127971: fix off-by-one read beyond the end of a string during search (python#132574) (python#136648)

(cherry picked from commit 85ec3b3)

Author: duaneg

Lib/test/string_tests.py

@@ -767,6 +767,15 @@ def test_replace(self):
         self.checkraises(TypeError, 'hello', 'replace', 42, 'h')
         self.checkraises(TypeError, 'hello', 'replace', 'h', 42)
 
+    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

Misc/NEWS.d/next/Core and Builtins/2025-04-16-12-01-13.gh-issue-127971.pMDOQ0.rst

@@ -0,0 +1 @@
+Fix off-by-one read beyond the end of a string in string search.

Objects/stringlib/fastsearch.h

@@ -588,7 +588,7 @@ 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 {
@@ -597,7 +597,7 @@ 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;
             }
         }
@@ -661,7 +661,7 @@ 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 {
@@ -670,7 +670,7 @@ 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;
             }
         }