[ruby/prism] Avoid reading out-of-bounds in pm_strnstr

stevenjohnstone · matzbot · commit 026140b3a209 · 2025-11-23T18:45:57.000Z
Fixes ruby/prism#3738. ruby/prism@37bb46ff5f
diff --git a/prism/prism.c b/prism/prism.c
@@ -22639,7 +22639,7 @@ static const char *
 pm_strnstr(const char *big, const char *little, size_t big_length) {
     size_t little_length = strlen(little);
 
-    for (const char *big_end = big + big_length; big < big_end; big++) {
+    for (const char *max = big + big_length - little_length; big <= max; big++) {
         if (*big == *little && memcmp(big, little, little_length) == 0) return big;
     }
 

Original file line number	Diff line number	Diff line change
`@@ -22639,7 +22639,7 @@ static const char *`
`22639`	`22639`	`pm_strnstr(const char big, const char little, size_t big_length) {`
`22640`	`22640`	`size_t little_length = strlen(little);`
`22641`	`22641`
`22642`		`- for (const char *big_end = big + big_length; big < big_end; big++) {`
	`22642`	`+ for (const char *max = big + big_length - little_length; big <= max; big++) {`
`22643`	`22643`	`if (big == little && memcmp(big, little, little_length) == 0) return big;`
`22644`	`22644`	`}`
`22645`	`22645`