Merge pull request #15117 from jketema/buffer

C++: Only consider the maximum buffer size for badly bounded write

Author: jketema

cpp/ql/src/Security/CWE/CWE-120/BadlyBoundedWrite.ql

@@ -24,7 +24,7 @@ import semmle.code.cpp.security.BufferWrite
 from BufferWrite bw, int destSize
 where
   bw.hasExplicitLimit() and // has an explicit size limit
-  destSize = getBufferSize(bw.getDest(), _) and
+  destSize = max(getBufferSize(bw.getDest(), _)) and
   bw.getExplicitLimit() > destSize // but it's larger than the destination
 select bw,
   "This '" + bw.getBWDesc() + "' operation is limited to " + bw.getExplicitLimit() +

cpp/ql/src/change-notes/2023-12-15-badly-bounded-write.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/badly-bounded-write` query could report false positives when a pointer was first initialized with a literal and later assigned a dynamically allocated array. These false positives now no longer occur.

cpp/ql/test/query-tests/Security/CWE/CWE-120/semmle/tests/BadlyBoundedWrite.expected

@@ -1,4 +1,2 @@
-| tests2.cpp:59:3:59:10 | call to snprintf | This 'call to snprintf' operation is limited to 13 bytes but the destination is only 2 bytes. |
-| tests2.cpp:63:3:63:10 | call to snprintf | This 'call to snprintf' operation is limited to 13 bytes but the destination is only 3 bytes. |
 | tests.c:43:3:43:10 | call to snprintf | This 'call to snprintf' operation is limited to 111 bytes but the destination is only 110 bytes. |
 | tests.c:46:3:46:10 | call to snprintf | This 'call to snprintf' operation is limited to 111 bytes but the destination is only 110 bytes. |

cpp/ql/test/query-tests/Security/CWE/CWE-120/semmle/tests/tests2.cpp

@@ -56,9 +56,9 @@ void test3() {
   dest1 = (char*)malloc(sizeof(src));
   if (!dest1)
     return;
-  snprintf(dest1, sizeof(src), "%s", src); // GOOD [FALSE POSITIVE]
+  snprintf(dest1, sizeof(src), "%s", src); // GOOD
   dest2 = (char*)malloc(3);
   if (!dest2)
     return;
-  snprintf(dest2, sizeof(src), "%s", src); // BAD (but with duplicate alerts)
+  snprintf(dest2, sizeof(src), "%s", src); // BAD [NOT DETECTED]
 }