[analyzer] Fix format attribute handling in GenericTaintChecker (#132765)

Previously `optin.taint.GenericTaint` misinterpreted the parameter
indices and produced false positives in situations when a [format
attribute](https://clang.llvm.org/docs/AttributeReference.html#format)
is applied on a non-static method. This commit fixes this bug

Author: NagyDonat

clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp

@@ -1080,7 +1080,23 @@ static bool getPrintfFormatArgumentNum(const CallEvent &Call,
   const ArgIdxTy CallNumArgs = fromArgumentCount(Call.getNumArgs());
 
   for (const auto *Format : FDecl->specific_attrs<FormatAttr>()) {
+    // The format attribute uses 1-based parameter indexing, for example
+    // plain `printf(const char *fmt, ...)` would be annotated with
+    // `__format__(__printf__, 1, 2)`, so we need to subtract 1 to get a
+    // 0-based index. (This checker uses 0-based parameter indices.)
     ArgNum = Format->getFormatIdx() - 1;
+    // The format attribute also counts the implicit `this` parameter of
+    // methods, so e.g. in `SomeClass::method(const char *fmt, ...)` could be
+    // annotated with `__format__(__printf__, 2, 3)`. This checker doesn't
+    // count the implicit `this` parameter, so in this case we need to subtract
+    // one again.
+    // FIXME: Apparently the implementation of the format attribute doesn't
+    // support methods with an explicit object parameter, so we cannot
+    // implement proper support for that rare case either.
+    const CXXMethodDecl *MDecl = dyn_cast<CXXMethodDecl>(FDecl);
+    if (MDecl && !MDecl->isStatic())
+      ArgNum--;
+
     if ((Format->getType()->getName() == "printf") && CallNumArgs > ArgNum)
       return true;
   }

clang/test/Analysis/taint-generic.cpp

@@ -161,3 +161,45 @@ void top() {
   clang_analyzer_isTainted(A.data); // expected-warning {{YES}}
 }
 } // namespace gh114270
+
+
+namespace format_attribute {
+__attribute__((__format__ (__printf__, 1, 2)))
+void log_freefunc(const char *fmt, ...);
+
+void test_format_attribute_freefunc() {
+  int n;
+  fscanf(stdin, "%d", &n); // Get a tainted value.
+                           
+  log_freefunc("This number is suspicious: %d\n", n); // no-warning
+}
+
+struct Foo {
+  // When the format attribute is applied to a method, argumet '1' is the
+  // implicit `this`, so e.g. in this case argument '2' specifies `fmt`.
+  // Specifying '1' instead of '2' would produce a compilation error:
+  // "format attribute cannot specify the implicit this argument as the format string"
+  __attribute__((__format__ (__printf__, 2, 3)))
+  void log_method(const char *fmt, ...);
+
+  void test_format_attribute_method() {
+    int n;
+    fscanf(stdin, "%d", &n); // Get a tainted value.
+                             
+    // The analyzer used to misinterpret the parameter indices in the format
+    // attribute when the format attribute is applied to a method.
+    log_method("This number is suspicious: %d\n", n); // no-warning
+  }
+
+  __attribute__((__format__ (__printf__, 1, 2)))
+  static void log_static_method(const char *fmt, ...);
+
+  void test_format_attribute_static_method() {
+    int n;
+    fscanf(stdin, "%d", &n); // Get a tainted value.
+                             
+    log_static_method("This number is suspicious: %d\n", n); // no-warning
+  }
+};
+
+} // namespace format_attribute