[clang] [Clang][counted_by] Honor counted_by in __bdos on direct struct access (PR #201161)

[Kees Cook via cfe-commits]

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@@ -0,0 +1,60 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O2 -Wno-missing-declarations \
+// RUN:   -emit-llvm -o - %s | FileCheck %s
+
+// See https://github.com/llvm/llvm-project/issues/200014
+//
+// __builtin_dynamic_object_size on a flexible array member must consult the
+// 'counted_by' attribute, even when the containing struct is accessed
+// directly (local or global variable) rather than through a pointer
+// dereference. Previously the AST constant evaluator would fold the call to
+// a layout-derived size (trailing padding for locals; trailing initializer
+// data for globals), bypassing the runtime counted_by path in CGBuiltin.
+
+typedef __SIZE_TYPE__ size_t;
+
+struct annotated_flex {
+    size_t  count;
+    char    induce_padding;
+    char    fam[] __attribute__((counted_by(count)));
+};
+
+struct annotated_flex gaf = {
+    .fam = "i am very long",
+    .count = 10,
+};
+
+extern size_t sink;
+
+// Global access: must consult gaf.count at runtime; the size must not be
+// folded to the static initializer length (15).
+//
+// CHECK-LABEL: define dso_local {{.*}}i64 @test_global()
+// CHECK:         load i64, ptr @gaf
+// CHECK-NOT:     ret i64 15
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kees wrote:

I always forget about `update_cc_test_checks.py`. I will get the tests improved.

https://github.com/llvm/llvm-project/pull/201161