selftests/bpf: Add test validating failure on ambiguous relocation value

Add test simulating ambiguous field size relocation, while fields themselves
are at the exact same offset.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200818223921.2911963-5-andriin@fb.com

tools/testing/selftests/bpf/prog_tests/core_reloc.c

@@ -452,6 +452,7 @@ static struct core_reloc_test_case test_cases[] = {
 	/* size relocation checks */
 	SIZE_CASE(size),
 	SIZE_CASE(size___diff_sz),
+	SIZE_ERR_CASE(size___err_ambiguous),
 };
 
 struct data {

tools/testing/selftests/bpf/progs/btf__core_reloc_size___err_ambiguous.c

@@ -0,0 +1,4 @@
+#include "core_reloc_types.h"
+
+void f(struct core_reloc_size___err_ambiguous1 x,
+       struct core_reloc_size___err_ambiguous2 y) {}

tools/testing/selftests/bpf/progs/core_reloc_types.h

@@ -809,3 +809,28 @@ struct core_reloc_size___diff_sz {
 	void *ptr_field;
 	enum { OTHER_VALUE = 0xFFFFFFFFFFFFFFFF } enum_field;
 };
+
+/* Error case of two candidates with the fields (int_field) at the same
+ * offset, but with differing final relocation values: size 4 vs size 1
+ */
+struct core_reloc_size___err_ambiguous1 {
+	/* int at offset 0 */
+	int int_field;
+
+	struct { int x; } struct_field;
+	union { int x; } union_field;
+	int arr_field[4];
+	void *ptr_field;
+	enum { VALUE___1 = 123 } enum_field;
+};
+
+struct core_reloc_size___err_ambiguous2 {
+	/* char at offset 0 */
+	char int_field;
+
+	struct { int x; } struct_field;
+	union { int x; } union_field;
+	int arr_field[4];
+	void *ptr_field;
+	enum { VALUE___2 = 123 } enum_field;
+};