selftests/bpf: Add BTF_KIND_FLOAT to test_core_reloc_size

Verify that bpf_core_field_size() is working correctly with floats.
Also document the required clang version.
Suggested-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210309005649.162480-2-iii@linux.ibm.com

tools/testing/selftests/bpf/README.rst

@@ -170,3 +170,12 @@ failures:
 .. _2: https://reviews.llvm.org/D85174
 .. _3: https://reviews.llvm.org/D83878
 .. _4: https://reviews.llvm.org/D83242
+
+Floating-point tests and Clang version
+======================================
+
+Certain selftests, e.g. core_reloc, require support for the floating-point
+types, which was introduced in `Clang 13`__. The older Clang versions will
+either crash when compiling these tests, or generate an incorrect BTF.
+
+__  https://reviews.llvm.org/D83289

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

@@ -266,6 +266,7 @@ static int duration = 0;
 		.arr_elem_sz = sizeof(((type *)0)->arr_field[0]),	\
 		.ptr_sz = 8, /* always 8-byte pointer for BPF */	\
 		.enum_sz = sizeof(((type *)0)->enum_field),		\
+		.float_sz = sizeof(((type *)0)->float_field),		\
 	}
 
 #define SIZE_CASE(name) {						\

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

@@ -807,6 +807,7 @@ struct core_reloc_size_output {
 	int arr_elem_sz;
 	int ptr_sz;
 	int enum_sz;
+	int float_sz;
 };
 
 struct core_reloc_size {
@@ -816,6 +817,7 @@ struct core_reloc_size {
 	int arr_field[4];
 	void *ptr_field;
 	enum { VALUE = 123 } enum_field;
+	float float_field;
 };
 
 struct core_reloc_size___diff_sz {
@@ -825,6 +827,7 @@ struct core_reloc_size___diff_sz {
 	char arr_field[10];
 	void *ptr_field;
 	enum { OTHER_VALUE = 0xFFFFFFFFFFFFFFFF } enum_field;
+	double float_field;
 };
 
 /* Error case of two candidates with the fields (int_field) at the same
@@ -839,6 +842,7 @@ struct core_reloc_size___err_ambiguous1 {
 	int arr_field[4];
 	void *ptr_field;
 	enum { VALUE___1 = 123 } enum_field;
+	float float_field;
 };
 
 struct core_reloc_size___err_ambiguous2 {
@@ -850,6 +854,7 @@ struct core_reloc_size___err_ambiguous2 {
 	int arr_field[4];
 	void *ptr_field;
 	enum { VALUE___2 = 123 } enum_field;
+	float float_field;
 };
 
 /*

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

@@ -21,6 +21,7 @@ struct core_reloc_size_output {
 	int arr_elem_sz;
 	int ptr_sz;
 	int enum_sz;
+	int float_sz;
 };
 
 struct core_reloc_size {
@@ -30,6 +31,7 @@ struct core_reloc_size {
 	int arr_field[4];
 	void *ptr_field;
 	enum { VALUE = 123 } enum_field;
+	float float_field;
 };
 
 SEC("raw_tracepoint/sys_enter")
@@ -45,6 +47,7 @@ int test_core_size(void *ctx)
 	out->arr_elem_sz = bpf_core_field_size(in->arr_field[0]);
 	out->ptr_sz = bpf_core_field_size(in->ptr_field);
 	out->enum_sz = bpf_core_field_size(in->enum_field);
+	out->float_sz = bpf_core_field_size(in->float_field);
 
 	return 0;
 }