bpf: selftests: test_loader: Support __btf_path() annotation

This commit adds support for per-prog btf_custom_path. This is necessary
for testing CO-RE relocations on non-vmlinux types using test_loader
infrastructure.

Acked-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Xu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

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

···
 #define __retval_unpriv(val)	__attribute__((btf_decl_tag("comment:test_retval_unpriv="#val)))
 #define __auxiliary		__attribute__((btf_decl_tag("comment:test_auxiliary")))
 #define __auxiliary_unpriv	__attribute__((btf_decl_tag("comment:test_auxiliary_unpriv")))
+#define __btf_path(path)	__attribute__((btf_decl_tag("comment:test_btf_path=" path)))
 
 /* Convenience macro for use with 'asm volatile' blocks */
 #define __naked __attribute__((naked))

tools/testing/selftests/bpf/test_loader.c

···
 #define TEST_TAG_RETVAL_PFX_UNPRIV "comment:test_retval_unpriv="
 #define TEST_TAG_AUXILIARY "comment:test_auxiliary"
 #define TEST_TAG_AUXILIARY_UNPRIV "comment:test_auxiliary_unpriv"
+#define TEST_BTF_PATH "comment:test_btf_path="
 
 /* Warning: duplicated in bpf_misc.h */
 #define POINTER_VALUE	0xcafe4all
···
 	const char *prog_name;
 	struct test_subspec priv;
 	struct test_subspec unpriv;
+	const char *btf_custom_path;
 	int log_level;
 	int prog_flags;
 	int mode_mask;
···
 					goto cleanup;
 				update_flags(&spec->prog_flags, flags, clear);
 			}
+		} else if (str_has_pfx(s, TEST_BTF_PATH)) {
+			spec->btf_custom_path = s + sizeof(TEST_BTF_PATH) - 1;
 		}
 	}
 
···
 			return;
 		}
 	}
+
+	/* Implicitly reset to NULL if next test case doesn't specify */
+	open_opts->btf_custom_path = spec->btf_custom_path;
 
 	tobj = bpf_object__open_mem(obj_bytes, obj_byte_cnt, open_opts);
 	if (!ASSERT_OK_PTR(tobj, "obj_open_mem")) /* shouldn't happen */