selftests/bpf: Annotate bpf_obj_new_impl() with __must_check

The verifier requires that pointers returned by bpf_obj_new_impl() are
either dropped or stored in a map. Therefore programs that do not use
its return values will fail to load. Make the compiler point out these
issues. Adjust selftests that check that the verifier does indeed spot
these bugs.

Note that now there two different bpf_obj_new_impl() declarations: one
with __must_check from bpf_experimental.h, and one without from
vmlinux.h. According to the GCC doc [1] this is fine and has the
desired effect:

    Compatible attribute specifications on distinct declarations of the
    same function are merged.

[1] https://gcc.gnu.org/onlinedocs/gcc-12.4.0/gcc/Function-Attributes.html

Link: https://lore.kernel.org/bpf/CAADnVQL6Q+QRv3_JwEd26biwGpFYcwD_=BjBJWLAtpgOP9CKRw@mail.gmail.com/
Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Ilya Leoshkevich <[email protected]>

Author: iii-i

tools/testing/selftests/bpf/bpf_experimental.h

@@ -8,6 +8,10 @@
 
 #define __contains(name, node) __attribute__((btf_decl_tag("contains:" #name ":" #node)))
 
+#ifndef __must_check
+#define __must_check __attribute__((__warn_unused_result__))
+#endif
+
 /* Description
  *	Allocates an object of the type represented by 'local_type_id' in
  *	program BTF. User may use the bpf_core_type_id_local macro to pass the
@@ -20,7 +24,7 @@
  *	A pointer to an object of the type corresponding to the passed in
  *	'local_type_id', or NULL on failure.
  */
-extern void *bpf_obj_new_impl(__u64 local_type_id, void *meta) __ksym;
+extern __must_check void *bpf_obj_new_impl(__u64 local_type_id, void *meta) __ksym;
 
 /* Convenience macro to wrap over bpf_obj_new_impl */
 #define bpf_obj_new(type) ((type *)bpf_obj_new_impl(bpf_core_type_id_local(type), NULL))

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

@@ -212,22 +212,33 @@ int map_compat_raw_tp_w(void *ctx)
 SEC("?tc")
 int obj_type_id_oor(void *ctx)
 {
-	bpf_obj_new_impl(~0UL, NULL);
+	void *f;
+
+	f = bpf_obj_new_impl(~0UL, NULL);
+	(void)f;
+
 	return 0;
 }
 
 SEC("?tc")
 int obj_new_no_composite(void *ctx)
 {
-	bpf_obj_new_impl(bpf_core_type_id_local(int), (void *)42);
+	void *f;
+
+	f = bpf_obj_new_impl(bpf_core_type_id_local(int), (void *)42);
+	(void)f;
+
 	return 0;
 }
 
 SEC("?tc")
 int obj_new_no_struct(void *ctx)
 {
+	void *f;
+
+	f = bpf_obj_new(union { int data; unsigned udata; });
+	(void)f;
 
-	bpf_obj_new(union { int data; unsigned udata; });
 	return 0;
 }
 
@@ -252,7 +263,11 @@ int new_null_ret(void *ctx)
 SEC("?tc")
 int obj_new_acq(void *ctx)
 {
-	bpf_obj_new(struct foo);
+	void *f;
+
+	f = bpf_obj_new(struct foo);
+	(void)f;
+
 	return 0;
 }