bpf: Allow kfuncs return 'void *'

Recognize return of 'void *' from kfunc as returning unknown scalar.

Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -12364,6 +12364,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 					meta.func_name);
 				return -EFAULT;
 			}
+		} else if (btf_type_is_void(ptr_type)) {
+			/* kfunc returning 'void *' is equivalent to returning scalar */
+			mark_reg_unknown(env, regs, BPF_REG_0);
 		} else if (!__btf_type_is_struct(ptr_type)) {
 			if (!meta.r0_size) {
 				__u32 sz;