bpf: Fix a verifier message for alloc size helper arg

The error message here is misleading, the argument will be rejected unless
it is a known constant.
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210112123913.2016804-1-jackmanb@google.com

kernel/bpf/verifier.c

@@ -4319,7 +4319,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			err = mark_chain_precision(env, regno);
 	} else if (arg_type_is_alloc_size(arg_type)) {
 		if (!tnum_is_const(reg->var_off)) {
-			verbose(env, "R%d unbounded size, use 'var &= const' or 'if (var < const)'\n",
+			verbose(env, "R%d is not a known constant'\n",
 				regno);
 			return -EACCES;
 		}