bpf: btf: Add BTF_FMODEL_SIGNED_ARG flag

s390x eBPF JIT needs to know whether a function return value is signed
and which function arguments are signed, in order to generate code
compliant with the s390x ABI.

Signed-off-by: Ilya Leoshkevich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/bpf.h

···
 /* The argument is a structure. */
 #define BTF_FMODEL_STRUCT_ARG		BIT(0)
 
+/* The argument is signed. */
+#define BTF_FMODEL_SIGNED_ARG		BIT(1)
+
 struct btf_func_model {
 	u8 ret_size;
+	u8 ret_flags;
 	u8 nr_args;
 	u8 arg_size[MAX_BPF_FUNC_ARGS];
 	u8 arg_flags[MAX_BPF_FUNC_ARGS];

include/linux/btf.h

···
 	return btf_type_is_int(t) && t->size <= sizeof(u64);
 }
 
+static inline u8 btf_int_encoding(const struct btf_type *t)
+{
+	return BTF_INT_ENCODING(*(u32 *)(t + 1));
+}
+
+static inline bool btf_type_is_signed_int(const struct btf_type *t)
+{
+	return btf_type_is_int(t) && (btf_int_encoding(t) & BTF_INT_SIGNED);
+}
+
 static inline bool btf_type_is_enum(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM;
···
 static inline u8 btf_int_offset(const struct btf_type *t)
 {
 	return BTF_INT_OFFSET(*(u32 *)(t + 1));
-}
-
-static inline u8 btf_int_encoding(const struct btf_type *t)
-{
-	return BTF_INT_ENCODING(*(u32 *)(t + 1));
 }
 
 static inline bool btf_type_is_scalar(const struct btf_type *t)

kernel/bpf/btf.c

···
 	return -EINVAL;
 }
 
+static u8 __get_type_fmodel_flags(const struct btf_type *t)
+{
+	u8 flags = 0;
+
+	if (__btf_type_is_struct(t))
+		flags |= BTF_FMODEL_STRUCT_ARG;
+	if (btf_type_is_signed_int(t))
+		flags |= BTF_FMODEL_SIGNED_ARG;
+
+	return flags;
+}
+
 int btf_distill_func_proto(struct bpf_verifier_log *log,
 			   struct btf *btf,
 			   const struct btf_type *func,
···
 			m->arg_flags[i] = 0;
 		}
 		m->ret_size = 8;
+		m->ret_flags = 0;
 		m->nr_args = MAX_BPF_FUNC_REG_ARGS;
 		return 0;
 	}
···
 		return -EINVAL;
 	}
 	m->ret_size = ret;
+	m->ret_flags = __get_type_fmodel_flags(t);
 
 	for (i = 0; i < nargs; i++) {
 		if (i == nargs - 1 && args[i].type == 0) {
···
 			return -EINVAL;
 		}
 		m->arg_size[i] = ret;
-		m->arg_flags[i] = __btf_type_is_struct(t) ? BTF_FMODEL_STRUCT_ARG : 0;
+		m->arg_flags[i] = __get_type_fmodel_flags(t);
 	}
 	m->nr_args = nargs;
 	return 0;