bpf: Allow union argument in trampoline based programs

Currently, functions with 'union' arguments cannot be traced with
fentry/fexit:

bpftrace -e 'fentry:release_pages { exit(); }' -v
AST node count: 6
Attaching 1 probe...
ERROR: Error loading BPF program for fentry_vmlinux_release_pages_1.
Kernel error log:
The function release_pages arg0 type UNION is unsupported.
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

ERROR: Loading BPF object(s) failed.

The type of the 'release_pages' argument is defined as:

typedef union {
	struct page **pages;
	struct folio **folios;
	struct encoded_page **encoded_pages;
} release_pages_arg __attribute__ ((__transparent_union__));

This patch relaxes the restriction by allowing function arguments of type
'union' to be traced in verifier.

Signed-off-by: Leon Hwang <[email protected]>
Reviewed-by: Amery Hung <[email protected]>

Author: Asphaltt

include/linux/bpf.h

@@ -1119,6 +1119,9 @@ struct bpf_prog_offload {
 /* The argument is signed. */
 #define BTF_FMODEL_SIGNED_ARG		BIT(1)
 
+/* The argument is a union. */
+#define BTF_FMODEL_UNION_ARG		BIT(2)
+
 struct btf_func_model {
 	u8 ret_size;
 	u8 ret_flags;

include/linux/btf.h

@@ -404,6 +404,11 @@ static inline bool btf_type_is_struct(const struct btf_type *t)
 	return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
 }
 
+static inline bool __btf_type_is_union(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_UNION;
+}
+
 static inline bool __btf_type_is_struct(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_STRUCT;

kernel/bpf/btf.c

@@ -6762,7 +6762,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	/* skip modifiers */
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
-	if (btf_type_is_small_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
+	if (btf_type_is_small_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
 		/* accessing a scalar */
 		return true;
 	if (!btf_type_is_ptr(t)) {
@@ -7334,7 +7334,7 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
 	if (btf_type_is_ptr(t))
 		/* kernel size of pointer. Not BPF's size of pointer*/
 		return sizeof(void *);
-	if (btf_type_is_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
+	if (btf_type_is_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
 		return t->size;
 	return -EINVAL;
 }
@@ -7347,6 +7347,8 @@ static u8 __get_type_fmodel_flags(const struct btf_type *t)
 		flags |= BTF_FMODEL_STRUCT_ARG;
 	if (btf_type_is_signed_int(t))
 		flags |= BTF_FMODEL_SIGNED_ARG;
+	if (__btf_type_is_union(t))
+		flags |= BTF_FMODEL_UNION_ARG;
 
 	return flags;
 }
@@ -7384,7 +7386,7 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 		return -EINVAL;
 	}
 	ret = __get_type_size(btf, func->type, &t);
-	if (ret < 0 || __btf_type_is_struct(t)) {
+	if (ret < 0 || btf_type_is_struct(t)) {
 		bpf_log(log,
 			"The function %s return type %s is unsupported.\n",
 			tname, btf_type_str(t));